Theory Polynomial
section ‹Polynomials as type over a ring structure›
theory Polynomial
imports
Complex_Main
"HOL-Library.More_List"
"HOL-Library.Infinite_Set"
Primes
begin
context semidom_modulo
begin
lemma not_dvd_imp_mod_neq_0:
‹a mod b ≠ 0› if ‹¬ b dvd a›
using that mod_0_imp_dvd [of a b] by blast
end
subsection ‹Auxiliary: operations for lists (later) representing coefficients›
definition cCons :: "'a::zero ⇒ 'a list ⇒ 'a list" (infixr ‹##› 65)
where "x ## xs = (if xs = [] ∧ x = 0 then [] else x # xs)"
lemma cCons_0_Nil_eq [simp]: "0 ## [] = []"
by (simp add: cCons_def)
lemma cCons_Cons_eq [simp]: "x ## y # ys = x # y # ys"
by (simp add: cCons_def)
lemma cCons_append_Cons_eq [simp]: "x ## xs @ y # ys = x # xs @ y # ys"
by (simp add: cCons_def)
lemma cCons_not_0_eq [simp]: "x ≠ 0 ⟹ x ## xs = x # xs"
by (simp add: cCons_def)
lemma strip_while_not_0_Cons_eq [simp]:
"strip_while (λx. x = 0) (x # xs) = x ## strip_while (λx. x = 0) xs"
proof (cases "x = 0")
case False
then show ?thesis by simp
next
case True
show ?thesis
proof (induct xs rule: rev_induct)
case Nil
with True show ?case by simp
next
case (snoc y ys)
then show ?case
by (cases "y = 0") (simp_all add: append_Cons [symmetric] del: append_Cons)
qed
qed
lemma tl_cCons [simp]: "tl (x ## xs) = xs"
by (simp add: cCons_def)
subsection ‹Definition of type ‹poly››
typedef (overloaded) 'a poly = "{f :: nat ⇒ 'a::zero. ∀⇩∞ n. f n = 0}"
morphisms coeff Abs_poly
by (auto intro!: ALL_MOST)
setup_lifting type_definition_poly
lemma poly_eq_iff: "p = q ⟷ (∀n. coeff p n = coeff q n)"
by (simp add: coeff_inject [symmetric] fun_eq_iff)
lemma poly_eqI: "(⋀n. coeff p n = coeff q n) ⟹ p = q"
by (simp add: poly_eq_iff)
lemma MOST_coeff_eq_0: "∀⇩∞ n. coeff p n = 0"
using coeff [of p] by simp
lemma coeff_Abs_poly:
assumes "⋀i. i > n ⟹ f i = 0"
shows "coeff (Abs_poly f) = f"
proof (rule Abs_poly_inverse, clarify)
have "eventually (λi. i > n) cofinite"
by (auto simp: MOST_nat)
thus "eventually (λi. f i = 0) cofinite"
by eventually_elim (use assms in auto)
qed
subsection ‹Degree of a polynomial›
definition degree :: "'a::zero poly ⇒ nat"
where "degree p = (LEAST n. ∀i>n. coeff p i = 0)"
lemma degree_cong:
assumes "⋀i. coeff p i = 0 ⟷ coeff q i = 0"
shows "degree p = degree q"
proof -
have "(λn. ∀i>n. poly.coeff p i = 0) = (λn. ∀i>n. poly.coeff q i = 0)"
using assms by (auto simp: fun_eq_iff)
thus ?thesis
by (simp only: degree_def)
qed
lemma coeff_Abs_poly_If_le:
"coeff (Abs_poly (λi. if i ≤ n then f i else 0)) = (λi. if i ≤ n then f i else 0)"
proof (rule Abs_poly_inverse, clarify)
have "eventually (λi. i > n) cofinite"
by (auto simp: MOST_nat)
thus "eventually (λi. (if i ≤ n then f i else 0) = 0) cofinite"
by eventually_elim auto
qed
lemma coeff_eq_0:
assumes "degree p < n"
shows "coeff p n = 0"
proof -
have "∃n. ∀i>n. coeff p i = 0"
using MOST_coeff_eq_0 by (simp add: MOST_nat)
then have "∀i>degree p. coeff p i = 0"
unfolding degree_def by (rule LeastI_ex)
with assms show ?thesis by simp
qed
lemma le_degree: "coeff p n ≠ 0 ⟹ n ≤ degree p"
by (erule contrapos_np, rule coeff_eq_0, simp)
lemma degree_le: "∀i>n. coeff p i = 0 ⟹ degree p ≤ n"
unfolding degree_def by (erule Least_le)
lemma less_degree_imp: "n < degree p ⟹ ∃i>n. coeff p i ≠ 0"
unfolding degree_def by (drule not_less_Least, simp)
subsection ‹The zero polynomial›
instantiation poly :: (zero) zero
begin
lift_definition zero_poly :: "'a poly"
is "λ_. 0"
by (rule MOST_I) simp
instance ..
end
lemma coeff_0 [simp]: "coeff 0 n = 0"
by transfer rule
lemma degree_0 [simp]: "degree 0 = 0"
by (rule order_antisym [OF degree_le le0]) simp
lemma leading_coeff_neq_0:
assumes "p ≠ 0"
shows "coeff p (degree p) ≠ 0"
proof (cases "degree p")
case 0
from ‹p ≠ 0› obtain n where "coeff p n ≠ 0"
by (auto simp add: poly_eq_iff)
then have "n ≤ degree p"
by (rule le_degree)
with ‹coeff p n ≠ 0› and ‹degree p = 0› show "coeff p (degree p) ≠ 0"
by simp
next
case (Suc n)
from ‹degree p = Suc n› have "n < degree p"
by simp
then have "∃i>n. coeff p i ≠ 0"
by (rule less_degree_imp)
then obtain i where "n < i" and "coeff p i ≠ 0"
by blast
from ‹degree p = Suc n› and ‹n < i› have "degree p ≤ i"
by simp
also from ‹coeff p i ≠ 0› have "i ≤ degree p"
by (rule le_degree)
finally have "degree p = i" .
with ‹coeff p i ≠ 0› show "coeff p (degree p) ≠ 0" by simp
qed
lemma leading_coeff_0_iff [simp]: "coeff p (degree p) = 0 ⟷ p = 0"
by (cases "p = 0") (simp_all add: leading_coeff_neq_0)
lemma degree_lessI:
assumes "p ≠ 0 ∨ n > 0" "∀k≥n. coeff p k = 0"
shows "degree p < n"
proof (cases "p = 0")
case False
show ?thesis
proof (rule ccontr)
assume *: "¬(degree p < n)"
define d where "d = degree p"
from ‹p ≠ 0› have "coeff p d ≠ 0"
by (auto simp: d_def)
moreover have "coeff p d = 0"
using assms(2) * by (auto simp: not_less)
ultimately show False by contradiction
qed
qed (use assms in auto)
lemma eq_zero_or_degree_less:
assumes "degree p ≤ n" and "coeff p n = 0"
shows "p = 0 ∨ degree p < n"
proof (cases n)
case 0
with ‹degree p ≤ n› and ‹coeff p n = 0› have "coeff p (degree p) = 0"
by simp
then have "p = 0" by simp
then show ?thesis ..
next
case (Suc m)
from ‹degree p ≤ n› have "∀i>n. coeff p i = 0"
by (simp add: coeff_eq_0)
with ‹coeff p n = 0› have "∀i≥n. coeff p i = 0"
by (simp add: le_less)
with ‹n = Suc m› have "∀i>m. coeff p i = 0"
by (simp add: less_eq_Suc_le)
then have "degree p ≤ m"
by (rule degree_le)
with ‹n = Suc m› have "degree p < n"
by (simp add: less_Suc_eq_le)
then show ?thesis ..
qed
lemma coeff_0_degree_minus_1: "coeff rrr dr = 0 ⟹ degree rrr ≤ dr ⟹ degree rrr ≤ dr - 1"
using eq_zero_or_degree_less by fastforce
subsection ‹List-style constructor for polynomials›
lift_definition pCons :: "'a::zero ⇒ 'a poly ⇒ 'a poly"
is "λa p. case_nat a (coeff p)"
by (rule MOST_SucD) (simp add: MOST_coeff_eq_0)
lemmas coeff_pCons = pCons.rep_eq
lemma coeff_pCons': "poly.coeff (pCons c p) n = (if n = 0 then c else poly.coeff p (n - 1))"
by transfer'(auto split: nat.splits)
lemma coeff_pCons_0 [simp]: "coeff (pCons a p) 0 = a"
by transfer simp
lemma coeff_pCons_Suc [simp]: "coeff (pCons a p) (Suc n) = coeff p n"
by (simp add: coeff_pCons)
lemma degree_pCons_le: "degree (pCons a p) ≤ Suc (degree p)"
by (rule degree_le) (simp add: coeff_eq_0 coeff_pCons split: nat.split)
lemma degree_pCons_eq: "p ≠ 0 ⟹ degree (pCons a p) = Suc (degree p)"
by (simp add: degree_pCons_le le_antisym le_degree)
lemma degree_pCons_0: "degree (pCons a 0) = 0"
proof -
have "degree (pCons a 0) ≤ Suc 0"
by (metis (no_types) degree_0 degree_pCons_le)
then show ?thesis
by (metis coeff_0 coeff_pCons_Suc degree_0 eq_zero_or_degree_less less_Suc0)
qed
lemma degree_pCons_eq_if [simp]: "degree (pCons a p) = (if p = 0 then 0 else Suc (degree p))"
by (simp add: degree_pCons_0 degree_pCons_eq)
lemma pCons_0_0 [simp]: "pCons 0 0 = 0"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)
lemma pCons_eq_iff [simp]: "pCons a p = pCons b q ⟷ a = b ∧ p = q"
proof safe
assume "pCons a p = pCons b q"
then have "coeff (pCons a p) 0 = coeff (pCons b q) 0"
by simp
then show "a = b"
by simp
next
assume "pCons a p = pCons b q"
then have "coeff (pCons a p) (Suc n) = coeff (pCons b q) (Suc n)" for n
by simp
then show "p = q"
by (simp add: poly_eq_iff)
qed
lemma pCons_eq_0_iff [simp]: "pCons a p = 0 ⟷ a = 0 ∧ p = 0"
using pCons_eq_iff [of a p 0 0] by simp
lemma pCons_cases [cases type: poly]:
obtains (pCons) a q where "p = pCons a q"
proof
show "p = pCons (coeff p 0) (Abs_poly (λn. coeff p (Suc n)))"
by transfer
(simp_all add: MOST_inj[where f=Suc and P="λn. p n = 0" for p] fun_eq_iff Abs_poly_inverse
split: nat.split)
qed
lemma pCons_induct [case_names 0 pCons, induct type: poly]:
assumes zero: "P 0"
assumes pCons: "⋀a p. a ≠ 0 ∨ p ≠ 0 ⟹ P p ⟹ P (pCons a p)"
shows "P p"
proof (induct p rule: measure_induct_rule [where f=degree])
case (less p)
obtain a q where "p = pCons a q" by (rule pCons_cases)
have "P q"
proof (cases "q = 0")
case True
then show "P q" by (simp add: zero)
next
case False
then have "degree (pCons a q) = Suc (degree q)"
by (rule degree_pCons_eq)
with ‹p = pCons a q› have "degree q < degree p"
by simp
then show "P q"
by (rule less.hyps)
qed
have "P (pCons a q)"
proof (cases "a ≠ 0 ∨ q ≠ 0")
case True
with ‹P q› show ?thesis by (auto intro: pCons)
next
case False
with zero show ?thesis by simp
qed
with ‹p = pCons a q› show ?case
by simp
qed
lemma degree_eq_zeroE:
fixes p :: "'a::zero poly"
assumes "degree p = 0"
obtains a where "p = pCons a 0"
proof -
obtain a q where p: "p = pCons a q"
by (cases p)
with assms have "q = 0"
by (cases "q = 0") simp_all
with p have "p = pCons a 0"
by simp
then show thesis ..
qed
subsection ‹Quickcheck generator for polynomials›
quickcheck_generator poly constructors: "0 :: _ poly", pCons
subsection ‹List-style syntax for polynomials›
syntax
"_poly" :: "args ⇒ 'a poly" (‹(‹indent=2 notation=‹mixfix polynomial enumeration››[:_:])›)
syntax_consts
"_poly" ⇌ pCons
translations
"[:x, xs:]" ⇌ "CONST pCons x [:xs:]"
"[:x:]" ⇌ "CONST pCons x 0"
subsection ‹Representation of polynomials by lists of coefficients›
primrec Poly :: "'a::zero list ⇒ 'a poly"
where
[code_post]: "Poly [] = 0"
| [code_post]: "Poly (a # as) = pCons a (Poly as)"
lemma Poly_replicate_0 [simp]: "Poly (replicate n 0) = 0"
by (induct n) simp_all
lemma Poly_eq_0: "Poly as = 0 ⟷ (∃n. as = replicate n 0)"
by (induct as) (auto simp add: Cons_replicate_eq)
lemma Poly_append_replicate_zero [simp]: "Poly (as @ replicate n 0) = Poly as"
by (induct as) simp_all
lemma Poly_snoc_zero [simp]: "Poly (as @ [0]) = Poly as"
using Poly_append_replicate_zero [of as 1] by simp
lemma Poly_cCons_eq_pCons_Poly [simp]: "Poly (a ## p) = pCons a (Poly p)"
by (simp add: cCons_def)
lemma Poly_on_rev_starting_with_0 [simp]: "hd as = 0 ⟹ Poly (rev (tl as)) = Poly (rev as)"
by (cases as) simp_all
lemma degree_Poly: "degree (Poly xs) ≤ length xs"
by (induct xs) simp_all
lemma coeff_Poly_eq [simp]: "coeff (Poly xs) = nth_default 0 xs"
by (induct xs) (simp_all add: fun_eq_iff coeff_pCons split: nat.splits)
definition coeffs :: "'a poly ⇒ 'a::zero list"
where "coeffs p = (if p = 0 then [] else map (λi. coeff p i) [0 ..< Suc (degree p)])"
lemma coeffs_eq_Nil [simp]: "coeffs p = [] ⟷ p = 0"
by (simp add: coeffs_def)
lemma not_0_coeffs_not_Nil: "p ≠ 0 ⟹ coeffs p ≠ []"
by simp
lemma coeffs_0_eq_Nil [simp]: "coeffs 0 = []"
by simp
lemma coeffs_pCons_eq_cCons [simp]: "coeffs (pCons a p) = a ## coeffs p"
proof -
have *: "∀m∈set ms. m > 0 ⟹ map (case_nat x f) ms = map f (map (λn. n - 1) ms)"
for ms :: "nat list" and f :: "nat ⇒ 'a" and x :: "'a"
by (induct ms) (auto split: nat.split)
show ?thesis
by (simp add: * coeffs_def upt_conv_Cons coeff_pCons map_decr_upt del: upt_Suc)
qed
lemma length_coeffs: "p ≠ 0 ⟹ length (coeffs p) = degree p + 1"
by (simp add: coeffs_def)
lemma coeffs_nth: "p ≠ 0 ⟹ n ≤ degree p ⟹ coeffs p ! n = coeff p n"
by (auto simp: coeffs_def simp del: upt_Suc)
lemma coeff_in_coeffs: "p ≠ 0 ⟹ n ≤ degree p ⟹ coeff p n ∈ set (coeffs p)"
using coeffs_nth [of p n, symmetric] by (simp add: length_coeffs)
lemma not_0_cCons_eq [simp]: "p ≠ 0 ⟹ a ## coeffs p = a # coeffs p"
by (simp add: cCons_def)
lemma Poly_coeffs [simp, code abstype]: "Poly (coeffs p) = p"
by (induct p) auto
lemma coeffs_Poly [simp]: "coeffs (Poly as) = strip_while (HOL.eq 0) as"
proof (induct as)
case Nil
then show ?case by simp
next
case (Cons a as)
from replicate_length_same [of as 0] have "(∀n. as ≠ replicate n 0) ⟷ (∃a∈set as. a ≠ 0)"
by (auto dest: sym [of _ as])
with Cons show ?case by auto
qed
lemma no_trailing_coeffs [simp]:
"no_trailing (HOL.eq 0) (coeffs p)"
by (induct p) auto
lemma strip_while_coeffs [simp]:
"strip_while (HOL.eq 0) (coeffs p) = coeffs p"
by simp
lemma coeffs_eq_iff: "p = q ⟷ coeffs p = coeffs q"
(is "?P ⟷ ?Q")
proof
assume ?P
then show ?Q by simp
next
assume ?Q
then have "Poly (coeffs p) = Poly (coeffs q)" by simp
then show ?P by simp
qed
lemma nth_default_coeffs_eq: "nth_default 0 (coeffs p) = coeff p"
by (simp add: fun_eq_iff coeff_Poly_eq [symmetric])
lemma [code]: "coeff p = nth_default 0 (coeffs p)"
by (simp add: nth_default_coeffs_eq)
lemma coeffs_eqI:
assumes coeff: "⋀n. coeff p n = nth_default 0 xs n"
assumes zero: "no_trailing (HOL.eq 0) xs"
shows "coeffs p = xs"
proof -
from coeff have "p = Poly xs"
by (simp add: poly_eq_iff)
with zero show ?thesis by simp
qed
lemma degree_eq_length_coeffs [code]: "degree p = length (coeffs p) - 1"
by (simp add: coeffs_def)
lemma length_coeffs_degree: "p ≠ 0 ⟹ length (coeffs p) = Suc (degree p)"
by (induct p) (auto simp: cCons_def)
lemma [code abstract]: "coeffs 0 = []"
by (fact coeffs_0_eq_Nil)
lemma [code abstract]: "coeffs (pCons a p) = a ## coeffs p"
by (fact coeffs_pCons_eq_cCons)
lemma set_coeffs_subset_singleton_0_iff [simp]:
"set (coeffs p) ⊆ {0} ⟷ p = 0"
by (auto simp add: coeffs_def intro: classical)
lemma set_coeffs_not_only_0 [simp]:
"set (coeffs p) ≠ {0}"
by (auto simp add: set_eq_subset)
lemma forall_coeffs_conv:
"(∀n. P (coeff p n)) ⟷ (∀c ∈ set (coeffs p). P c)" if "P 0"
using that by (auto simp add: coeffs_def)
(metis atLeastLessThan_iff coeff_eq_0 not_less_iff_gr_or_eq zero_le)
instantiation poly :: ("{zero, equal}") equal
begin
definition [code]: "HOL.equal (p::'a poly) q ⟷ HOL.equal (coeffs p) (coeffs q)"
instance
by standard (simp add: equal equal_poly_def coeffs_eq_iff)
end
lemma [code nbe]: "HOL.equal (p :: _ poly) p ⟷ True"
by (fact equal_refl)
definition is_zero :: "'a::zero poly ⇒ bool"
where [code]: "is_zero p ⟷ List.null (coeffs p)"
lemma is_zero_null [code_abbrev]: "is_zero p ⟷ p = 0"
by (simp add: is_zero_def null_def)
text ‹Reconstructing the polynomial from the list›
definition poly_of_list :: "'a::comm_monoid_add list ⇒ 'a poly"
where [simp]: "poly_of_list = Poly"
lemma poly_of_list_impl [code abstract]: "coeffs (poly_of_list as) = strip_while (HOL.eq 0) as"
by simp
subsection ‹Fold combinator for polynomials›
definition fold_coeffs :: "('a::zero ⇒ 'b ⇒ 'b) ⇒ 'a poly ⇒ 'b ⇒ 'b"
where "fold_coeffs f p = foldr f (coeffs p)"
lemma fold_coeffs_0_eq [simp]: "fold_coeffs f 0 = id"
by (simp add: fold_coeffs_def)
lemma fold_coeffs_pCons_eq [simp]: "f 0 = id ⟹ fold_coeffs f (pCons a p) = f a ∘ fold_coeffs f p"
by (simp add: fold_coeffs_def cCons_def fun_eq_iff)
lemma fold_coeffs_pCons_0_0_eq [simp]: "fold_coeffs f (pCons 0 0) = id"
by (simp add: fold_coeffs_def)
lemma fold_coeffs_pCons_coeff_not_0_eq [simp]:
"a ≠ 0 ⟹ fold_coeffs f (pCons a p) = f a ∘ fold_coeffs f p"
by (simp add: fold_coeffs_def)
lemma fold_coeffs_pCons_not_0_0_eq [simp]:
"p ≠ 0 ⟹ fold_coeffs f (pCons a p) = f a ∘ fold_coeffs f p"
by (simp add: fold_coeffs_def)
subsection ‹Canonical morphism on polynomials -- evaluation›
definition poly :: ‹'a::comm_semiring_0 poly ⇒ 'a ⇒ 'a›
where ‹poly p a = horner_sum id a (coeffs p)›
lemma poly_eq_fold_coeffs:
‹poly p = fold_coeffs (λa f x. a + x * f x) p (λx. 0)›
by (induction p) (auto simp add: fun_eq_iff poly_def)
lemma poly_0 [simp]: "poly 0 x = 0"
by (simp add: poly_def)
lemma poly_pCons [simp]: "poly (pCons a p) x = a + x * poly p x"
by (cases "p = 0 ∧ a = 0") (auto simp add: poly_def)
lemma poly_altdef: "poly p x = (∑i≤degree p. coeff p i * x ^ i)"
for x :: "'a::{comm_semiring_0,semiring_1}"
proof (induction p rule: pCons_induct)
case 0
then show ?case
by simp
next
case (pCons a p)
show ?case
proof (cases "p = 0")
case True
then show ?thesis by simp
next
case False
let ?p' = "pCons a p"
note poly_pCons[of a p x]
also note pCons.IH
also have "a + x * (∑i≤degree p. coeff p i * x ^ i) =
coeff ?p' 0 * x^0 + (∑i≤degree p. coeff ?p' (Suc i) * x^Suc i)"
by (simp add: field_simps sum_distrib_left coeff_pCons)
also note sum.atMost_Suc_shift[symmetric]
also note degree_pCons_eq[OF ‹p ≠ 0›, of a, symmetric]
finally show ?thesis .
qed
qed
lemma poly_0_coeff_0: "poly p 0 = coeff p 0"
by (cases p) (auto simp: poly_altdef)
subsection ‹Monomials›
lift_definition monom :: "'a ⇒ nat ⇒ 'a::zero poly"
is "λa m n. if m = n then a else 0"
by (simp add: MOST_iff_cofinite)
lemma coeff_monom [simp]: "coeff (monom a m) n = (if m = n then a else 0)"
by transfer rule
lemma monom_0: "monom a 0 = [:a:]"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)
lemma monom_Suc: "monom a (Suc n) = pCons 0 (monom a n)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)
lemma monom_eq_0 [simp]: "monom 0 n = 0"
by (rule poly_eqI) simp
lemma monom_eq_0_iff [simp]: "monom a n = 0 ⟷ a = 0"
by (simp add: poly_eq_iff)
lemma monom_eq_iff [simp]: "monom a n = monom b n ⟷ a = b"
by (simp add: poly_eq_iff)
lemma degree_monom_le: "degree (monom a n) ≤ n"
by (rule degree_le, simp)
lemma degree_monom_eq: "a ≠ 0 ⟹ degree (monom a n) = n"
by (metis coeff_monom leading_coeff_0_iff)
lemma coeffs_monom [code abstract]:
"coeffs (monom a n) = (if a = 0 then [] else replicate n 0 @ [a])"
by (induct n) (simp_all add: monom_0 monom_Suc)
lemma fold_coeffs_monom [simp]: "a ≠ 0 ⟹ fold_coeffs f (monom a n) = f 0 ^^ n ∘ f a"
by (simp add: fold_coeffs_def coeffs_monom fun_eq_iff)
lemma poly_monom: "poly (monom a n) x = a * x ^ n"
for a x :: "'a::comm_semiring_1"
by (cases "a = 0", simp_all) (induct n, simp_all add: mult.left_commute poly_eq_fold_coeffs)
lemma monom_eq_iff': "monom c n = monom d m ⟷ c = d ∧ (c = 0 ∨ n = m)"
by (auto simp: poly_eq_iff)
lemma monom_eq_const_iff: "monom c n = [:d:] ⟷ c = d ∧ (c = 0 ∨ n = 0)"
using monom_eq_iff'[of c n d 0] by (simp add: monom_0)
subsection ‹Leading coefficient›
abbreviation lead_coeff:: "'a::zero poly ⇒ 'a"
where "lead_coeff p ≡ coeff p (degree p)"
lemma lead_coeff_pCons[simp]:
"p ≠ 0 ⟹ lead_coeff (pCons a p) = lead_coeff p"
"p = 0 ⟹ lead_coeff (pCons a p) = a"
by auto
lemma lead_coeff_monom [simp]: "lead_coeff (monom c n) = c"
by (cases "c = 0") (simp_all add: degree_monom_eq)
lemma last_coeffs_eq_coeff_degree:
"last (coeffs p) = lead_coeff p" if "p ≠ 0"
using that by (simp add: coeffs_def)
subsection ‹Addition and subtraction›
instantiation poly :: (comm_monoid_add) comm_monoid_add
begin
lift_definition plus_poly :: "'a poly ⇒ 'a poly ⇒ 'a poly"
is "λp q n. coeff p n + coeff q n"
proof -
fix q p :: "'a poly"
show "∀⇩∞n. coeff p n + coeff q n = 0"
using MOST_coeff_eq_0[of p] MOST_coeff_eq_0[of q] by eventually_elim simp
qed
lemma coeff_add [simp]: "coeff (p + q) n = coeff p n + coeff q n"
by (simp add: plus_poly.rep_eq)
instance
proof
fix p q r :: "'a poly"
show "(p + q) + r = p + (q + r)"
by (simp add: poly_eq_iff add.assoc)
show "p + q = q + p"
by (simp add: poly_eq_iff add.commute)
show "0 + p = p"
by (simp add: poly_eq_iff)
qed
end
instantiation poly :: (cancel_comm_monoid_add) cancel_comm_monoid_add
begin
lift_definition minus_poly :: "'a poly ⇒ 'a poly ⇒ 'a poly"
is "λp q n. coeff p n - coeff q n"
proof -
fix q p :: "'a poly"
show "∀⇩∞n. coeff p n - coeff q n = 0"
using MOST_coeff_eq_0[of p] MOST_coeff_eq_0[of q] by eventually_elim simp
qed
lemma coeff_diff [simp]: "coeff (p - q) n = coeff p n - coeff q n"
by (simp add: minus_poly.rep_eq)
instance
proof
fix p q r :: "'a poly"
show "p + q - p = q"
by (simp add: poly_eq_iff)
show "p - q - r = p - (q + r)"
by (simp add: poly_eq_iff diff_diff_eq)
qed
end
instantiation poly :: (ab_group_add) ab_group_add
begin
lift_definition uminus_poly :: "'a poly ⇒ 'a poly"
is "λp n. - coeff p n"
proof -
fix p :: "'a poly"
show "∀⇩∞n. - coeff p n = 0"
using MOST_coeff_eq_0 by simp
qed
lemma coeff_minus [simp]: "coeff (- p) n = - coeff p n"
by (simp add: uminus_poly.rep_eq)
instance
proof
fix p q :: "'a poly"
show "- p + p = 0"
by (simp add: poly_eq_iff)
show "p - q = p + - q"
by (simp add: poly_eq_iff)
qed
end
lemma add_pCons [simp]: "pCons a p + pCons b q = pCons (a + b) (p + q)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)
lemma minus_pCons [simp]: "- pCons a p = pCons (- a) (- p)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)
lemma diff_pCons [simp]: "pCons a p - pCons b q = pCons (a - b) (p - q)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)
lemma degree_add_le_max: "degree (p + q) ≤ max (degree p) (degree q)"
by (rule degree_le) (auto simp add: coeff_eq_0)
lemma degree_add_le: "degree p ≤ n ⟹ degree q ≤ n ⟹ degree (p + q) ≤ n"
by (auto intro: order_trans degree_add_le_max)
lemma degree_add_less: "degree p < n ⟹ degree q < n ⟹ degree (p + q) < n"
by (auto intro: le_less_trans degree_add_le_max)
lemma degree_add_eq_right: assumes "degree p < degree q" shows "degree (p + q) = degree q"
proof (cases "q = 0")
case False
show ?thesis
proof (rule order_antisym)
show "degree (p + q) ≤ degree q"
by (simp add: assms degree_add_le order.strict_implies_order)
show "degree q ≤ degree (p + q)"
by (simp add: False assms coeff_eq_0 le_degree)
qed
qed (use assms in auto)
lemma degree_add_eq_left: "degree q < degree p ⟹ degree (p + q) = degree p"
using degree_add_eq_right [of q p] by (simp add: add.commute)
lemma degree_minus [simp]: "degree (- p) = degree p"
by (simp add: degree_def)
lemma lead_coeff_add_le: "degree p < degree q ⟹ lead_coeff (p + q) = lead_coeff q"
by (metis coeff_add coeff_eq_0 monoid_add_class.add.left_neutral degree_add_eq_right)
lemma lead_coeff_minus: "lead_coeff (- p) = - lead_coeff p"
by (metis coeff_minus degree_minus)
lemma degree_diff_le_max: "degree (p - q) ≤ max (degree p) (degree q)"
for p q :: "'a::ab_group_add poly"
using degree_add_le [where p=p and q="-q"] by simp
lemma degree_diff_le: "degree p ≤ n ⟹ degree q ≤ n ⟹ degree (p - q) ≤ n"
for p q :: "'a::ab_group_add poly"
using degree_add_le [of p n "- q"] by simp
lemma degree_diff_less: "degree p < n ⟹ degree q < n ⟹ degree (p - q) < n"
for p q :: "'a::ab_group_add poly"
using degree_add_less [of p n "- q"] by simp
lemma add_monom: "monom a n + monom b n = monom (a + b) n"
by (rule poly_eqI) simp
lemma diff_monom: "monom a n - monom b n = monom (a - b) n"
by (rule poly_eqI) simp
lemma minus_monom: "- monom a n = monom (- a) n"
by (rule poly_eqI) simp
lemma coeff_sum: "coeff (∑x∈A. p x) i = (∑x∈A. coeff (p x) i)"
by (induct A rule: infinite_finite_induct) simp_all
lemma monom_sum: "monom (∑x∈A. a x) n = (∑x∈A. monom (a x) n)"
by (rule poly_eqI) (simp add: coeff_sum)
fun plus_coeffs :: "'a::comm_monoid_add list ⇒ 'a list ⇒ 'a list"
where
"plus_coeffs xs [] = xs"
| "plus_coeffs [] ys = ys"
| "plus_coeffs (x # xs) (y # ys) = (x + y) ## plus_coeffs xs ys"
lemma coeffs_plus_eq_plus_coeffs [code abstract]:
"coeffs (p + q) = plus_coeffs (coeffs p) (coeffs q)"
proof -
have *: "nth_default 0 (plus_coeffs xs ys) n = nth_default 0 xs n + nth_default 0 ys n"
for xs ys :: "'a list" and n
proof (induct xs ys arbitrary: n rule: plus_coeffs.induct)
case (3 x xs y ys n)
then show ?case
by (cases n) (auto simp add: cCons_def)
qed simp_all
have **: "no_trailing (HOL.eq 0) (plus_coeffs xs ys)"
if "no_trailing (HOL.eq 0) xs" and "no_trailing (HOL.eq 0) ys"
for xs ys :: "'a list"
using that by (induct xs ys rule: plus_coeffs.induct) (simp_all add: cCons_def)
show ?thesis
by (rule coeffs_eqI) (auto simp add: * nth_default_coeffs_eq intro: **)
qed
lemma coeffs_uminus [code abstract]:
"coeffs (- p) = map uminus (coeffs p)"
proof -
have eq_0: "HOL.eq 0 ∘ uminus = HOL.eq (0::'a)"
by (simp add: fun_eq_iff)
show ?thesis
by (rule coeffs_eqI) (simp_all add: nth_default_map_eq nth_default_coeffs_eq no_trailing_map eq_0)
qed
lemma [code]: "p - q = p + - q"
for p q :: "'a::ab_group_add poly"
by (fact diff_conv_add_uminus)
lemma poly_add [simp]: "poly (p + q) x = poly p x + poly q x"
proof (induction p arbitrary: q)
case (pCons a p)
then show ?case
by (cases q) (simp add: algebra_simps)
qed auto
lemma poly_minus [simp]: "poly (- p) x = - poly p x"
for x :: "'a::comm_ring"
by (induct p) simp_all
lemma poly_diff [simp]: "poly (p - q) x = poly p x - poly q x"
for x :: "'a::comm_ring"
using poly_add [of p "- q" x] by simp
lemma poly_sum: "poly (∑k∈A. p k) x = (∑k∈A. poly (p k) x)"
by (induct A rule: infinite_finite_induct) simp_all
lemma poly_sum_list: "poly (∑p←ps. p) y = (∑p←ps. poly p y)"
by (induction ps) auto
lemma poly_sum_mset: "poly (∑x∈#A. p x) y = (∑x∈#A. poly (p x) y)"
by (induction A) auto
lemma degree_sum_le: "finite S ⟹ (⋀p. p ∈ S ⟹ degree (f p) ≤ n) ⟹ degree (sum f S) ≤ n"
proof (induct S rule: finite_induct)
case empty
then show ?case by simp
next
case (insert p S)
then have "degree (sum f S) ≤ n" "degree (f p) ≤ n"
by auto
then show ?case
unfolding sum.insert[OF insert(1-2)] by (metis degree_add_le)
qed
lemma degree_sum_less:
assumes "⋀x. x ∈ A ⟹ degree (f x) < n" "n > 0"
shows "degree (sum f A) < n"
using assms by (induction rule: infinite_finite_induct) (auto intro!: degree_add_less)
lemma poly_as_sum_of_monoms':
assumes "degree p ≤ n"
shows "(∑i≤n. monom (coeff p i) i) = p"
proof -
have eq: "⋀i. {..n} ∩ {i} = (if i ≤ n then {i} else {})"
by auto
from assms show ?thesis
by (simp add: poly_eq_iff coeff_sum coeff_eq_0 sum.If_cases eq
if_distrib[where f="λx. x * a" for a])
qed
lemma poly_as_sum_of_monoms: "(∑i≤degree p. monom (coeff p i) i) = p"
by (intro poly_as_sum_of_monoms' order_refl)
lemma Poly_snoc: "Poly (xs @ [x]) = Poly xs + monom x (length xs)"
by (induct xs) (simp_all add: monom_0 monom_Suc)
subsection ‹Multiplication by a constant, polynomial multiplication and the unit polynomial›
lift_definition smult :: "'a::comm_semiring_0 ⇒ 'a poly ⇒ 'a poly"
is "λa p n. a * coeff p n"
proof -
fix a :: 'a and p :: "'a poly"
show "∀⇩∞ i. a * coeff p i = 0"
using MOST_coeff_eq_0[of p] by eventually_elim simp
qed
lemma coeff_smult [simp]: "coeff (smult a p) n = a * coeff p n"
by (simp add: smult.rep_eq)
lemma degree_smult_le: "degree (smult a p) ≤ degree p"
by (rule degree_le) (simp add: coeff_eq_0)
lemma smult_smult [simp]: "smult a (smult b p) = smult (a * b) p"
by (rule poly_eqI) (simp add: mult.assoc)
lemma smult_0_right [simp]: "smult a 0 = 0"
by (rule poly_eqI) simp
lemma smult_0_left [simp]: "smult 0 p = 0"
by (rule poly_eqI) simp
lemma smult_1_left [simp]: "smult (1::'a::comm_semiring_1) p = p"
by (rule poly_eqI) simp
lemma smult_add_right: "smult a (p + q) = smult a p + smult a q"
by (rule poly_eqI) (simp add: algebra_simps)
lemma smult_add_left: "smult (a + b) p = smult a p + smult b p"
by (rule poly_eqI) (simp add: algebra_simps)
lemma smult_minus_right [simp]: "smult a (- p) = - smult a p"
for a :: "'a::comm_ring"
by (rule poly_eqI) simp
lemma smult_minus_left [simp]: "smult (- a) p = - smult a p"
for a :: "'a::comm_ring"
by (rule poly_eqI) simp
lemma smult_diff_right: "smult a (p - q) = smult a p - smult a q"
for a :: "'a::comm_ring"
by (rule poly_eqI) (simp add: algebra_simps)
lemma smult_diff_left: "smult (a - b) p = smult a p - smult b p"
for a b :: "'a::comm_ring"
by (rule poly_eqI) (simp add: algebra_simps)
lemmas smult_distribs =
smult_add_left smult_add_right
smult_diff_left smult_diff_right
lemma smult_pCons [simp]: "smult a (pCons b p) = pCons (a * b) (smult a p)"
by (rule poly_eqI) (simp add: coeff_pCons split: nat.split)
lemma smult_monom: "smult a (monom b n) = monom (a * b) n"
by (induct n) (simp_all add: monom_0 monom_Suc)
lemma smult_Poly: "smult c (Poly xs) = Poly (map ((*) c) xs)"
by (auto simp: poly_eq_iff nth_default_def)
lemma degree_smult_eq [simp]: "degree (smult a p) = (if a = 0 then 0 else degree p)"
for a :: "'a::{comm_semiring_0,semiring_no_zero_divisors}"
by (cases "a = 0") (simp_all add: degree_def)
lemma smult_eq_0_iff [simp]: "smult a p = 0 ⟷ a = 0 ∨ p = 0"
for a :: "'a::{comm_semiring_0,semiring_no_zero_divisors}"
by (simp add: poly_eq_iff)
lemma coeffs_smult [code abstract]:
"coeffs (smult a p) = (if a = 0 then [] else map (Groups.times a) (coeffs p))"
for p :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
proof -
have eq_0: "HOL.eq 0 ∘ times a = HOL.eq (0::'a)" if "a ≠ 0"
using that by (simp add: fun_eq_iff)
show ?thesis
by (rule coeffs_eqI) (auto simp add: no_trailing_map nth_default_map_eq nth_default_coeffs_eq eq_0)
qed
lemma smult_eq_iff:
fixes b :: "'a :: field"
assumes "b ≠ 0"
shows "smult a p = smult b q ⟷ smult (a / b) p = q"
(is "?lhs ⟷ ?rhs")
proof
assume ?lhs
also from assms have "smult (inverse b) … = q"
by simp
finally show ?rhs
by (simp add: field_simps)
next
assume ?rhs
with assms show ?lhs by auto
qed
instantiation poly :: (comm_semiring_0) comm_semiring_0
begin
definition "p * q = fold_coeffs (λa p. smult a q + pCons 0 p) p 0"
lemma mult_poly_0_left: "(0::'a poly) * q = 0"
by (simp add: times_poly_def)
lemma mult_pCons_left [simp]: "pCons a p * q = smult a q + pCons 0 (p * q)"
by (cases "p = 0 ∧ a = 0") (auto simp add: times_poly_def)
lemma mult_poly_0_right: "p * (0::'a poly) = 0"
by (induct p) (simp_all add: mult_poly_0_left)
lemma mult_pCons_right [simp]: "p * pCons a q = smult a p + pCons 0 (p * q)"
by (induct p) (simp_all add: mult_poly_0_left algebra_simps)
lemmas mult_poly_0 = mult_poly_0_left mult_poly_0_right
lemma mult_smult_left [simp]: "smult a p * q = smult a (p * q)"
by (induct p) (simp_all add: mult_poly_0 smult_add_right)
lemma mult_smult_right [simp]: "p * smult a q = smult a (p * q)"
by (induct q) (simp_all add: mult_poly_0 smult_add_right)
lemma mult_poly_add_left: "(p + q) * r = p * r + q * r"
for p q r :: "'a poly"
by (induct r) (simp_all add: mult_poly_0 smult_distribs algebra_simps)
instance
proof
fix p q r :: "'a poly"
show 0: "0 * p = 0"
by (rule mult_poly_0_left)
show "p * 0 = 0"
by (rule mult_poly_0_right)
show "(p + q) * r = p * r + q * r"
by (rule mult_poly_add_left)
show "(p * q) * r = p * (q * r)"
by (induct p) (simp_all add: mult_poly_0 mult_poly_add_left)
show "p * q = q * p"
by (induct p) (simp_all add: mult_poly_0)
qed
end
lemma coeff_mult_degree_sum:
"coeff (p * q) (degree p + degree q) = coeff p (degree p) * coeff q (degree q)"
by (induct p) (simp_all add: coeff_eq_0)
instance poly :: ("{comm_semiring_0,semiring_no_zero_divisors}") semiring_no_zero_divisors
proof
fix p q :: "'a poly"
assume "p ≠ 0" and "q ≠ 0"
have "coeff (p * q) (degree p + degree q) = coeff p (degree p) * coeff q (degree q)"
by (rule coeff_mult_degree_sum)
also from ‹p ≠ 0› ‹q ≠ 0› have "coeff p (degree p) * coeff q (degree q) ≠ 0"
by simp
finally have "∃n. coeff (p * q) n ≠ 0" ..
then show "p * q ≠ 0"
by (simp add: poly_eq_iff)
qed
instance poly :: (comm_semiring_0_cancel) comm_semiring_0_cancel ..
lemma coeff_mult: "coeff (p * q) n = (∑i≤n. coeff p i * coeff q (n-i))"
proof (induct p arbitrary: n)
case 0
show ?case by simp
next
case (pCons a p n)
then show ?case
by (cases n) (simp_all add: sum.atMost_Suc_shift del: sum.atMost_Suc)
qed
lemma coeff_mult_0: "coeff (p * q) 0 = coeff p 0 * coeff q 0"
by (simp add: coeff_mult)
lemma degree_mult_le: "degree (p * q) ≤ degree p + degree q"
proof (rule degree_le)
show "∀i>degree p + degree q. coeff (p * q) i = 0"
by (induct p) (simp_all add: coeff_eq_0 coeff_pCons split: nat.split)
qed
lemma mult_monom: "monom a m * monom b n = monom (a * b) (m + n)"
by (induct m) (simp add: monom_0 smult_monom, simp add: monom_Suc)
instantiation poly :: (comm_semiring_1) comm_semiring_1
begin
lift_definition one_poly :: "'a poly"
is "λn. of_bool (n = 0)"
by (rule MOST_SucD) simp
lemma coeff_1 [simp]:
"coeff 1 n = of_bool (n = 0)"
by (simp add: one_poly.rep_eq)
lemma one_pCons:
"1 = [:1:]"
by (simp add: poly_eq_iff coeff_pCons split: nat.splits)
lemma pCons_one:
"[:1:] = 1"
by (simp add: one_pCons)
instance
by standard (simp_all add: one_pCons)
end
lemma poly_1 [simp]:
"poly 1 x = 1"
by (simp add: one_pCons)
lemma one_poly_eq_simps [simp]:
"1 = [:1:] ⟷ True"
"[:1:] = 1 ⟷ True"
by (simp_all add: one_pCons)
lemma degree_1 [simp]:
"degree 1 = 0"
by (simp add: one_pCons)
lemma coeffs_1_eq [simp, code abstract]:
"coeffs 1 = [1]"
by (simp add: one_pCons)
lemma smult_one [simp]:
"smult c 1 = [:c:]"
by (simp add: one_pCons)
lemma monom_eq_1 [simp]:
"monom 1 0 = 1"
by (simp add: monom_0 one_pCons)
lemma monom_eq_1_iff:
"monom c n = 1 ⟷ c = 1 ∧ n = 0"
using monom_eq_const_iff [of c n 1] by auto
lemma monom_altdef:
"monom c n = smult c ([:0, 1:] ^ n)"
by (induct n) (simp_all add: monom_0 monom_Suc)
instance poly :: ("{comm_semiring_1,semiring_1_no_zero_divisors}") semiring_1_no_zero_divisors ..
instance poly :: (comm_ring) comm_ring ..
instance poly :: (comm_ring_1) comm_ring_1 ..
instance poly :: (comm_ring_1) comm_semiring_1_cancel ..
lemma prod_smult: "(∏x∈A. smult (c x) (p x)) = smult (prod c A) (prod p A)"
by (induction A rule: infinite_finite_induct) (auto simp: mult_ac)
lemma degree_power_le: "degree (p ^ n) ≤ degree p * n"
by (induct n) (auto intro: order_trans degree_mult_le)
lemma coeff_0_power: "coeff (p ^ n) 0 = coeff p 0 ^ n"
by (induct n) (simp_all add: coeff_mult)
lemma poly_smult [simp]: "poly (smult a p) x = a * poly p x"
by (induct p) (simp_all add: algebra_simps)
lemma poly_mult [simp]: "poly (p * q) x = poly p x * poly q x"
by (induct p) (simp_all add: algebra_simps)
lemma poly_power [simp]: "poly (p ^ n) x = poly p x ^ n"
for p :: "'a::comm_semiring_1 poly"
by (induct n) simp_all
lemma poly_prod: "poly (∏k∈A. p k) x = (∏k∈A. poly (p k) x)"
by (induct A rule: infinite_finite_induct) simp_all
lemma poly_prod_list: "poly (∏p←ps. p) y = (∏p←ps. poly p y)"
by (induction ps) auto
lemma poly_prod_mset: "poly (∏x∈#A. p x) y = (∏x∈#A. poly (p x) y)"
by (induction A) auto
lemma poly_const_pow: "[: c :] ^ n = [: c ^ n :]"
by (induction n) (auto simp: algebra_simps)
lemma monom_power: "monom c n ^ k = monom (c ^ k) (n * k)"
by (induction k) (auto simp: mult_monom)
lemma degree_prod_sum_le: "finite S ⟹ degree (prod f S) ≤ sum (degree ∘ f) S"
proof (induct S rule: finite_induct)
case empty
then show ?case by simp
next
case (insert a S)
show ?case
unfolding prod.insert[OF insert(1-2)] sum.insert[OF insert(1-2)]
by (rule le_trans[OF degree_mult_le]) (use insert in auto)
qed
lemma coeff_0_prod_list: "coeff (prod_list xs) 0 = prod_list (map (λp. coeff p 0) xs)"
by (induct xs) (simp_all add: coeff_mult)
lemma coeff_monom_mult: "coeff (monom c n * p) k = (if k < n then 0 else c * coeff p (k - n))"
proof -
have "coeff (monom c n * p) k = (∑i≤k. (if n = i then c else 0) * coeff p (k - i))"
by (simp add: coeff_mult)
also have "… = (∑i≤k. (if n = i then c * coeff p (k - i) else 0))"
by (intro sum.cong) simp_all
also have "… = (if k < n then 0 else c * coeff p (k - n))"
by simp
finally show ?thesis .
qed
lemma monom_1_dvd_iff': "monom 1 n dvd p ⟷ (∀k<n. coeff p k = 0)"
proof
assume "monom 1 n dvd p"
then obtain r where "p = monom 1 n * r"
by (rule dvdE)
then show "∀k<n. coeff p k = 0"
by (simp add: coeff_mult)
next
assume zero: "(∀k<n. coeff p k = 0)"
define r where "r = Abs_poly (λk. coeff p (k + n))"
have "∀⇩∞k. coeff p (k + n) = 0"
by (subst cofinite_eq_sequentially, subst eventually_sequentially_seg,
subst cofinite_eq_sequentially [symmetric]) transfer
then have coeff_r [simp]: "coeff r k = coeff p (k + n)" for k
unfolding r_def by (subst poly.Abs_poly_inverse) simp_all
have "p = monom 1 n * r"
by (rule poly_eqI, subst coeff_monom_mult) (simp_all add: zero)
then show "monom 1 n dvd p" by simp
qed
subsection ‹Mapping polynomials›
definition map_poly :: "('a :: zero ⇒ 'b :: zero) ⇒ 'a poly ⇒ 'b poly"
where "map_poly f p = Poly (map f (coeffs p))"
lemma map_poly_0 [simp]: "map_poly f 0 = 0"
by (simp add: map_poly_def)
lemma map_poly_1: "map_poly f 1 = [:f 1:]"
by (simp add: map_poly_def)
lemma map_poly_1' [simp]: "f 1 = 1 ⟹ map_poly f 1 = 1"
by (simp add: map_poly_def one_pCons)
lemma coeff_map_poly:
assumes "f 0 = 0"
shows "coeff (map_poly f p) n = f (coeff p n)"
by (auto simp: assms map_poly_def nth_default_def coeffs_def not_less Suc_le_eq coeff_eq_0
simp del: upt_Suc)
lemma coeffs_map_poly [code abstract]:
"coeffs (map_poly f p) = strip_while ((=) 0) (map f (coeffs p))"
by (simp add: map_poly_def)
lemma coeffs_map_poly':
assumes "⋀x. x ≠ 0 ⟹ f x ≠ 0"
shows "coeffs (map_poly f p) = map f (coeffs p)"
using assms
by (auto simp add: coeffs_map_poly strip_while_idem_iff
last_coeffs_eq_coeff_degree no_trailing_unfold last_map)
lemma set_coeffs_map_poly:
"(⋀x. f x = 0 ⟷ x = 0) ⟹ set (coeffs (map_poly f p)) = f ` set (coeffs p)"
by (simp add: coeffs_map_poly')
lemma degree_map_poly:
assumes "⋀x. x ≠ 0 ⟹ f x ≠ 0"
shows "degree (map_poly f p) = degree p"
by (simp add: degree_eq_length_coeffs coeffs_map_poly' assms)
lemma map_poly_eq_0_iff:
assumes "f 0 = 0" "⋀x. x ∈ set (coeffs p) ⟹ x ≠ 0 ⟹ f x ≠ 0"
shows "map_poly f p = 0 ⟷ p = 0"
proof -
have "(coeff (map_poly f p) n = 0) = (coeff p n = 0)" for n
proof -
have "coeff (map_poly f p) n = f (coeff p n)"
by (simp add: coeff_map_poly assms)
also have "… = 0 ⟷ coeff p n = 0"
proof (cases "n < length (coeffs p)")
case True
then have "coeff p n ∈ set (coeffs p)"
by (auto simp: coeffs_def simp del: upt_Suc)
with assms show "f (coeff p n) = 0 ⟷ coeff p n = 0"
by auto
next
case False
then show ?thesis
by (auto simp: assms length_coeffs nth_default_coeffs_eq [symmetric] nth_default_def)
qed
finally show ?thesis .
qed
then show ?thesis by (auto simp: poly_eq_iff)
qed
lemma map_poly_smult:
assumes "f 0 = 0""⋀c x. f (c * x) = f c * f x"
shows "map_poly f (smult c p) = smult (f c) (map_poly f p)"
by (intro poly_eqI) (simp_all add: assms coeff_map_poly)
lemma map_poly_pCons:
assumes "f 0 = 0"
shows "map_poly f (pCons c p) = pCons (f c) (map_poly f p)"
by (intro poly_eqI) (simp_all add: assms coeff_map_poly coeff_pCons split: nat.splits)
lemma map_poly_map_poly:
assumes "f 0 = 0" "g 0 = 0"
shows "map_poly f (map_poly g p) = map_poly (f ∘ g) p"
by (intro poly_eqI) (simp add: coeff_map_poly assms)
lemma map_poly_id [simp]: "map_poly id p = p"
by (simp add: map_poly_def)
lemma map_poly_id' [simp]: "map_poly (λx. x) p = p"
by (simp add: map_poly_def)
lemma map_poly_cong:
assumes "(⋀x. x ∈ set (coeffs p) ⟹ f x = g x)"
shows "map_poly f p = map_poly g p"
proof -
from assms have "map f (coeffs p) = map g (coeffs p)"
by (intro map_cong) simp_all
then show ?thesis
by (simp only: coeffs_eq_iff coeffs_map_poly)
qed
lemma map_poly_monom: "f 0 = 0 ⟹ map_poly f (monom c n) = monom (f c) n"
by (intro poly_eqI) (simp_all add: coeff_map_poly)
lemma map_poly_idI:
assumes "⋀x. x ∈ set (coeffs p) ⟹ f x = x"
shows "map_poly f p = p"
using map_poly_cong[OF assms, of _ id] by simp
lemma map_poly_idI':
assumes "⋀x. x ∈ set (coeffs p) ⟹ f x = x"
shows "p = map_poly f p"
using map_poly_cong[OF assms, of _ id] by simp
lemma smult_conv_map_poly: "smult c p = map_poly (λx. c * x) p"
by (intro poly_eqI) (simp_all add: coeff_map_poly)
lemma poly_cnj: "cnj (poly p z) = poly (map_poly cnj p) (cnj z)"
by (simp add: poly_altdef degree_map_poly coeff_map_poly)
lemma poly_cnj_real:
assumes "⋀n. poly.coeff p n ∈ ℝ"
shows "cnj (poly p z) = poly p (cnj z)"
proof -
from assms have "map_poly cnj p = p"
by (intro poly_eqI) (auto simp: coeff_map_poly Reals_cnj_iff)
with poly_cnj[of p z] show ?thesis by simp
qed
lemma real_poly_cnj_root_iff:
assumes "⋀n. poly.coeff p n ∈ ℝ"
shows "poly p (cnj z) = 0 ⟷ poly p z = 0"
proof -
have "poly p (cnj z) = cnj (poly p z)"
by (simp add: poly_cnj_real assms)
also have "… = 0 ⟷ poly p z = 0" by simp
finally show ?thesis .
qed
lemma sum_to_poly: "(∑x∈A. [:f x:]) = [:∑x∈A. f x:]"
by (induction A rule: infinite_finite_induct) auto
lemma diff_to_poly: "[:c:] - [:d:] = [:c - d:]"
by (simp add: poly_eq_iff mult_ac)
lemma mult_to_poly: "[:c:] * [:d:] = [:c * d:]"
by (simp add: poly_eq_iff mult_ac)
lemma prod_to_poly: "(∏x∈A. [:f x:]) = [:∏x∈A. f x:]"
by (induction A rule: infinite_finite_induct) (auto simp: mult_to_poly mult_ac)
lemma poly_map_poly_cnj [simp]: "poly (map_poly cnj p) x = cnj (poly p (cnj x))"
by (induction p) (auto simp: map_poly_pCons)
subsection ‹Conversions›
lemma of_nat_poly:
"of_nat n = [:of_nat n:]"
by (induct n) (simp_all add: one_pCons)
lemma of_nat_monom:
"of_nat n = monom (of_nat n) 0"
by (simp add: of_nat_poly monom_0)
lemma degree_of_nat [simp]:
"degree (of_nat n) = 0"
by (simp add: of_nat_poly)
lemma lead_coeff_of_nat [simp]:
"lead_coeff (of_nat n) = of_nat n"
by (simp add: of_nat_poly)
lemma of_int_poly:
"of_int k = [:of_int k:]"
by (simp only: of_int_of_nat of_nat_poly) simp
lemma of_int_monom:
"of_int k = monom (of_int k) 0"
by (simp add: of_int_poly monom_0)
lemma degree_of_int [simp]:
"degree (of_int k) = 0"
by (simp add: of_int_poly)
lemma lead_coeff_of_int [simp]:
"lead_coeff (of_int k) = of_int k"
by (simp add: of_int_poly)
lemma poly_of_nat [simp]: "poly (of_nat n) x = of_nat n"
by (simp add: of_nat_poly)
lemma poly_of_int [simp]: "poly (of_int n) x = of_int n"
by (simp add: of_int_poly)
lemma poly_numeral [simp]: "poly (numeral n) x = numeral n"
by (metis of_nat_numeral poly_of_nat)
lemma numeral_poly: "numeral n = [:numeral n:]"
proof -
have "numeral n = of_nat (numeral n)"
by simp
also have "… = [:of_nat (numeral n):]"
by (simp add: of_nat_poly)
finally show ?thesis
by simp
qed
lemma numeral_monom:
"numeral n = monom (numeral n) 0"
by (simp add: numeral_poly monom_0)
lemma degree_numeral [simp]:
"degree (numeral n) = 0"
by (simp add: numeral_poly)
lemma lead_coeff_numeral [simp]:
"lead_coeff (numeral n) = numeral n"
by (simp add: numeral_poly)
lemma coeff_linear_poly_power:
fixes c :: "'a :: semiring_1"
assumes "i ≤ n"
shows "coeff ([:a, b:] ^ n) i = of_nat (n choose i) * b ^ i * a ^ (n - i)"
proof -
have "[:a, b:] = monom b 1 + [:a:]"
by (simp add: monom_altdef)
also have "coeff (… ^ n) i = (∑k≤n. a^(n-k) * of_nat (n choose k) * (if k = i then b ^ k else 0))"
by (subst binomial_ring) (simp add: coeff_sum of_nat_poly monom_power poly_const_pow mult_ac)
also have "… = (∑k∈{i}. a ^ (n - i) * b ^ i * of_nat (n choose k))"
using assms by (intro sum.mono_neutral_cong_right) (auto simp: mult_ac)
finally show *: ?thesis by (simp add: mult_ac)
qed
subsection ‹Lemmas about divisibility›
lemma dvd_smult:
assumes "p dvd q"
shows "p dvd smult a q"
proof -
from assms obtain k where "q = p * k" ..
then have "smult a q = p * smult a k" by simp
then show "p dvd smult a q" ..
qed
lemma dvd_smult_cancel: "p dvd smult a q ⟹ a ≠ 0 ⟹ p dvd q"
for a :: "'a::field"
by (drule dvd_smult [where a="inverse a"]) simp
lemma dvd_smult_iff: "a ≠ 0 ⟹ p dvd smult a q ⟷ p dvd q"
for a :: "'a::field"
by (safe elim!: dvd_smult dvd_smult_cancel)
lemma smult_dvd_cancel:
assumes "smult a p dvd q"
shows "p dvd q"
proof -
from assms obtain k where "q = smult a p * k" ..
then have "q = p * smult a k" by simp
then show "p dvd q" ..
qed
lemma smult_dvd: "p dvd q ⟹ a ≠ 0 ⟹ smult a p dvd q"
for a :: "'a::field"
by (rule smult_dvd_cancel [where a="inverse a"]) simp
lemma smult_dvd_iff: "smult a p dvd q ⟷ (if a = 0 then q = 0 else p dvd q)"
for a :: "'a::field"
by (auto elim: smult_dvd smult_dvd_cancel)
lemma is_unit_smult_iff: "smult c p dvd 1 ⟷ c dvd 1 ∧ p dvd 1"
proof -
have "smult c p = [:c:] * p" by simp
also have "… dvd 1 ⟷ c dvd 1 ∧ p dvd 1"
proof safe
assume *: "[:c:] * p dvd 1"
then show "p dvd 1"
by (rule dvd_mult_right)
from * obtain q where q: "1 = [:c:] * p * q"
by (rule dvdE)
have "c dvd c * (coeff p 0 * coeff q 0)"
by simp
also have "… = coeff ([:c:] * p * q) 0"
by (simp add: mult.assoc coeff_mult)
also note q [symmetric]
finally have "c dvd coeff 1 0" .
then show "c dvd 1" by simp
next
assume "c dvd 1" "p dvd 1"
from this(1) obtain d where "1 = c * d"
by (rule dvdE)
then have "1 = [:c:] * [:d:]"
by (simp add: one_pCons ac_simps)
then have "[:c:] dvd 1"
by (rule dvdI)
from mult_dvd_mono[OF this ‹p dvd 1›] show "[:c:] * p dvd 1"
by simp
qed
finally show ?thesis .
qed
subsection ‹Polynomials form an integral domain›
instance poly :: (idom) idom ..
instance poly :: ("{ring_char_0, comm_ring_1}") ring_char_0
by standard (auto simp add: of_nat_poly intro: injI)
lemma semiring_char_poly [simp]: "CHAR('a :: comm_semiring_1 poly) = CHAR('a)"
by (rule CHAR_eqI) (auto simp: of_nat_poly of_nat_eq_0_iff_char_dvd)
instance poly :: ("{semiring_prime_char,comm_semiring_1}") semiring_prime_char
by (rule semiring_prime_charI) auto
instance poly :: ("{comm_semiring_prime_char,comm_semiring_1}") comm_semiring_prime_char
by standard
instance poly :: ("{comm_ring_prime_char,comm_semiring_1}") comm_ring_prime_char
by standard
instance poly :: ("{idom_prime_char,comm_semiring_1}") idom_prime_char
by standard
lemma degree_mult_eq: "p ≠ 0 ⟹ q ≠ 0 ⟹ degree (p * q) = degree p + degree q"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
by (rule order_antisym [OF degree_mult_le le_degree]) (simp add: coeff_mult_degree_sum)
lemma degree_prod_sum_eq:
"(⋀x. x ∈ A ⟹ f x ≠ 0) ⟹
degree (prod f A :: 'a :: idom poly) = (∑x∈A. degree (f x))"
by (induction A rule: infinite_finite_induct) (auto simp: degree_mult_eq)
lemma dvd_imp_degree:
‹degree x ≤ degree y› if ‹x dvd y› ‹x ≠ 0› ‹y ≠ 0›
for x y :: ‹'a::{comm_semiring_1,semiring_no_zero_divisors} poly›
proof -
from ‹x dvd y› obtain z where ‹y = x * z› ..
with ‹x ≠ 0› ‹y ≠ 0› show ?thesis
by (simp add: degree_mult_eq)
qed
lemma degree_prod_eq_sum_degree:
fixes A :: "'a set"
and f :: "'a ⇒ 'b::idom poly"
assumes f0: "∀i∈A. f i ≠ 0"
shows "degree (∏i∈A. (f i)) = (∑i∈A. degree (f i))"
using assms
by (induction A rule: infinite_finite_induct) (auto simp: degree_mult_eq)
lemma degree_mult_eq_0:
"degree (p * q) = 0 ⟷ p = 0 ∨ q = 0 ∨ (p ≠ 0 ∧ q ≠ 0 ∧ degree p = 0 ∧ degree q = 0)"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
by (auto simp: degree_mult_eq)
lemma degree_power_eq: "p ≠ 0 ⟹ degree ((p :: 'a :: idom poly) ^ n) = n * degree p"
by (induction n) (simp_all add: degree_mult_eq)
lemma degree_mult_right_le:
fixes p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
assumes "q ≠ 0"
shows "degree p ≤ degree (p * q)"
using assms by (cases "p = 0") (simp_all add: degree_mult_eq)
lemma coeff_degree_mult: "coeff (p * q) (degree (p * q)) = coeff q (degree q) * coeff p (degree p)"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
by (cases "p = 0 ∨ q = 0") (auto simp: degree_mult_eq coeff_mult_degree_sum mult_ac)
lemma dvd_imp_degree_le: "p dvd q ⟹ q ≠ 0 ⟹ degree p ≤ degree q"
for p q :: "'a::{comm_semiring_1,semiring_no_zero_divisors} poly"
by (erule dvdE, hypsubst, subst degree_mult_eq) auto
lemma divides_degree:
fixes p q :: "'a ::{comm_semiring_1,semiring_no_zero_divisors} poly"
assumes "p dvd q"
shows "degree p ≤ degree q ∨ q = 0"
by (metis dvd_imp_degree_le assms)
lemma const_poly_dvd_iff:
fixes c :: "'a::{comm_semiring_1,semiring_no_zero_divisors}"
shows "[:c:] dvd p ⟷ (∀n. c dvd coeff p n)"
proof (cases "c = 0 ∨ p = 0")
case True
then show ?thesis
by (auto intro!: poly_eqI)
next
case False
show ?thesis
proof
assume "[:c:] dvd p"
then show "∀n. c dvd coeff p n"
by (auto simp: coeffs_def)
next
assume *: "∀n. c dvd coeff p n"
define mydiv where "mydiv x y = (SOME z. x = y * z)" for x y :: 'a
have mydiv: "x = y * mydiv x y" if "y dvd x" for x y
using that unfolding mydiv_def dvd_def by (rule someI_ex)
define q where "q = Poly (map (λa. mydiv a c) (coeffs p))"
from False * have "p = q * [:c:]"
by (intro poly_eqI)
(auto simp: q_def nth_default_def not_less length_coeffs_degree coeffs_nth
intro!: coeff_eq_0 mydiv)
then show "[:c:] dvd p"
by (simp only: dvd_triv_right)
qed
qed
lemma const_poly_dvd_const_poly_iff [simp]: "[:a:] dvd [:b:] ⟷ a dvd b"
for a b :: "'a::{comm_semiring_1,semiring_no_zero_divisors}"
by (subst const_poly_dvd_iff) (auto simp: coeff_pCons split: nat.splits)
lemma lead_coeff_mult: "lead_coeff (p * q) = lead_coeff p * lead_coeff q"
for p q :: "'a::{comm_semiring_0, semiring_no_zero_divisors} poly"
by (cases "p = 0 ∨ q = 0") (auto simp: coeff_mult_degree_sum degree_mult_eq)
lemma lead_coeff_prod: "lead_coeff (prod f A) = (∏x∈A. lead_coeff (f x))"
for f :: "'a ⇒ 'b::{comm_semiring_1, semiring_no_zero_divisors} poly"
by (induction A rule: infinite_finite_induct) (auto simp: lead_coeff_mult)
lemma lead_coeff_smult: "lead_coeff (smult c p) = c * lead_coeff p"
for p :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
proof -
have "smult c p = [:c:] * p" by simp
also have "lead_coeff … = c * lead_coeff p"
by (subst lead_coeff_mult) simp_all
finally show ?thesis .
qed
lemma lead_coeff_1 [simp]: "lead_coeff 1 = 1"
by simp
lemma lead_coeff_power: "lead_coeff (p ^ n) = lead_coeff p ^ n"
for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors} poly"
by (induct n) (simp_all add: lead_coeff_mult)
subsection ‹Polynomials form an ordered integral domain›
definition pos_poly :: "'a::linordered_semidom poly ⇒ bool"
where "pos_poly p ⟷ 0 < coeff p (degree p)"
lemma pos_poly_pCons: "pos_poly (pCons a p) ⟷ pos_poly p ∨ (p = 0 ∧ 0 < a)"
by (simp add: pos_poly_def)
lemma not_pos_poly_0 [simp]: "¬ pos_poly 0"
by (simp add: pos_poly_def)
lemma pos_poly_add: "pos_poly p ⟹ pos_poly q ⟹ pos_poly (p + q)"
proof (induction p arbitrary: q)
case (pCons a p)
then show ?case
by (cases q; force simp add: pos_poly_pCons add_pos_pos)
qed auto
lemma pos_poly_mult: "pos_poly p ⟹ pos_poly q ⟹ pos_poly (p * q)"
by (simp add: pos_poly_def coeff_degree_mult)
lemma pos_poly_total: "p = 0 ∨ pos_poly p ∨ pos_poly (- p)"
for p :: "'a::linordered_idom poly"
by (induct p) (auto simp: pos_poly_pCons)
lemma pos_poly_coeffs [code]: "pos_poly p ⟷ (let as = coeffs p in as ≠ [] ∧ last as > 0)"
(is "?lhs ⟷ ?rhs")
proof
assume ?rhs
then show ?lhs
by (auto simp add: pos_poly_def last_coeffs_eq_coeff_degree)
next
assume ?lhs
then have *: "0 < coeff p (degree p)"
by (simp add: pos_poly_def)
then have "p ≠ 0"
by auto
with * show ?rhs
by (simp add: last_coeffs_eq_coeff_degree)
qed
instantiation poly :: (linordered_idom) linordered_idom
begin
definition "x < y ⟷ pos_poly (y - x)"
definition "x ≤ y ⟷ x = y ∨ pos_poly (y - x)"
definition "¦x::'a poly¦ = (if x < 0 then - x else x)"
definition "sgn (x::'a poly) = (if x = 0 then 0 else if 0 < x then 1 else - 1)"
instance
proof
fix x y z :: "'a poly"
show "x < y ⟷ x ≤ y ∧ ¬ y ≤ x"
unfolding less_eq_poly_def less_poly_def
using pos_poly_add by force
then show "x ≤ y ⟹ y ≤ x ⟹ x = y"
using less_eq_poly_def less_poly_def by force
show "x ≤ x"
by (simp add: less_eq_poly_def)
show "x ≤ y ⟹ y ≤ z ⟹ x ≤ z"
using less_eq_poly_def pos_poly_add by fastforce
show "x ≤ y ⟹ z + x ≤ z + y"
by (simp add: less_eq_poly_def)
show "x ≤ y ∨ y ≤ x"
unfolding less_eq_poly_def
using pos_poly_total [of "x - y"]
by auto
show "x < y ⟹ 0 < z ⟹ z * x < z * y"
by (simp add: less_poly_def right_diff_distrib [symmetric] pos_poly_mult)
show "¦x¦ = (if x < 0 then - x else x)"
by (rule abs_poly_def)
show "sgn x = (if x = 0 then 0 else if 0 < x then 1 else - 1)"
by (rule sgn_poly_def)
qed
end
text ‹TODO: Simplification rules for comparisons›
subsection ‹Synthetic division and polynomial roots›
subsubsection ‹Synthetic division›
text ‹Synthetic division is simply division by the linear polynomial \<^term>‹x - c›.›
definition synthetic_divmod :: "'a::comm_semiring_0 poly ⇒ 'a ⇒ 'a poly × 'a"
where "synthetic_divmod p c = fold_coeffs (λa (q, r). (pCons r q, a + c * r)) p (0, 0)"
definition synthetic_div :: "'a::comm_semiring_0 poly ⇒ 'a ⇒ 'a poly"
where "synthetic_div p c = fst (synthetic_divmod p c)"
lemma synthetic_divmod_0 [simp]: "synthetic_divmod 0 c = (0, 0)"
by (simp add: synthetic_divmod_def)
lemma synthetic_divmod_pCons [simp]:
"synthetic_divmod (pCons a p) c = (λ(q, r). (pCons r q, a + c * r)) (synthetic_divmod p c)"
by (cases "p = 0 ∧ a = 0") (auto simp add: synthetic_divmod_def)
lemma synthetic_div_0 [simp]: "synthetic_div 0 c = 0"
by (simp add: synthetic_div_def)
lemma synthetic_div_unique_lemma: "smult c p = pCons a p ⟹ p = 0"
by (induct p arbitrary: a) simp_all
lemma snd_synthetic_divmod: "snd (synthetic_divmod p c) = poly p c"
by (induct p) (simp_all add: split_def)
lemma synthetic_div_pCons [simp]:
"synthetic_div (pCons a p) c = pCons (poly p c) (synthetic_div p c)"
by (simp add: synthetic_div_def split_def snd_synthetic_divmod)
lemma synthetic_div_eq_0_iff: "synthetic_div p c = 0 ⟷ degree p = 0"
proof (induct p)
case 0
then show ?case by simp
next
case (pCons a p)
then show ?case by (cases p) simp
qed
lemma degree_synthetic_div: "degree (synthetic_div p c) = degree p - 1"
by (induct p) (simp_all add: synthetic_div_eq_0_iff)
lemma synthetic_div_correct:
"p + smult c (synthetic_div p c) = pCons (poly p c) (synthetic_div p c)"
by (induct p) simp_all
lemma synthetic_div_unique: "p + smult c q = pCons r q ⟹ r = poly p c ∧ q = synthetic_div p c"
proof (induction p arbitrary: q r)
case 0
then show ?case
using synthetic_div_unique_lemma by fastforce
next
case (pCons a p)
then show ?case
by (cases q; force)
qed
lemma synthetic_div_correct': "[:-c, 1:] * synthetic_div p c + [:poly p c:] = p"
for c :: "'a::comm_ring_1"
using synthetic_div_correct [of p c] by (simp add: algebra_simps)
subsubsection ‹Polynomial roots›
lemma poly_eq_0_iff_dvd: "poly p c = 0 ⟷ [:- c, 1:] dvd p"
(is "?lhs ⟷ ?rhs")
for c :: "'a::comm_ring_1"
proof
assume ?lhs
with synthetic_div_correct' [of c p] have "p = [:-c, 1:] * synthetic_div p c" by simp
then show ?rhs ..
next
assume ?rhs
then obtain k where "p = [:-c, 1:] * k" by (rule dvdE)
then show ?lhs by simp
qed
lemma dvd_iff_poly_eq_0: "[:c, 1:] dvd p ⟷ poly p (- c) = 0"
for c :: "'a::comm_ring_1"
by (simp add: poly_eq_0_iff_dvd)
lemma poly_roots_finite: "p ≠ 0 ⟹ finite {x. poly p x = 0}"
for p :: "'a::{comm_ring_1,ring_no_zero_divisors} poly"
proof (induct n ≡ "degree p" arbitrary: p)
case 0
then obtain a where "a ≠ 0" and "p = [:a:]"
by (cases p) (simp split: if_splits)
then show "finite {x. poly p x = 0}"
by simp
next
case (Suc n)
show "finite {x. poly p x = 0}"
proof (cases "∃x. poly p x = 0")
case False
then show "finite {x. poly p x = 0}" by simp
next
case True
then obtain a where "poly p a = 0" ..
then have "[:-a, 1:] dvd p"
by (simp only: poly_eq_0_iff_dvd)
then obtain k where k: "p = [:-a, 1:] * k" ..
with ‹p ≠ 0› have "k ≠ 0"
by auto
with k have "degree p = Suc (degree k)"
by (simp add: degree_mult_eq del: mult_pCons_left)
with ‹Suc n = degree p› have "n = degree k"
by simp
from this ‹k ≠ 0› have "finite {x. poly k x = 0}"
by (rule Suc.hyps)
then have "finite (insert a {x. poly k x = 0})"
by simp
then show "finite {x. poly p x = 0}"
by (simp add: k Collect_disj_eq del: mult_pCons_left)
qed
qed
lemma poly_eq_poly_eq_iff: "poly p = poly q ⟷ p = q"
(is "?lhs ⟷ ?rhs")
for p q :: "'a::{comm_ring_1,ring_no_zero_divisors,ring_char_0} poly"
proof
assume ?rhs
then show ?lhs by simp
next
assume ?lhs
have "poly p = poly 0 ⟷ p = 0" for p :: "'a poly"
proof (cases "p = 0")
case False
then show ?thesis
by (auto simp add: infinite_UNIV_char_0 dest: poly_roots_finite)
qed auto
from ‹?lhs› and this [of "p - q"] show ?rhs
by auto
qed
lemma poly_all_0_iff_0: "(∀x. poly p x = 0) ⟷ p = 0"
for p :: "'a::{ring_char_0,comm_ring_1,ring_no_zero_divisors} poly"
by (auto simp add: poly_eq_poly_eq_iff [symmetric])
lemma card_poly_roots_bound:
fixes p :: "'a::{comm_ring_1,ring_no_zero_divisors} poly"
assumes "p ≠ 0"
shows "card {x. poly p x = 0} ≤ degree p"
using assms
proof (induction "degree p" arbitrary: p rule: less_induct)
case (less p)
show ?case
proof (cases "∃x. poly p x = 0")
case False
hence "{x. poly p x = 0} = {}" by blast
thus ?thesis by simp
next
case True
then obtain x where x: "poly p x = 0" by blast
hence "[:-x, 1:] dvd p" by (subst (asm) poly_eq_0_iff_dvd)
then obtain q where q: "p = [:-x, 1:] * q" by (auto simp: dvd_def)
with ‹p ≠ 0› have [simp]: "q ≠ 0" by auto
have deg: "degree p = Suc (degree q)"
by (subst q, subst degree_mult_eq) auto
have "card {x. poly p x = 0} ≤ card (insert x {x. poly q x = 0})"
by (intro card_mono) (auto intro: poly_roots_finite simp: q)
also have "… ≤ Suc (card {x. poly q x = 0})"
by (rule card_insert_le_m1) auto
also from deg have "card {x. poly q x = 0} ≤ degree q"
using ‹p ≠ 0› and q by (intro less) auto
also have "Suc … = degree p" by (simp add: deg)
finally show ?thesis by - simp_all
qed
qed
lemma poly_eqI_degree:
fixes p q :: "'a :: {comm_ring_1, ring_no_zero_divisors} poly"
assumes "⋀x. x ∈ A ⟹ poly p x = poly q x"
assumes "card A > degree p" "card A > degree q"
shows "p = q"
proof (rule ccontr)
assume neq: "p ≠ q"
have "degree (p - q) ≤ max (degree p) (degree q)"
by (rule degree_diff_le_max)
also from assms have "… < card A" by linarith
also have "… ≤ card {x. poly (p - q) x = 0}"
using neq and assms by (intro card_mono poly_roots_finite) auto
finally have "degree (p - q) < card {x. poly (p - q) x = 0}" .
moreover have "degree (p - q) ≥ card {x. poly (p - q) x = 0}"
using neq by (intro card_poly_roots_bound) auto
ultimately show False by linarith
qed
subsubsection ‹Order of polynomial roots›
definition order :: "'a::idom ⇒ 'a poly ⇒ nat"
where "order a p = (LEAST n. ¬ [:-a, 1:] ^ Suc n dvd p)"
lemma coeff_linear_power: "coeff ([:a, 1:] ^ n) n = 1"
for a :: "'a::comm_semiring_1"
proof (induct n)
case (Suc n)
have "degree ([:a, 1:] ^ n) ≤ 1 * n"
by (metis One_nat_def degree_pCons_eq_if degree_power_le one_neq_zero one_pCons)
then have "coeff ([:a, 1:] ^ n) (Suc n) = 0"
by (simp add: coeff_eq_0)
then show ?case
using Suc.hyps by fastforce
qed auto
lemma degree_linear_power: "degree ([:a, 1:] ^ n) = n"
for a :: "'a::comm_semiring_1"
proof (rule order_antisym)
show "degree ([:a, 1:] ^ n) ≤ n"
by (metis One_nat_def degree_pCons_eq_if degree_power_le mult.left_neutral one_neq_zero one_pCons)
qed (simp add: coeff_linear_power le_degree)
lemma order_1: "[:-a, 1:] ^ order a p dvd p"
proof (cases "p = 0")
case False
show ?thesis
proof (cases "order a p")
case (Suc n)
then show ?thesis
by (metis lessI not_less_Least order_def)
qed auto
qed auto
lemma order_2:
assumes "p ≠ 0"
shows "¬ [:-a, 1:] ^ Suc (order a p) dvd p"
proof -
have False if "[:- a, 1:] ^ Suc (degree p) dvd p"
using dvd_imp_degree_le [OF that]
by (metis Suc_n_not_le_n assms degree_linear_power)
then show ?thesis
unfolding order_def
by (metis (no_types, lifting) LeastI)
qed
lemma order: "p ≠ 0 ⟹ [:-a, 1:] ^ order a p dvd p ∧ ¬ [:-a, 1:] ^ Suc (order a p) dvd p"
by (rule conjI [OF order_1 order_2])
lemma order_degree:
assumes p: "p ≠ 0"
shows "order a p ≤ degree p"
proof -
have "order a p = degree ([:-a, 1:] ^ order a p)"
by (simp only: degree_linear_power)
also from order_1 p have "… ≤ degree p"
by (rule dvd_imp_degree_le)
finally show ?thesis .
qed
lemma order_root: "poly p a = 0 ⟷ p = 0 ∨ order a p ≠ 0" (is "?lhs = ?rhs")
proof
show "?lhs ⟹ ?rhs"
by (metis One_nat_def order_2 poly_eq_0_iff_dvd power_one_right)
show "?rhs ⟹ ?lhs"
by (meson dvd_power dvd_trans neq0_conv order_1 poly_0 poly_eq_0_iff_dvd)
qed
lemma order_0I: "poly p a ≠ 0 ⟹ order a p = 0"
by (subst (asm) order_root) auto
lemma order_unique_lemma:
fixes p :: "'a::idom poly"
assumes "[:-a, 1:] ^ n dvd p" "¬ [:-a, 1:] ^ Suc n dvd p"
shows "order a p = n"
unfolding Polynomial.order_def
by (metis (mono_tags, lifting) Least_equality assms not_less_eq_eq power_le_dvd)
lemma order_mult:
assumes "p * q ≠ 0" shows "order a (p * q) = order a p + order a q"
proof -
define i where "i ≡ order a p"
define j where "j ≡ order a q"
define t where "t ≡ [:-a, 1:]"
have t_dvd_iff: "⋀u. t dvd u ⟷ poly u a = 0"
by (simp add: t_def dvd_iff_poly_eq_0)
have dvd: "t ^ i dvd p" "t ^ j dvd q" and "¬ t ^ Suc i dvd p" "¬ t ^ Suc j dvd q"
using assms i_def j_def order_1 order_2 t_def by auto
then have "¬ t ^ Suc(i + j) dvd p * q"
by (elim dvdE) (simp add: power_add t_dvd_iff)
moreover have "t ^ (i + j) dvd p * q"
using dvd by (simp add: mult_dvd_mono power_add)
ultimately show "order a (p * q) = i + j"
using order_unique_lemma t_def by blast
qed
lemma order_smult:
assumes "c ≠ 0"
shows "order x (smult c p) = order x p"
proof (cases "p = 0")
case True
then show ?thesis
by simp
next
case False
have "smult c p = [:c:] * p" by simp
also from assms False have "order x … = order x [:c:] + order x p"
by (subst order_mult) simp_all
also have "order x [:c:] = 0"
by (rule order_0I) (use assms in auto)
finally show ?thesis
by simp
qed
lemma order_gt_0_iff: "p ≠ 0 ⟹ order x p > 0 ⟷ poly p x = 0"
by (subst order_root) auto
lemma order_eq_0_iff: "p ≠ 0 ⟹ order x p = 0 ⟷ poly p x ≠ 0"
by (subst order_root) auto
text ‹Next three lemmas contributed by Wenda Li›
lemma order_1_eq_0 [simp]:"order x 1 = 0"
by (metis order_root poly_1 zero_neq_one)
lemma order_uminus[simp]: "order x (-p) = order x p"
by (metis neg_equal_0_iff_equal order_smult smult_1_left smult_minus_left)
lemma order_power_n_n: "order a ([:-a,1:]^n)=n"
proof (induct n)
case 0
then show ?case
by (metis order_root poly_1 power_0 zero_neq_one)
next
case (Suc n)
have "order a ([:- a, 1:] ^ Suc n) = order a ([:- a, 1:] ^ n) + order a [:-a,1:]"
by (metis (no_types, opaque_lifting) One_nat_def add_Suc_right monoid_add_class.add.right_neutral
one_neq_zero order_mult pCons_eq_0_iff power_add power_eq_0_iff power_one_right)
moreover have "order a [:-a,1:] = 1"
unfolding order_def
proof (rule Least_equality, rule notI)
assume "[:- a, 1:] ^ Suc 1 dvd [:- a, 1:]"
then have "degree ([:- a, 1:] ^ Suc 1) ≤ degree ([:- a, 1:])"
by (rule dvd_imp_degree_le) auto
then show False
by auto
next
fix y
assume *: "¬ [:- a, 1:] ^ Suc y dvd [:- a, 1:]"
show "1 ≤ y"
proof (rule ccontr)
assume "¬ 1 ≤ y"
then have "y = 0" by auto
then have "[:- a, 1:] ^ Suc y dvd [:- a, 1:]" by auto
with * show False by auto
qed
qed
ultimately show ?case
using Suc by auto
qed
lemma order_0_monom [simp]: "c ≠ 0 ⟹ order 0 (monom c n) = n"
using order_power_n_n[of 0 n] by (simp add: monom_altdef order_smult)
lemma dvd_imp_order_le: "q ≠ 0 ⟹ p dvd q ⟹ Polynomial.order a p ≤ Polynomial.order a q"
by (auto simp: order_mult)
text ‹Now justify the standard squarefree decomposition, i.e. ‹f / gcd f f'›.›
lemma order_divides: "[:-a, 1:] ^ n dvd p ⟷ p = 0 ∨ n ≤ order a p"
by (meson dvd_0_right not_less_eq_eq order_1 order_2 power_le_dvd)
lemma order_decomp:
assumes "p ≠ 0"
shows "∃q. p = [:- a, 1:] ^ order a p * q ∧ ¬ [:- a, 1:] dvd q"
proof -
from assms have *: "[:- a, 1:] ^ order a p dvd p"
and **: "¬ [:- a, 1:] ^ Suc (order a p) dvd p"
by (auto dest: order)
from * obtain q where q: "p = [:- a, 1:] ^ order a p * q" ..
with ** have "¬ [:- a, 1:] ^ Suc (order a p) dvd [:- a, 1:] ^ order a p * q"
by simp
then have "¬ [:- a, 1:] ^ order a p * [:- a, 1:] dvd [:- a, 1:] ^ order a p * q"
by simp
with idom_class.dvd_mult_cancel_left [of "[:- a, 1:] ^ order a p" "[:- a, 1:]" q]
have "¬ [:- a, 1:] dvd q" by auto
with q show ?thesis by blast
qed
lemma monom_1_dvd_iff: "p ≠ 0 ⟹ monom 1 n dvd p ⟷ n ≤ order 0 p"
using order_divides[of 0 n p] by (simp add: monom_altdef)
lemma poly_root_order_induct [case_names 0 no_roots root]:
fixes p :: "'a :: idom poly"
assumes "P 0" "⋀p. (⋀x. poly p x ≠ 0) ⟹ P p"
"⋀p x n. n > 0 ⟹ poly p x ≠ 0 ⟹ P p ⟹ P ([:-x, 1:] ^ n * p)"
shows "P p"
proof (induction "degree p" arbitrary: p rule: less_induct)
case (less p)
consider "p = 0" | "p ≠ 0" "∃x. poly p x = 0" | "⋀x. poly p x ≠ 0" by blast
thus ?case
proof cases
case 3
with assms(2)[of p] show ?thesis by simp
next
case 2
then obtain x where x: "poly p x = 0" by auto
have "[:-x, 1:] ^ order x p dvd p" by (intro order_1)
then obtain q where q: "p = [:-x, 1:] ^ order x p * q" by (auto simp: dvd_def)
with 2 have [simp]: "q ≠ 0" by auto
have order_pos: "order x p > 0"
using ‹p ≠ 0› and x by (auto simp: order_root)
have "order x p = order x p + order x q"
by (subst q, subst order_mult) (auto simp: order_power_n_n)
hence [simp]: "order x q = 0" by simp
have deg: "degree p = order x p + degree q"
by (subst q, subst degree_mult_eq) (auto simp: degree_power_eq)
with order_pos have "degree q < degree p" by simp
hence "P q" by (rule less)
with order_pos have "P ([:-x, 1:] ^ order x p * q)"
by (intro assms(3)) (auto simp: order_root)
with q show ?thesis by simp
qed (simp_all add: assms(1))
qed
context
includes multiset.lifting
begin
lift_definition proots :: "('a :: idom) poly ⇒ 'a multiset" is
"λ(p :: 'a poly) (x :: 'a). if p = 0 then 0 else order x p"
proof -
fix p :: "'a poly"
show "finite {x. 0 < (if p = 0 then 0 else order x p)}"
by (cases "p = 0")
(auto simp: order_gt_0_iff intro: finite_subset[OF _ poly_roots_finite[of p]])
qed
lemma proots_0 [simp]: "proots (0 :: 'a :: idom poly) = {#}"
by transfer' auto
lemma proots_1 [simp]: "proots (1 :: 'a :: idom poly) = {#}"
by transfer' auto
lemma proots_const [simp]: "proots [: x :] = 0"
by transfer' (auto split: if_splits simp: fun_eq_iff order_eq_0_iff)
lemma proots_numeral [simp]: "proots (numeral n) = 0"
by (simp add: numeral_poly)
lemma count_proots [simp]:
"p ≠ 0 ⟹ count (proots p) a = order a p"
by transfer' auto
lemma set_count_proots [simp]:
"p ≠ 0 ⟹ set_mset (proots p) = {x. poly p x = 0}"
by (auto simp: set_mset_def order_gt_0_iff)
lemma proots_uminus [simp]: "proots (-p) = proots p"
by (cases "p = 0"; rule multiset_eqI) auto
lemma proots_smult [simp]: "c ≠ 0 ⟹ proots (smult c p) = proots p"
by (cases "p = 0"; rule multiset_eqI) (auto simp: order_smult)
lemma proots_mult:
assumes "p ≠ 0" "q ≠ 0"
shows "proots (p * q) = proots p + proots q"
using assms by (intro multiset_eqI) (auto simp: order_mult)
lemma proots_prod:
assumes "⋀x. x ∈ A ⟹ f x ≠ 0"
shows "proots (∏x∈A. f x) = (∑x∈A. proots (f x))"
using assms by (induction A rule: infinite_finite_induct) (auto simp: proots_mult)
lemma proots_prod_mset:
assumes "0 ∉# A"
shows "proots (∏p∈#A. p) = (∑p∈#A. proots p)"
using assms by (induction A) (auto simp: proots_mult)
lemma proots_prod_list:
assumes "0 ∉ set ps"
shows "proots (∏p←ps. p) = (∑p←ps. proots p)"
using assms by (induction ps) (auto simp: proots_mult prod_list_zero_iff)
lemma proots_power: "proots (p ^ n) = repeat_mset n (proots p)"
proof (cases "p = 0")
case False
thus ?thesis
by (induction n) (auto simp: proots_mult)
qed (auto simp: power_0_left)
lemma proots_linear_factor [simp]: "proots [:x, 1:] = {#-x#}"
proof -
have "order (-x) [:x, 1:] > 0"
by (subst order_gt_0_iff) auto
moreover have "order (-x) [:x, 1:] ≤ degree [:x, 1:]"
by (rule order_degree) auto
moreover have "order y [:x, 1:] = 0" if "y ≠ -x" for y
by (rule order_0I) (use that in ‹auto simp: add_eq_0_iff›)
ultimately show ?thesis
by (intro multiset_eqI) auto
qed
lemma size_proots_le: "size (proots p) ≤ degree p"
proof (induction p rule: poly_root_order_induct)
case (no_roots p)
hence "proots p = 0"
by (simp add: multiset_eqI order_root)
thus ?case by simp
next
case (root p x n)
have [simp]: "p ≠ 0"
using root.hyps by auto
from root.IH show ?case
by (auto simp: proots_mult proots_power degree_mult_eq degree_power_eq)
qed auto
end
subsection ‹Additional induction rules on polynomials›
text ‹
An induction rule for induction over the roots of a polynomial with a certain property.
(e.g. all positive roots)
›
lemma poly_root_induct [case_names 0 no_roots root]:
fixes p :: "'a :: idom poly"
assumes "Q 0"
and "⋀p. (⋀a. P a ⟹ poly p a ≠ 0) ⟹ Q p"
and "⋀a p. P a ⟹ Q p ⟹ Q ([:a, -1:] * p)"
shows "Q p"
proof (induction "degree p" arbitrary: p rule: less_induct)
case (less p)
show ?case
proof (cases "p = 0")
case True
with assms(1) show ?thesis by simp
next
case False
show ?thesis
proof (cases "∃a. P a ∧ poly p a = 0")
case False
then show ?thesis by (intro assms(2)) blast
next
case True
then obtain a where a: "P a" "poly p a = 0"
by blast
then have "-[:-a, 1:] dvd p"
by (subst minus_dvd_iff) (simp add: poly_eq_0_iff_dvd)
then obtain q where q: "p = [:a, -1:] * q" by (elim dvdE) simp
with False have "q ≠ 0" by auto
have "degree p = Suc (degree q)"
by (subst q, subst degree_mult_eq) (simp_all add: ‹q ≠ 0›)
then have "Q q" by (intro less) simp
with a(1) have "Q ([:a, -1:] * q)"
by (rule assms(3))
with q show ?thesis by simp
qed
qed
qed
lemma dropWhile_replicate_append:
"dropWhile ((=) a) (replicate n a @ ys) = dropWhile ((=) a) ys"
by (induct n) simp_all
lemma Poly_append_replicate_0: "Poly (xs @ replicate n 0) = Poly xs"
by (subst coeffs_eq_iff) (simp_all add: strip_while_def dropWhile_replicate_append)
text ‹
An induction rule for simultaneous induction over two polynomials,
prepending one coefficient in each step.
›
lemma poly_induct2 [case_names 0 pCons]:
assumes "P 0 0" "⋀a p b q. P p q ⟹ P (pCons a p) (pCons b q)"
shows "P p q"
proof -
define n where "n = max (length (coeffs p)) (length (coeffs q))"
define xs where "xs = coeffs p @ (replicate (n - length (coeffs p)) 0)"
define ys where "ys = coeffs q @ (replicate (n - length (coeffs q)) 0)"
have "length xs = length ys"
by (simp add: xs_def ys_def n_def)
then have "P (Poly xs) (Poly ys)"
by (induct rule: list_induct2) (simp_all add: assms)
also have "Poly xs = p"
by (simp add: xs_def Poly_append_replicate_0)
also have "Poly ys = q"
by (simp add: ys_def Poly_append_replicate_0)
finally show ?thesis .
qed
subsection ‹Composition of polynomials›
definition pcompose :: "'a::comm_semiring_0 poly ⇒ 'a poly ⇒ 'a poly"
where "pcompose p q = fold_coeffs (λa c. [:a:] + q * c) p 0"
notation pcompose (infixl ‹∘⇩p› 71)
lemma pcompose_0 [simp]: "pcompose 0 q = 0"
by (simp add: pcompose_def)
lemma pcompose_pCons: "pcompose (pCons a p) q = [:a:] + q * pcompose p q"
by (cases "p = 0 ∧ a = 0") (auto simp add: pcompose_def)
lemma pcompose_altdef: "pcompose p q = poly (map_poly (λx. [:x:]) p) q"
by (induction p) (simp_all add: map_poly_pCons pcompose_pCons)
lemma coeff_pcompose_0 [simp]:
"coeff (pcompose p q) 0 = poly p (coeff q 0)"
by (induction p) (simp_all add: coeff_mult_0 pcompose_pCons)
lemma pcompose_1: "pcompose 1 p = 1"
for p :: "'a::comm_semiring_1 poly"
by (auto simp: one_pCons pcompose_pCons)
lemma poly_pcompose: "poly (pcompose p q) x = poly p (poly q x)"
by (induct p) (simp_all add: pcompose_pCons)
lemma degree_pcompose_le: "degree (pcompose p q) ≤ degree p * degree q"
proof (induction p)
case (pCons a p)
then show ?case
proof (clarsimp simp add: pcompose_pCons)
assume "degree (p ∘⇩p q) ≤ degree p * degree q" "p ≠ 0"
then have "degree (q * p ∘⇩p q) ≤ degree q + degree p * degree q"
by (meson add_le_cancel_left degree_mult_le dual_order.trans pCons.IH)
then show "degree ([:a:] + q * p ∘⇩p q) ≤ degree q + degree p * degree q"
by (simp add: degree_add_le)
qed
qed auto
lemma pcompose_add: "pcompose (p + q) r = pcompose p r + pcompose q r"
for p q r :: "'a::{comm_semiring_0, ab_semigroup_add} poly"
proof (induction p q rule: poly_induct2)
case 0
then show ?case by simp
next
case (pCons a p b q)
have "pcompose (pCons a p + pCons b q) r = [:a + b:] + r * pcompose p r + r * pcompose q r"
by (simp_all add: pcompose_pCons pCons.IH algebra_simps)
also have "[:a + b:] = [:a:] + [:b:]" by simp
also have "… + r * pcompose p r + r * pcompose q r = pcompose (pCons a p) r + pcompose (pCons b q) r"
by (simp only: pcompose_pCons add_ac)
finally show ?case .
qed
lemma pcompose_uminus: "pcompose (-p) r = -pcompose p r"
for p r :: "'a::comm_ring poly"
by (induct p) (simp_all add: pcompose_pCons)
lemma pcompose_diff: "pcompose (p - q) r = pcompose p r - pcompose q r"
for p q r :: "'a::comm_ring poly"
using pcompose_add[of p "-q"] by (simp add: pcompose_uminus)
lemma pcompose_smult: "pcompose (smult a p) r = smult a (pcompose p r)"
for p r :: "'a::comm_semiring_0 poly"
by (induct p) (simp_all add: pcompose_pCons pcompose_add smult_add_right)
lemma pcompose_mult: "pcompose (p * q) r = pcompose p r * pcompose q r"
for p q r :: "'a::comm_semiring_0 poly"
by (induct p arbitrary: q) (simp_all add: pcompose_add pcompose_smult pcompose_pCons algebra_simps)
lemma pcompose_assoc: "pcompose p (pcompose q r) = pcompose (pcompose p q) r"
for p q r :: "'a::comm_semiring_0 poly"
by (induct p arbitrary: q) (simp_all add: pcompose_pCons pcompose_add pcompose_mult)
lemma pcompose_idR[simp]: "pcompose p [: 0, 1 :] = p"
for p :: "'a::comm_semiring_1 poly"
by (induct p) (simp_all add: pcompose_pCons)
lemma pcompose_sum: "pcompose (sum f A) p = sum (λi. pcompose (f i) p) A"
by (induct A rule: infinite_finite_induct) (simp_all add: pcompose_1 pcompose_add)
lemma pcompose_prod: "pcompose (prod f A) p = prod (λi. pcompose (f i) p) A"
by (induct A rule: infinite_finite_induct) (simp_all add: pcompose_1 pcompose_mult)
lemma pcompose_const [simp]: "pcompose [:a:] q = [:a:]"
by (subst pcompose_pCons) simp
lemma pcompose_0': "pcompose p 0 = [:coeff p 0:]"
by (induct p) (auto simp add: pcompose_pCons)
lemma degree_pcompose: "degree (pcompose p q) = degree p * degree q"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
proof (induct p)
case 0
then show ?case by auto
next
case (pCons a p)
consider "degree (q * pcompose p q) = 0" | "degree (q * pcompose p q) > 0"
by blast
then show ?case
proof cases
case prems: 1
show ?thesis
proof (cases "p = 0")
case True
then show ?thesis by auto
next
case False
from prems have "degree q = 0 ∨ pcompose p q = 0"
by (auto simp add: degree_mult_eq_0)
moreover have False if "pcompose p q = 0" "degree q ≠ 0"
proof -
from pCons.hyps(2) that have "degree p = 0"
by auto
then obtain a1 where "p = [:a1:]"
by (metis degree_pCons_eq_if old.nat.distinct(2) pCons_cases)
with ‹pcompose p q = 0› ‹p ≠ 0› show False
by auto
qed
ultimately have "degree (pCons a p) * degree q = 0"
by auto
moreover have "degree (pcompose (pCons a p) q) = 0"
proof -
from prems have "0 = max (degree [:a:]) (degree (q * pcompose p q))"
by simp
also have "… ≥ degree ([:a:] + q * pcompose p q)"
by (rule degree_add_le_max)
finally show ?thesis
by (auto simp add: pcompose_pCons)
qed
ultimately show ?thesis by simp
qed
next
case prems: 2
then have "p ≠ 0" "q ≠ 0" "pcompose p q ≠ 0"
by auto
from prems degree_add_eq_right [of "[:a:]"]
have "degree (pcompose (pCons a p) q) = degree (q * pcompose p q)"
by (auto simp: pcompose_pCons)
with pCons.hyps(2) degree_mult_eq[OF ‹q≠0› ‹pcompose p q≠0›] show ?thesis
by auto
qed
qed
lemma pcompose_eq_0:
fixes p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
assumes "pcompose p q = 0" "degree q > 0"
shows "p = 0"
proof -
from assms degree_pcompose [of p q] have "degree p = 0"
by auto
then obtain a where "p = [:a:]"
by (metis degree_pCons_eq_if gr0_conv_Suc neq0_conv pCons_cases)
with assms(1) have "a = 0"
by auto
with ‹p = [:a:]› show ?thesis
by simp
qed
lemma pcompose_eq_0_iff:
fixes p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
assumes "degree q > 0"
shows "pcompose p q = 0 ⟷ p = 0"
using pcompose_eq_0[OF _ assms] by auto
lemma coeff_pcompose_linear:
"coeff (pcompose p [:0, a :: 'a :: comm_semiring_1:]) i = a ^ i * coeff p i"
by (induction p arbitrary: i) (auto simp: pcompose_pCons coeff_pCons mult_ac split: nat.splits)
lemma lead_coeff_comp:
fixes p q :: "'a::{comm_semiring_1,semiring_no_zero_divisors} poly"
assumes "degree q > 0"
shows "lead_coeff (pcompose p q) = lead_coeff p * lead_coeff q ^ (degree p)"
proof (induct p)
case 0
then show ?case by auto
next
case (pCons a p)
consider "degree (q * pcompose p q) = 0" | "degree (q * pcompose p q) > 0"
by blast
then show ?case
proof cases
case prems: 1
then have "pcompose p q = 0"
by (metis assms degree_0 degree_mult_eq_0 neq0_conv)
with pcompose_eq_0[OF _ ‹degree q > 0›] have "p = 0"
by simp
then show ?thesis
by auto
next
case prems: 2
then have "degree [:a:] < degree (q * pcompose p q)"
by simp
then have "lead_coeff ([:a:] + q * p ∘⇩p q) = lead_coeff (q * p ∘⇩p q)"
by (rule lead_coeff_add_le)
then have "lead_coeff (pcompose (pCons a p) q) = lead_coeff (q * pcompose p q)"
by (simp add: pcompose_pCons)
also have "… = lead_coeff q * (lead_coeff p * lead_coeff q ^ degree p)"
using pCons.hyps(2) lead_coeff_mult[of q "pcompose p q"] by simp
also have "… = lead_coeff p * lead_coeff q ^ (degree p + 1)"
by (auto simp: mult_ac)
finally show ?thesis by auto
qed
qed
lemma coeff_pcompose_monom_linear [simp]:
fixes p :: "'a :: comm_ring_1 poly"
shows "coeff (pcompose p (monom c (Suc 0))) k = c ^ k * coeff p k"
by (induction p arbitrary: k)
(auto simp: coeff_pCons coeff_monom_mult pcompose_pCons split: nat.splits)
lemma of_nat_mult_conv_smult: "of_nat n * P = smult (of_nat n) P"
by (simp add: monom_0 of_nat_monom)
lemma numeral_mult_conv_smult: "numeral n * P = smult (numeral n) P"
by (simp add: numeral_poly)
lemma sum_order_le_degree:
assumes "p ≠ 0"
shows "(∑x | poly p x = 0. order x p) ≤ degree p"
using assms
proof (induction "degree p" arbitrary: p rule: less_induct)
case (less p)
show ?case
proof (cases "∃x. poly p x = 0")
case False
thus ?thesis
by auto
next
case True
then obtain x where x: "poly p x = 0"
by auto
have "[:-x, 1:] ^ order x p dvd p"
by (simp add: order_1)
then obtain q where q: "p = [:-x, 1:] ^ order x p * q"
by (elim dvdE)
have [simp]: "q ≠ 0"
using q less.prems by auto
have "order x p = order x p + order x q"
by (subst q, subst order_mult) (auto simp: order_power_n_n)
hence "order x q = 0"
by auto
hence [simp]: "poly q x ≠ 0"
by (simp add: order_root)
have deg_p: "degree p = degree q + order x p"
by (subst q, subst degree_mult_eq) (auto simp: degree_power_eq)
moreover have "order x p > 0"
using x less.prems by (simp add: order_root)
ultimately have "degree q < degree p"
by linarith
hence "(∑x | poly q x = 0. order x q) ≤ degree q"
by (intro less.hyps) auto
hence "order x p + (∑x | poly q x = 0. order x q) ≤ degree p"
by (simp add: deg_p)
also have "{y. poly q y = 0} = {y. poly p y = 0} - {x}"
by (subst q) auto
also have "(∑y ∈ {y. poly p y = 0} - {x}. order y q) =
(∑y ∈ {y. poly p y = 0} - {x}. order y p)"
by (intro sum.cong refl, subst q)
(auto simp: order_mult order_power_n_n intro!: order_0I)
also have "order x p + … = (∑y ∈ insert x ({y. poly p y = 0} - {x}). order y p)"
using ‹p ≠ 0› by (subst sum.insert) (auto simp: poly_roots_finite)
also have "insert x ({y. poly p y = 0} - {x}) = {y. poly p y = 0}"
using ‹poly p x = 0› by auto
finally show ?thesis .
qed
qed
subsection ‹Closure properties of coefficients›
context
fixes R :: "'a :: comm_semiring_1 set"
assumes R_0: "0 ∈ R"
assumes R_plus: "⋀x y. x ∈ R ⟹ y ∈ R ⟹ x + y ∈ R"
assumes R_mult: "⋀x y. x ∈ R ⟹ y ∈ R ⟹ x * y ∈ R"
begin
lemma coeff_mult_semiring_closed:
assumes "⋀i. coeff p i ∈ R" "⋀i. coeff q i ∈ R"
shows "coeff (p * q) i ∈ R"
proof -
have R_sum: "sum f A ∈ R" if "⋀x. x ∈ A ⟹ f x ∈ R" for A and f :: "nat ⇒ 'a"
using that by (induction A rule: infinite_finite_induct) (auto intro: R_0 R_plus)
show ?thesis
unfolding coeff_mult by (auto intro!: R_sum R_mult assms)
qed
lemma coeff_pcompose_semiring_closed:
assumes "⋀i. coeff p i ∈ R" "⋀i. coeff q i ∈ R"
shows "coeff (pcompose p q) i ∈ R"
using assms(1)
proof (induction p arbitrary: i)
case (pCons a p i)
have [simp]: "a ∈ R"
using pCons.prems[of 0] by auto
have "coeff p i ∈ R" for i
using pCons.prems[of "Suc i"] by auto
hence "coeff (p ∘⇩p q) i ∈ R" for i
using pCons.prems by (intro pCons.IH)
thus ?case
by (auto simp: pcompose_pCons coeff_pCons split: nat.splits
intro!: assms R_plus coeff_mult_semiring_closed)
qed auto
end
subsection ‹Shifting polynomials›
definition poly_shift :: "nat ⇒ 'a::zero poly ⇒ 'a poly"
where "poly_shift n p = Abs_poly (λi. coeff p (i + n))"
lemma nth_default_drop: "nth_default x (drop n xs) m = nth_default x xs (m + n)"
by (auto simp add: nth_default_def add_ac)
lemma nth_default_take: "nth_default x (take n xs) m = (if m < n then nth_default x xs m else x)"
by (auto simp add: nth_default_def add_ac)
lemma coeff_poly_shift: "coeff (poly_shift n p) i = coeff p (i + n)"
proof -
from MOST_coeff_eq_0[of p] obtain m where "∀k>m. coeff p k = 0"
by (auto simp: MOST_nat)
then have "∀k>m. coeff p (k + n) = 0"
by auto
then have "∀⇩∞k. coeff p (k + n) = 0"
by (auto simp: MOST_nat)
then show ?thesis
by (simp add: poly_shift_def poly.Abs_poly_inverse)
qed
lemma poly_shift_id [simp]: "poly_shift 0 = (λx. x)"
by (simp add: poly_eq_iff fun_eq_iff coeff_poly_shift)
lemma poly_shift_0 [simp]: "poly_shift n 0 = 0"
by (simp add: poly_eq_iff coeff_poly_shift)
lemma poly_shift_1: "poly_shift n 1 = (if n = 0 then 1 else 0)"
by (simp add: poly_eq_iff coeff_poly_shift)
lemma poly_shift_monom: "poly_shift n (monom c m) = (if m ≥ n then monom c (m - n) else 0)"
by (auto simp add: poly_eq_iff coeff_poly_shift)
lemma coeffs_shift_poly [code abstract]:
"coeffs (poly_shift n p) = drop n (coeffs p)"
proof (cases "p = 0")
case True
then show ?thesis by simp
next
case False
then show ?thesis
by (intro coeffs_eqI)
(simp_all add: coeff_poly_shift nth_default_drop nth_default_coeffs_eq)
qed
subsection ‹Truncating polynomials›
definition poly_cutoff
where "poly_cutoff n p = Abs_poly (λk. if k < n then coeff p k else 0)"
lemma coeff_poly_cutoff: "coeff (poly_cutoff n p) k = (if k < n then coeff p k else 0)"
unfolding poly_cutoff_def
by (subst poly.Abs_poly_inverse) (auto simp: MOST_nat intro: exI[of _ n])
lemma poly_cutoff_0 [simp]: "poly_cutoff n 0 = 0"
by (simp add: poly_eq_iff coeff_poly_cutoff)
lemma poly_cutoff_1 [simp]: "poly_cutoff n 1 = (if n = 0 then 0 else 1)"
by (simp add: poly_eq_iff coeff_poly_cutoff)
lemma coeffs_poly_cutoff [code abstract]:
"coeffs (poly_cutoff n p) = strip_while ((=) 0) (take n (coeffs p))"
proof (cases "strip_while ((=) 0) (take n (coeffs p)) = []")
case True
then have "coeff (poly_cutoff n p) k = 0" for k
unfolding coeff_poly_cutoff
by (auto simp: nth_default_coeffs_eq [symmetric] nth_default_def set_conv_nth)
then have "poly_cutoff n p = 0"
by (simp add: poly_eq_iff)
then show ?thesis
by (subst True) simp_all
next
case False
have "no_trailing ((=) 0) (strip_while ((=) 0) (take n (coeffs p)))"
by simp
with False have "last (strip_while ((=) 0) (take n (coeffs p))) ≠ 0"
unfolding no_trailing_unfold by auto
then show ?thesis
by (intro coeffs_eqI)
(simp_all add: coeff_poly_cutoff nth_default_take nth_default_coeffs_eq)
qed
subsection ‹Reflecting polynomials›
definition reflect_poly :: "'a::zero poly ⇒ 'a poly"
where "reflect_poly p = Poly (rev (coeffs p))"
lemma coeffs_reflect_poly [code abstract]:
"coeffs (reflect_poly p) = rev (dropWhile ((=) 0) (coeffs p))"
by (simp add: reflect_poly_def)
lemma reflect_poly_0 [simp]: "reflect_poly 0 = 0"
by (simp add: reflect_poly_def)
lemma reflect_poly_1 [simp]: "reflect_poly 1 = 1"
by (simp add: reflect_poly_def one_pCons)
lemma coeff_reflect_poly:
"coeff (reflect_poly p) n = (if n > degree p then 0 else coeff p (degree p - n))"
by (cases "p = 0")
(auto simp add: reflect_poly_def nth_default_def
rev_nth degree_eq_length_coeffs coeffs_nth not_less
dest: le_imp_less_Suc)
lemma coeff_0_reflect_poly_0_iff [simp]: "coeff (reflect_poly p) 0 = 0 ⟷ p = 0"
by (simp add: coeff_reflect_poly)
lemma reflect_poly_at_0_eq_0_iff [simp]: "poly (reflect_poly p) 0 = 0 ⟷ p = 0"
by (simp add: coeff_reflect_poly poly_0_coeff_0)
lemma reflect_poly_pCons':
"p ≠ 0 ⟹ reflect_poly (pCons c p) = reflect_poly p + monom c (Suc (degree p))"
by (intro poly_eqI)
(auto simp: coeff_reflect_poly coeff_pCons not_less Suc_diff_le split: nat.split)
lemma reflect_poly_const [simp]: "reflect_poly [:a:] = [:a:]"
by (cases "a = 0") (simp_all add: reflect_poly_def)
lemma poly_reflect_poly_nz:
"x ≠ 0 ⟹ poly (reflect_poly p) x = x ^ degree p * poly p (inverse x)"
for x :: "'a::field"
by (induct rule: pCons_induct) (simp_all add: field_simps reflect_poly_pCons' poly_monom)
lemma coeff_0_reflect_poly [simp]: "coeff (reflect_poly p) 0 = lead_coeff p"
by (simp add: coeff_reflect_poly)
lemma poly_reflect_poly_0 [simp]: "poly (reflect_poly p) 0 = lead_coeff p"
by (simp add: poly_0_coeff_0)
lemma reflect_poly_reflect_poly [simp]: "coeff p 0 ≠ 0 ⟹ reflect_poly (reflect_poly p) = p"
by (cases p rule: pCons_cases) (simp add: reflect_poly_def )
lemma degree_reflect_poly_le: "degree (reflect_poly p) ≤ degree p"
by (simp add: degree_eq_length_coeffs coeffs_reflect_poly length_dropWhile_le diff_le_mono)
lemma reflect_poly_pCons: "a ≠ 0 ⟹ reflect_poly (pCons a p) = Poly (rev (a # coeffs p))"
by (subst coeffs_eq_iff) (simp add: coeffs_reflect_poly)
lemma degree_reflect_poly_eq [simp]: "coeff p 0 ≠ 0 ⟹ degree (reflect_poly p) = degree p"
by (cases p rule: pCons_cases) (simp add: reflect_poly_pCons degree_eq_length_coeffs)
lemma reflect_poly_eq_0_iff [simp]: "reflect_poly p = 0 ⟷ p = 0"
using coeff_0_reflect_poly_0_iff by fastforce
lemma reflect_poly_mult: "reflect_poly (p * q) = reflect_poly p * reflect_poly q"
for p q :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
proof (cases "p = 0 ∨ q = 0")
case False
then have [simp]: "p ≠ 0" "q ≠ 0" by auto
show ?thesis
proof (rule poly_eqI)
show "coeff (reflect_poly (p * q)) i = coeff (reflect_poly p * reflect_poly q) i" for i
proof (cases "i ≤ degree (p * q)")
case True
define A where "A = {..i} ∩ {i - degree q..degree p}"
define B where "B = {..degree p} ∩ {degree p - i..degree (p*q) - i}"
let ?f = "λj. degree p - j"
from True have "coeff (reflect_poly (p * q)) i = coeff (p * q) (degree (p * q) - i)"
by (simp add: coeff_reflect_poly)
also have "… = (∑j≤degree (p * q) - i. coeff p j * coeff q (degree (p * q) - i - j))"
by (simp add: coeff_mult)
also have "… = (∑j∈B. coeff p j * coeff q (degree (p * q) - i - j))"
by (intro sum.mono_neutral_right) (auto simp: B_def degree_mult_eq not_le coeff_eq_0)
also from True have "… = (∑j∈A. coeff p (degree p - j) * coeff q (degree q - (i - j)))"
by (intro sum.reindex_bij_witness[of _ ?f ?f])
(auto simp: A_def B_def degree_mult_eq add_ac)
also have "… =
(∑j≤i.
if j ∈ {i - degree q..degree p}
then coeff p (degree p - j) * coeff q (degree q - (i - j))
else 0)"
by (subst sum.inter_restrict [symmetric]) (simp_all add: A_def)
also have "… = coeff (reflect_poly p * reflect_poly q) i"
by (fastforce simp: coeff_mult coeff_reflect_poly intro!: sum.cong)
finally show ?thesis .
qed (auto simp: coeff_mult coeff_reflect_poly coeff_eq_0 degree_mult_eq intro!: sum.neutral)
qed
qed auto
lemma reflect_poly_smult: "reflect_poly (smult c p) = smult c (reflect_poly p)"
for p :: "'a::{comm_semiring_0,semiring_no_zero_divisors} poly"
using reflect_poly_mult[of "[:c:]" p] by simp
lemma reflect_poly_power: "reflect_poly (p ^ n) = reflect_poly p ^ n"
for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors} poly"
by (induct n) (simp_all add: reflect_poly_mult)
lemma reflect_poly_prod: "reflect_poly (prod f A) = prod (λx. reflect_poly (f x)) A"
for f :: "_ ⇒ _::{comm_semiring_0,semiring_no_zero_divisors} poly"
by (induct A rule: infinite_finite_induct) (simp_all add: reflect_poly_mult)
lemma reflect_poly_prod_list: "reflect_poly (prod_list xs) = prod_list (map reflect_poly xs)"
for xs :: "_::{comm_semiring_0,semiring_no_zero_divisors} poly list"
by (induct xs) (simp_all add: reflect_poly_mult)
lemma reflect_poly_Poly_nz:
"no_trailing (HOL.eq 0) xs ⟹ reflect_poly (Poly xs) = Poly (rev xs)"
by (simp add: reflect_poly_def)
lemmas reflect_poly_simps =
reflect_poly_0 reflect_poly_1 reflect_poly_const reflect_poly_smult reflect_poly_mult
reflect_poly_power reflect_poly_prod reflect_poly_prod_list
subsection ‹Derivatives›
function pderiv :: "('a :: {comm_semiring_1,semiring_no_zero_divisors}) poly ⇒ 'a poly"
where "pderiv (pCons a p) = (if p = 0 then 0 else p + pCons 0 (pderiv p))"
by (auto intro: pCons_cases)
termination pderiv
by (relation "measure degree") simp_all
declare pderiv.simps[simp del]
lemma pderiv_0 [simp]: "pderiv 0 = 0"
using pderiv.simps [of 0 0] by simp
lemma pderiv_pCons: "pderiv (pCons a p) = p + pCons 0 (pderiv p)"
by (simp add: pderiv.simps)
lemma pderiv_1 [simp]: "pderiv 1 = 0"
by (simp add: one_pCons pderiv_pCons)
lemma pderiv_of_nat [simp]: "pderiv (of_nat n) = 0"
and pderiv_numeral [simp]: "pderiv (numeral m) = 0"
by (simp_all add: of_nat_poly numeral_poly pderiv_pCons)
lemma coeff_pderiv: "coeff (pderiv p) n = of_nat (Suc n) * coeff p (Suc n)"
by (induct p arbitrary: n)
(auto simp add: pderiv_pCons coeff_pCons algebra_simps split: nat.split)
fun pderiv_coeffs_code :: "'a::{comm_semiring_1,semiring_no_zero_divisors} ⇒ 'a list ⇒ 'a list"
where
"pderiv_coeffs_code f (x # xs) = cCons (f * x) (pderiv_coeffs_code (f+1) xs)"
| "pderiv_coeffs_code f [] = []"
definition pderiv_coeffs :: "'a::{comm_semiring_1,semiring_no_zero_divisors} list ⇒ 'a list"
where "pderiv_coeffs xs = pderiv_coeffs_code 1 (tl xs)"
lemma pderiv_coeffs_code:
"nth_default 0 (pderiv_coeffs_code f xs) n = (f + of_nat n) * nth_default 0 xs n"
proof (induct xs arbitrary: f n)
case Nil
then show ?case by simp
next
case (Cons x xs)
show ?case
proof (cases n)
case 0
then show ?thesis
by (cases "pderiv_coeffs_code (f + 1) xs = [] ∧ f * x = 0") (auto simp: cCons_def)
next
case n: (Suc m)
show ?thesis
proof (cases "pderiv_coeffs_code (f + 1) xs = [] ∧ f * x = 0")
case False
then have "nth_default 0 (pderiv_coeffs_code f (x # xs)) n =
nth_default 0 (pderiv_coeffs_code (f + 1) xs) m"
by (auto simp: cCons_def n)
also have "… = (f + of_nat n) * nth_default 0 xs m"
by (simp add: Cons n add_ac)
finally show ?thesis
by (simp add: n)
next
case True
have empty: "pderiv_coeffs_code g xs = [] ⟹ g + of_nat m = 0 ∨ nth_default 0 xs m = 0" for g
proof (induct xs arbitrary: g m)
case Nil
then show ?case by simp
next
case (Cons x xs)
from Cons(2) have empty: "pderiv_coeffs_code (g + 1) xs = []" and g: "g = 0 ∨ x = 0"
by (auto simp: cCons_def split: if_splits)
note IH = Cons(1)[OF empty]
from IH[of m] IH[of "m - 1"] g show ?case
by (cases m) (auto simp: field_simps)
qed
from True have "nth_default 0 (pderiv_coeffs_code f (x # xs)) n = 0"
by (auto simp: cCons_def n)
moreover from True have "(f + of_nat n) * nth_default 0 (x # xs) n = 0"
by (simp add: n) (use empty[of "f+1"] in ‹auto simp: field_simps›)
ultimately show ?thesis by simp
qed
qed
qed
lemma coeffs_pderiv_code [code abstract]: "coeffs (pderiv p) = pderiv_coeffs (coeffs p)"
unfolding pderiv_coeffs_def
proof (rule coeffs_eqI, unfold pderiv_coeffs_code coeff_pderiv, goal_cases)
case (1 n)
have id: "coeff p (Suc n) = nth_default 0 (map (λi. coeff p (Suc i)) [0..<degree p]) n"
by (cases "n < degree p") (auto simp: nth_default_def coeff_eq_0)
show ?case
unfolding coeffs_def map_upt_Suc by (auto simp: id)
next
case 2
obtain n :: 'a and xs where defs: "tl (coeffs p) = xs" "1 = n"
by simp
from 2 show ?case
unfolding defs by (induct xs arbitrary: n) (auto simp: cCons_def)
qed
lemma pderiv_eq_0_iff: "pderiv p = 0 ⟷ degree p = 0"
for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors,semiring_char_0} poly"
proof (cases "degree p")
case 0
then show ?thesis
by (metis degree_eq_zeroE pderiv.simps)
next
case (Suc n)
then show ?thesis
using coeff_0 coeff_pderiv degree_0 leading_coeff_0_iff mult_eq_0_iff nat.distinct(1) of_nat_eq_0_iff
by (metis coeff_0 coeff_pderiv degree_0 leading_coeff_0_iff mult_eq_0_iff nat.distinct(1) of_nat_eq_0_iff)
qed
lemma degree_pderiv: "degree (pderiv p) = degree p - 1"
for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors,semiring_char_0} poly"
proof -
have "degree p - 1 ≤ degree (pderiv p)"
proof (cases "degree p")
case (Suc n)
then show ?thesis
by (metis coeff_pderiv degree_0 diff_Suc_1 le_degree leading_coeff_0_iff mult_eq_0_iff nat.distinct(1) of_nat_eq_0_iff)
qed auto
moreover have "∀i>degree p - 1. coeff (pderiv p) i = 0"
by (simp add: coeff_eq_0 coeff_pderiv)
ultimately show ?thesis
using order_antisym [OF degree_le] by blast
qed
lemma not_dvd_pderiv:
fixes p :: "'a::{comm_semiring_1,semiring_no_zero_divisors,semiring_char_0} poly"
assumes "degree p ≠ 0"
shows "¬ p dvd pderiv p"
proof
assume dvd: "p dvd pderiv p"
then obtain q where p: "pderiv p = p * q"
unfolding dvd_def by auto
from dvd have le: "degree p ≤ degree (pderiv p)"
by (simp add: assms dvd_imp_degree_le pderiv_eq_0_iff)
from assms and this [unfolded degree_pderiv]
show False by auto
qed
lemma dvd_pderiv_iff [simp]: "p dvd pderiv p ⟷ degree p = 0"
for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors,semiring_char_0} poly"
using not_dvd_pderiv[of p] by (auto simp: pderiv_eq_0_iff [symmetric])
lemma pderiv_singleton [simp]: "pderiv [:a:] = 0"
by (simp add: pderiv_pCons)
lemma pderiv_add: "pderiv (p + q) = pderiv p + pderiv q"
by (rule poly_eqI) (simp add: coeff_pderiv algebra_simps)
lemma pderiv_minus: "pderiv (- p :: 'a :: idom poly) = - pderiv p"
by (rule poly_eqI) (simp add: coeff_pderiv algebra_simps)
lemma pderiv_diff: "pderiv ((p :: _ :: idom poly) - q) = pderiv p - pderiv q"
by (rule poly_eqI) (simp add: coeff_pderiv algebra_simps)
lemma pderiv_smult: "pderiv (smult a p) = smult a (pderiv p)"
by (rule poly_eqI) (simp add: coeff_pderiv algebra_simps)
lemma pderiv_mult: "pderiv (p * q) = p * pderiv q + q * pderiv p"
by (induct p) (auto simp: pderiv_add pderiv_smult pderiv_pCons algebra_simps)
lemma pderiv_power_Suc: "pderiv (p ^ Suc n) = smult (of_nat (Suc n)) (p ^ n) * pderiv p"
proof (induction n)
case (Suc n)
then show ?case
by (simp add: pderiv_mult smult_add_left algebra_simps)
qed auto
lemma pderiv_power:
"pderiv (p ^ n) = smult (of_nat n) (p ^ (n - 1) * pderiv p)"
by (cases n) (simp_all add: pderiv_power_Suc del: power_Suc)
lemma pderiv_monom:
"pderiv (monom c n) = monom (of_nat n * c) (n - 1)"
by (cases n)
(simp_all add: monom_altdef pderiv_power_Suc pderiv_smult pderiv_pCons mult_ac del: power_Suc)
lemma pderiv_pcompose: "pderiv (pcompose p q) = pcompose (pderiv p) q * pderiv q"
by (induction p rule: pCons_induct)
(auto simp: pcompose_pCons pderiv_add pderiv_mult pderiv_pCons pcompose_add algebra_simps)
lemma pderiv_prod: "pderiv (prod f (as)) = (∑a∈as. prod f (as - {a}) * pderiv (f a))"
proof (induct as rule: infinite_finite_induct)
case (insert a as)
then have id: "prod f (insert a as) = f a * prod f as"
"⋀g. sum g (insert a as) = g a + sum g as"
"insert a as - {a} = as"
by auto
have "prod f (insert a as - {b}) = f a * prod f (as - {b})" if "b ∈ as" for b
proof -
from ‹a ∉ as› that have *: "insert a as - {b} = insert a (as - {b})"
by auto
show ?thesis
unfolding * by (subst prod.insert) (use insert in auto)
qed
then show ?case
unfolding id pderiv_mult insert(3) sum_distrib_left
by (auto simp add: ac_simps intro!: sum.cong)
qed auto
lemma coeff_higher_pderiv:
"coeff ((pderiv ^^ m) f) n = pochhammer (of_nat (Suc n)) m * coeff f (n + m)"
by (induction m arbitrary: n) (simp_all add: coeff_pderiv pochhammer_rec algebra_simps)
lemma higher_pderiv_0 [simp]: "(pderiv ^^ n) 0 = 0"
by (induction n) simp_all
lemma higher_pderiv_add: "(pderiv ^^ n) (p + q) = (pderiv ^^ n) p + (pderiv ^^ n) q"
by (induction n arbitrary: p q) (simp_all del: funpow.simps add: funpow_Suc_right pderiv_add)
lemma higher_pderiv_smult: "(pderiv ^^ n) (smult c p) = smult c ((pderiv ^^ n) p)"
by (induction n arbitrary: p) (simp_all del: funpow.simps add: funpow_Suc_right pderiv_smult)
lemma higher_pderiv_monom:
"m ≤ n + 1 ⟹ (pderiv ^^ m) (monom c n) = monom (pochhammer (int n - int m + 1) m * c) (n - m)"
proof (induction m arbitrary: c n)
case (Suc m)
thus ?case
by (cases n)
(simp_all del: funpow.simps add: funpow_Suc_right pderiv_monom pochhammer_rec' Suc.IH)
qed simp_all
lemma higher_pderiv_monom_eq_zero:
"m > n + 1 ⟹ (pderiv ^^ m) (monom c n) = 0"
proof (induction m arbitrary: c n)
case (Suc m)
thus ?case
by (cases n)
(simp_all del: funpow.simps add: funpow_Suc_right pderiv_monom pochhammer_rec' Suc.IH)
qed simp_all
lemma higher_pderiv_sum: "(pderiv ^^ n) (sum f A) = (∑x∈A. (pderiv ^^ n) (f x))"
by (induction A rule: infinite_finite_induct) (simp_all add: higher_pderiv_add)
lemma higher_pderiv_sum_mset: "(pderiv ^^ n) (sum_mset A) = (∑p∈#A. (pderiv ^^ n) p)"
by (induction A) (simp_all add: higher_pderiv_add)
lemma higher_pderiv_sum_list: "(pderiv ^^ n) (sum_list ps) = (∑p←ps. (pderiv ^^ n) p)"
by (induction ps) (simp_all add: higher_pderiv_add)
lemma degree_higher_pderiv: "Polynomial.degree ((pderiv ^^ n) p) = Polynomial.degree p - n"
for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors,semiring_char_0} poly"
by (induction n) (auto simp: degree_pderiv)
lemma DERIV_pow2: "DERIV (λx. x ^ Suc n) x :> real (Suc n) * (x ^ n)"
by (rule DERIV_cong, rule DERIV_pow) simp
declare DERIV_pow2 [simp] DERIV_pow [simp]
lemma DERIV_add_const: "DERIV f x :> D ⟹ DERIV (λx. a + f x :: 'a::real_normed_field) x :> D"
by (rule DERIV_cong, rule DERIV_add) auto
lemma poly_DERIV [simp]: "DERIV (λx. poly p x) x :> poly (pderiv p) x"
by (induct p) (auto intro!: derivative_eq_intros simp add: pderiv_pCons)
lemma poly_isCont[simp]:
fixes x::"'a::real_normed_field"
shows "isCont (λx. poly p x) x"
by (rule poly_DERIV [THEN DERIV_isCont])
lemma tendsto_poly [tendsto_intros]: "(f ⤏ a) F ⟹ ((λx. poly p (f x)) ⤏ poly p a) F"
for f :: "_ ⇒ 'a::real_normed_field"
by (rule isCont_tendsto_compose [OF poly_isCont])
lemma continuous_within_poly: "continuous (at z within s) (poly p)"
for z :: "'a::{real_normed_field}"
by (simp add: continuous_within tendsto_poly)
lemma continuous_poly [continuous_intros]: "continuous F f ⟹ continuous F (λx. poly p (f x))"
for f :: "_ ⇒ 'a::real_normed_field"
unfolding continuous_def by (rule tendsto_poly)
lemma continuous_on_poly [continuous_intros]:
fixes p :: "'a :: {real_normed_field} poly"
assumes "continuous_on A f"
shows "continuous_on A (λx. poly p (f x))"
by (metis DERIV_continuous_on assms continuous_on_compose2 poly_DERIV subset_UNIV)
text ‹Consequences of the derivative theorem above.›
lemma poly_differentiable[simp]: "(λx. poly p x) differentiable (at x)"
for x :: real
by (simp add: real_differentiable_def) (blast intro: poly_DERIV)
lemma poly_IVT_pos: "a < b ⟹ poly p a < 0 ⟹ 0 < poly p b ⟹ ∃x. a < x ∧ x < b ∧ poly p x = 0"
for a b :: real
using IVT [of "poly p" a 0 b] by (auto simp add: order_le_less)
lemma poly_IVT_neg: "a < b ⟹ 0 < poly p a ⟹ poly p b < 0 ⟹ ∃x. a < x ∧ x < b ∧ poly p x = 0"
for a b :: real
using poly_IVT_pos [where p = "- p"] by simp
lemma poly_IVT: "a < b ⟹ poly p a * poly p b < 0 ⟹ ∃x>a. x < b ∧ poly p x = 0"
for p :: "real poly"
by (metis less_not_sym mult_less_0_iff poly_IVT_neg poly_IVT_pos)
lemma poly_MVT: "a < b ⟹ ∃x. a < x ∧ x < b ∧ poly p b - poly p a = (b - a) * poly (pderiv p) x"
for a b :: real
by (simp add: MVT2)
lemma poly_MVT':
fixes a b :: real
assumes "{min a b..max a b} ⊆ A"
shows "∃x∈A. poly p b - poly p a = (b - a) * poly (pderiv p) x"
proof (cases a b rule: linorder_cases)
case less
from poly_MVT[OF less, of p] obtain x
where "a < x" "x < b" "poly p b - poly p a = (b - a) * poly (pderiv p) x"
by auto
then show ?thesis by (intro bexI[of _ x]) (auto intro!: subsetD[OF assms])
next
case greater
from poly_MVT[OF greater, of p] obtain x
where "b < x" "x < a" "poly p a - poly p b = (a - b) * poly (pderiv p) x" by auto
then show ?thesis by (intro bexI[of _ x]) (auto simp: algebra_simps intro!: subsetD[OF assms])
qed (use assms in auto)
lemma poly_pinfty_gt_lc:
fixes p :: "real poly"
assumes "lead_coeff p > 0"
shows "∃n. ∀ x ≥ n. poly p x ≥ lead_coeff p"
using assms
proof (induct p)
case 0
then show ?case by auto
next
case (pCons a p)
from this(1) consider "a ≠ 0" "p = 0" | "p ≠ 0" by auto
then show ?case
proof cases
case 1
then show ?thesis by auto
next
case 2
with pCons obtain n1 where gte_lcoeff: "∀x≥n1. lead_coeff p ≤ poly p x"
by auto
from pCons(3) ‹p ≠ 0› have gt_0: "lead_coeff p > 0" by auto
define n where "n = max n1 (1 + ¦a¦ / lead_coeff p)"
have "lead_coeff (pCons a p) ≤ poly (pCons a p) x" if "n ≤ x" for x
proof -
from gte_lcoeff that have "lead_coeff p ≤ poly p x"
by (auto simp: n_def)
with gt_0 have "¦a¦ / lead_coeff p ≥ ¦a¦ / poly p x" and "poly p x > 0"
by (auto intro: frac_le)
with ‹n ≤ x›[unfolded n_def] have "x ≥ 1 + ¦a¦ / poly p x"
by auto
with ‹lead_coeff p ≤ poly p x› ‹poly p x > 0› ‹p ≠ 0›
show "lead_coeff (pCons a p) ≤ poly (pCons a p) x"
by (auto simp: field_simps)
qed
then show ?thesis by blast
qed
qed
lemma lemma_order_pderiv1:
"pderiv ([:- a, 1:] ^ Suc n * q) = [:- a, 1:] ^ Suc n * pderiv q +
smult (of_nat (Suc n)) (q * [:- a, 1:] ^ n)"
by (simp only: pderiv_mult pderiv_power_Suc) (simp del: power_Suc of_nat_Suc add: pderiv_pCons)
lemma lemma_order_pderiv:
fixes p :: "'a :: field_char_0 poly"
assumes n: "0 < n"
and pd: "pderiv p ≠ 0"
and pe: "p = [:- a, 1:] ^ n * q"
and nd: "¬ [:- a, 1:] dvd q"
shows "n = Suc (order a (pderiv p))"
proof -
from assms have "pderiv ([:- a, 1:] ^ n * q) ≠ 0"
by auto
from assms obtain n' where "n = Suc n'" "0 < Suc n'" "pderiv ([:- a, 1:] ^ Suc n' * q) ≠ 0"
by (cases n) auto
have "order a (pderiv ([:- a, 1:] ^ Suc n' * q)) = n'"
proof (rule order_unique_lemma)
show "[:- a, 1:] ^ n' dvd pderiv ([:- a, 1:] ^ Suc n' * q)"
unfolding lemma_order_pderiv1
proof (rule dvd_add)
show "[:- a, 1:] ^ n' dvd [:- a, 1:] ^ Suc n' * pderiv q"
by (metis dvdI dvd_mult2 power_Suc2)
show "[:- a, 1:] ^ n' dvd smult (of_nat (Suc n')) (q * [:- a, 1:] ^ n')"
by (metis dvd_smult dvd_triv_right)
qed
have "k dvd k * pderiv q + smult (of_nat (Suc n')) l ⟹ k dvd l" for k l
by (auto simp del: of_nat_Suc simp: dvd_add_right_iff dvd_smult_iff)
then show "¬ [:- a, 1:] ^ Suc n' dvd pderiv ([:- a, 1:] ^ Suc n' * q)"
unfolding lemma_order_pderiv1
by (metis nd dvd_mult_cancel_right power_not_zero pCons_eq_0_iff power_Suc zero_neq_one)
qed
then show ?thesis
by (metis ‹n = Suc n'› pe)
qed
lemma order_pderiv: "order a p = Suc (order a (pderiv p))"
if "pderiv p ≠ 0" "order a p ≠ 0"
for p :: "'a::field_char_0 poly"
proof (cases "p = 0")
case False
obtain q where "p = [:- a, 1:] ^ order a p * q ∧ ¬ [:- a, 1:] dvd q"
using False order_decomp by blast
then show ?thesis
using lemma_order_pderiv that by blast
qed (use that in auto)
lemma poly_squarefree_decomp_order:
fixes p :: "'a::field_char_0 poly"
assumes "pderiv p ≠ 0"
and p: "p = q * d"
and p': "pderiv p = e * d"
and d: "d = r * p + s * pderiv p"
shows "order a q = (if order a p = 0 then 0 else 1)"
proof (rule classical)
assume 1: "¬ ?thesis"
from ‹pderiv p ≠ 0› have "p ≠ 0" by auto
with p have "order a p = order a q + order a d"
by (simp add: order_mult)
with 1 have "order a p ≠ 0"
by (auto split: if_splits)
from ‹pderiv p ≠ 0› ‹pderiv p = e * d› have oapp: "order a (pderiv p) = order a e + order a d"
by (simp add: order_mult)
from ‹pderiv p ≠ 0› ‹order a p ≠ 0› have oap: "order a p = Suc (order a (pderiv p))"
by (rule order_pderiv)
from ‹p ≠ 0› ‹p = q * d› have "d ≠ 0"
by simp
have "[:- a, 1:] ^ order a (pderiv p) dvd r * p"
by (metis dvd_trans dvd_triv_right oap order_1 power_Suc)
then have "([:-a, 1:] ^ (order a (pderiv p))) dvd d"
by (simp add: d order_1)
with ‹d ≠ 0› have "order a (pderiv p) ≤ order a d"
by (simp add: order_divides)
show ?thesis
using ‹order a p = order a q + order a d›
and oapp oap
and ‹order a (pderiv p) ≤ order a d›
by auto
qed
lemma poly_squarefree_decomp_order2:
"pderiv p ≠ 0 ⟹ p = q * d ⟹ pderiv p = e * d ⟹
d = r * p + s * pderiv p ⟹ ∀a. order a q = (if order a p = 0 then 0 else 1)"
for p :: "'a::field_char_0 poly"
by (blast intro: poly_squarefree_decomp_order)
lemma order_pderiv2:
"pderiv p ≠ 0 ⟹ order a p ≠ 0 ⟹ order a (pderiv p) = n ⟷ order a p = Suc n"
for p :: "'a::field_char_0 poly"
by (auto dest: order_pderiv)
definition rsquarefree :: "'a::idom poly ⇒ bool"
where "rsquarefree p ⟷ p ≠ 0 ∧ (∀a. order a p = 0 ∨ order a p = 1)"
lemma pderiv_iszero: "pderiv p = 0 ⟹ ∃h. p = [:h:]"
for p :: "'a::{semidom,semiring_char_0} poly"
by (cases p) (auto simp: pderiv_eq_0_iff split: if_splits)
lemma rsquarefree_roots: "rsquarefree p ⟷ (∀a. ¬ (poly p a = 0 ∧ poly (pderiv p) a = 0))"
for p :: "'a::field_char_0 poly"
proof (cases "p = 0")
case False
show ?thesis
proof (cases "pderiv p = 0")
case True
with ‹p ≠ 0› pderiv_iszero show ?thesis
by (force simp add: order_0I rsquarefree_def)
next
case False
with ‹p ≠ 0› order_pderiv2 show ?thesis
by (force simp add: rsquarefree_def order_root)
qed
qed (simp add: rsquarefree_def)
lemma rsquarefree_root_order:
assumes "rsquarefree p" "poly p z = 0" "p ≠ 0"
shows "order z p = 1"
proof -
from assms have "order z p ∈ {0, 1}" by (auto simp: rsquarefree_def)
moreover from assms have "order z p > 0" by (auto simp: order_root)
ultimately show "order z p = 1" by auto
qed
lemma poly_squarefree_decomp:
fixes p :: "'a::field_char_0 poly"
assumes "pderiv p ≠ 0"
and "p = q * d"
and "pderiv p = e * d"
and "d = r * p + s * pderiv p"
shows "rsquarefree q ∧ (∀a. poly q a = 0 ⟷ poly p a = 0)"
proof -
from ‹pderiv p ≠ 0› have "p ≠ 0" by auto
with ‹p = q * d› have "q ≠ 0" by simp
from assms have "∀a. order a q = (if order a p = 0 then 0 else 1)"
by (rule poly_squarefree_decomp_order2)
with ‹p ≠ 0› ‹q ≠ 0› show ?thesis
by (simp add: rsquarefree_def order_root)
qed
lemma has_field_derivative_poly [derivative_intros]:
assumes "(f has_field_derivative f') (at x within A)"
shows "((λx. poly p (f x)) has_field_derivative
(f' * poly (pderiv p) (f x))) (at x within A)"
using DERIV_chain[OF poly_DERIV assms, of p] by (simp add: o_def mult_ac)
subsection ‹Algebraic numbers›
lemma intpolyE:
assumes "⋀i. poly.coeff p i ∈ ℤ"
obtains q where "p = map_poly of_int q"
proof -
have "∀i∈{..Polynomial.degree p}. ∃x. poly.coeff p i = of_int x"
using assms by (auto simp: Ints_def)
from bchoice[OF this] obtain f
where f: "⋀i. i ≤ Polynomial.degree p ⟹ poly.coeff p i = of_int (f i)" by blast
define q where "q = Poly (map f [0..<Suc (Polynomial.degree p)])"
have "p = map_poly of_int q"
by (intro poly_eqI)
(auto simp: coeff_map_poly q_def nth_default_def f coeff_eq_0 simp del: upt_Suc)
with that show ?thesis by blast
qed
lemma ratpolyE:
assumes "⋀i. poly.coeff p i ∈ ℚ"
obtains q where "p = map_poly of_rat q"
proof -
have "∀i∈{..Polynomial.degree p}. ∃x. poly.coeff p i = of_rat x"
using assms by (auto simp: Rats_def)
from bchoice[OF this] obtain f
where f: "⋀i. i ≤ Polynomial.degree p ⟹ poly.coeff p i = of_rat (f i)" by blast
define q where "q = Poly (map f [0..<Suc (Polynomial.degree p)])"
have "p = map_poly of_rat q"
by (intro poly_eqI)
(auto simp: coeff_map_poly q_def nth_default_def f coeff_eq_0 simp del: upt_Suc)
with that show ?thesis by blast
qed
text ‹
Algebraic numbers can be defined in two equivalent ways: all real numbers that are
roots of rational polynomials or of integer polynomials. The Algebraic-Numbers AFP entry
uses the rational definition, but we need the integer definition.
The equivalence is obvious since any rational polynomial can be multiplied with the
LCM of its coefficients, yielding an integer polynomial with the same roots.
›
definition algebraic :: "'a :: field_char_0 ⇒ bool"
where "algebraic x ⟷ (∃p. (∀i. coeff p i ∈ ℤ) ∧ p ≠ 0 ∧ poly p x = 0)"
lemma algebraicI: "(⋀i. coeff p i ∈ ℤ) ⟹ p ≠ 0 ⟹ poly p x = 0 ⟹ algebraic x"
unfolding algebraic_def by blast
lemma algebraicE:
assumes "algebraic x"
obtains p where "⋀i. coeff p i ∈ ℤ" "p ≠ 0" "poly p x = 0"
using assms unfolding algebraic_def by blast
lemma algebraic_altdef: "algebraic x ⟷ (∃p. (∀i. coeff p i ∈ ℚ) ∧ p ≠ 0 ∧ poly p x = 0)"
for p :: "'a::field_char_0 poly"
proof safe
fix p
assume rat: "∀i. coeff p i ∈ ℚ" and root: "poly p x = 0" and nz: "p ≠ 0"
define cs where "cs = coeffs p"
from rat have "∀c∈range (coeff p). ∃c'. c = of_rat c'"
unfolding Rats_def by blast
then obtain f where f: "coeff p i = of_rat (f (coeff p i))" for i
by (subst (asm) bchoice_iff) blast
define cs' where "cs' = map (quotient_of ∘ f) (coeffs p)"
define d where "d = Lcm (set (map snd cs'))"
define p' where "p' = smult (of_int d) p"
have "coeff p' n ∈ ℤ" for n
proof (cases "n ≤ degree p")
case True
define c where "c = coeff p n"
define a where "a = fst (quotient_of (f (coeff p n)))"
define b where "b = snd (quotient_of (f (coeff p n)))"
have b_pos: "b > 0"
unfolding b_def using quotient_of_denom_pos' by simp
have "coeff p' n = of_int d * coeff p n"
by (simp add: p'_def)
also have "coeff p n = of_rat (of_int a / of_int b)"
unfolding a_def b_def
by (subst quotient_of_div [of "f (coeff p n)", symmetric]) (simp_all add: f [symmetric])
also have "of_int d * … = of_rat (of_int (a*d) / of_int b)"
by (simp add: of_rat_mult of_rat_divide)
also from nz True have "b ∈ snd ` set cs'"
by (force simp: cs'_def o_def b_def coeffs_def simp del: upt_Suc)
then have "b dvd (a * d)"
by (simp add: d_def)
then have "of_int (a * d) / of_int b ∈ (ℤ :: rat set)"
by (rule of_int_divide_in_Ints)
then have "of_rat (of_int (a * d) / of_int b) ∈ ℤ" by (elim Ints_cases) auto
finally show ?thesis .
next
case False
then show ?thesis
by (auto simp: p'_def not_le coeff_eq_0)
qed
moreover have "set (map snd cs') ⊆ {0<..}"
unfolding cs'_def using quotient_of_denom_pos' by (auto simp: coeffs_def simp del: upt_Suc)
then have "d ≠ 0"
unfolding d_def by (induct cs') simp_all
with nz have "p' ≠ 0" by (simp add: p'_def)
moreover from root have "poly p' x = 0"
by (simp add: p'_def)
ultimately show "algebraic x"
unfolding algebraic_def by blast
next
assume "algebraic x"
then obtain p where p: "coeff p i ∈ ℤ" "poly p x = 0" "p ≠ 0" for i
by (force simp: algebraic_def)
moreover have "coeff p i ∈ ℤ ⟹ coeff p i ∈ ℚ" for i
by (elim Ints_cases) simp
ultimately show "∃p. (∀i. coeff p i ∈ ℚ) ∧ p ≠ 0 ∧ poly p x = 0" by auto
qed
lemma algebraicI': "(⋀i. coeff p i ∈ ℚ) ⟹ p ≠ 0 ⟹ poly p x = 0 ⟹ algebraic x"
unfolding algebraic_altdef by blast
lemma algebraicE':
assumes "algebraic (x :: 'a :: field_char_0)"
obtains p where "p ≠ 0" "poly (map_poly of_int p) x = 0"
proof -
from assms obtain q where q: "⋀i. coeff q i ∈ ℤ" "q ≠ 0" "poly q x = 0"
by (erule algebraicE)
moreover from this(1) obtain q' where q': "q = map_poly of_int q'" by (erule intpolyE)
moreover have "q' ≠ 0"
using q' q by auto
ultimately show ?thesis by (intro that[of q']) simp_all
qed
lemma algebraicE'_nonzero:
assumes "algebraic (x :: 'a :: field_char_0)" "x ≠ 0"
obtains p where "p ≠ 0" "coeff p 0 ≠ 0" "poly (map_poly of_int p) x = 0"
proof -
from assms(1) obtain p where p: "p ≠ 0" "poly (map_poly of_int p) x = 0"
by (erule algebraicE')
define n :: nat where "n = order 0 p"
have "monom 1 n dvd p" by (simp add: monom_1_dvd_iff p n_def)
then obtain q where q: "p = monom 1 n * q" by (erule dvdE)
have [simp]: "map_poly of_int (monom 1 n * q) = monom (1 :: 'a) n * map_poly of_int q"
by (induction n) (auto simp: monom_0 monom_Suc map_poly_pCons)
from p have "q ≠ 0" "poly (map_poly of_int q) x = 0" by (auto simp: q poly_monom assms(2))
moreover from this have "order 0 p = n + order 0 q" by (simp add: q order_mult)
hence "order 0 q = 0" by (simp add: n_def)
with ‹q ≠ 0› have "poly q 0 ≠ 0" by (simp add: order_root)
ultimately show ?thesis using that[of q] by (auto simp: poly_0_coeff_0)
qed
lemma rat_imp_algebraic: "x ∈ ℚ ⟹ algebraic x"
proof (rule algebraicI')
show "poly [:-x, 1:] x = 0"
by simp
qed (auto simp: coeff_pCons split: nat.splits)
lemma algebraic_0 [simp, intro]: "algebraic 0"
and algebraic_1 [simp, intro]: "algebraic 1"
and algebraic_numeral [simp, intro]: "algebraic (numeral n)"
and algebraic_of_nat [simp, intro]: "algebraic (of_nat k)"
and algebraic_of_int [simp, intro]: "algebraic (of_int m)"
by (simp_all add: rat_imp_algebraic)
lemma algebraic_ii [simp, intro]: "algebraic 𝗂"
proof (rule algebraicI)
show "poly [:1, 0, 1:] 𝗂 = 0"
by simp
qed (auto simp: coeff_pCons split: nat.splits)
lemma algebraic_minus [intro]:
assumes "algebraic x"
shows "algebraic (-x)"
proof -
from assms obtain p where p: "∀i. coeff p i ∈ ℤ" "poly p x = 0" "p ≠ 0"
by (elim algebraicE) auto
define s where "s = (if even (degree p) then 1 else -1 :: 'a)"
define q where "q = Polynomial.smult s (pcompose p [:0, -1:])"
have "poly q (-x) = 0"
using p by (auto simp: q_def poly_pcompose s_def)
moreover have "q ≠ 0"
using p by (auto simp: q_def s_def pcompose_eq_0_iff)
find_theorems "pcompose _ _ = 0"
moreover have "coeff q i ∈ ℤ" for i
proof -
have "coeff (pcompose p [:0, -1:]) i ∈ ℤ"
using p by (intro coeff_pcompose_semiring_closed) (auto simp: coeff_pCons split: nat.splits)
thus ?thesis by (simp add: q_def s_def)
qed
ultimately show ?thesis
by (auto simp: algebraic_def)
qed
lemma algebraic_minus_iff [simp]:
"algebraic (-x) ⟷ algebraic (x :: 'a :: field_char_0)"
using algebraic_minus[of x] algebraic_minus[of "-x"] by auto
lemma algebraic_inverse [intro]:
assumes "algebraic x"
shows "algebraic (inverse x)"
proof (cases "x = 0")
case [simp]: False
from assms obtain p where p: "∀i. coeff p i ∈ ℤ" "poly p x = 0" "p ≠ 0"
by (elim algebraicE) auto
show ?thesis
proof (rule algebraicI)
show "poly (reflect_poly p) (inverse x) = 0"
using assms p by (simp add: poly_reflect_poly_nz)
qed (use assms p in ‹auto simp: coeff_reflect_poly›)
qed auto
lemma algebraic_root:
assumes "algebraic y"
and "poly p x = y" and "∀i. coeff p i ∈ ℤ" and "lead_coeff p = 1" and "degree p > 0"
shows "algebraic x"
proof -
from assms obtain q where q: "poly q y = 0" "∀i. coeff q i ∈ ℤ" "q ≠ 0"
by (elim algebraicE) auto
show ?thesis
proof (rule algebraicI)
from assms q show "pcompose q p ≠ 0"
by (auto simp: pcompose_eq_0_iff)
from assms q show "coeff (pcompose q p) i ∈ ℤ" for i
by (intro allI coeff_pcompose_semiring_closed) auto
show "poly (pcompose q p) x = 0"
using assms q by (simp add: poly_pcompose)
qed
qed
lemma algebraic_abs_real [simp]:
"algebraic ¦x :: real¦ ⟷ algebraic x"
by (auto simp: abs_if)
lemma algebraic_nth_root_real [intro]:
assumes "algebraic x"
shows "algebraic (root n x)"
proof (cases "n = 0")
case False
show ?thesis
proof (rule algebraic_root)
show "poly (monom 1 n) (root n x) = (if even n then ¦x¦ else x)"
using sgn_power_root[of n x] False
by (auto simp add: poly_monom sgn_if split: if_splits)
qed (use False assms in ‹auto simp: degree_monom_eq›)
qed auto
lemma algebraic_sqrt [intro]: "algebraic x ⟹ algebraic (sqrt x)"
by (auto simp: sqrt_def)
lemma algebraic_csqrt [intro]: "algebraic x ⟹ algebraic (csqrt x)"
by (rule algebraic_root[where p = "monom 1 2"])
(auto simp: poly_monom degree_monom_eq)
lemma algebraic_cnj [intro]:
assumes "algebraic x"
shows "algebraic (cnj x)"
proof -
from assms obtain p where p: "poly p x = 0" "∀i. coeff p i ∈ ℤ" "p ≠ 0"
by (elim algebraicE) auto
show ?thesis
proof (rule algebraicI)
show "poly (map_poly cnj p) (cnj x) = 0"
using p by simp
show "map_poly cnj p ≠ 0"
using p by (auto simp: map_poly_eq_0_iff)
show "coeff (map_poly cnj p) i ∈ ℤ" for i
using p by (auto simp: coeff_map_poly)
qed
qed
lemma algebraic_cnj_iff [simp]: "algebraic (cnj x) ⟷ algebraic x"
using algebraic_cnj[of x] algebraic_cnj[of "cnj x"] by auto
lemma algebraic_of_real [intro]:
assumes "algebraic x"
shows "algebraic (of_real x)"
proof -
from assms obtain p where p: "p ≠ 0" "poly (map_poly of_int p) x = 0" by (erule algebraicE')
have 1: "map_poly of_int p ≠ (0 :: 'a poly)"
using p by (metis coeff_0 coeff_map_poly leading_coeff_0_iff of_int_eq_0_iff)
have "poly (map_poly of_int p) (of_real x :: 'a) = of_real (poly (map_poly of_int p) x)"
by (simp add: poly_altdef degree_map_poly coeff_map_poly)
also note p(2)
finally have 2: "poly (map_poly of_int p) (of_real x :: 'a) = 0"
by simp
from 1 2 show "algebraic (of_real x :: 'a)"
by (intro algebraicI[of "map_poly of_int p"]) (auto simp: coeff_map_poly)
qed
lemma algebraic_of_real_iff [simp]:
"algebraic (of_real x :: 'a :: {real_algebra_1,field_char_0}) ⟷ algebraic x"
proof
assume "algebraic (of_real x :: 'a)"
then obtain p where p: "p ≠ 0" "poly (map_poly of_int p) (of_real x :: 'a) = 0"
by (erule algebraicE')
have 1: "(map_poly of_int p :: real poly) ≠ 0"
using p by (metis coeff_0 coeff_map_poly leading_coeff_0_iff of_int_0 of_int_eq_iff)
note p(2)
also have "poly (map_poly of_int p) (of_real x :: 'a) = of_real (poly (map_poly of_int p) x)"
by (simp add: poly_altdef degree_map_poly coeff_map_poly)
also have "… = 0 ⟷ poly (map_poly of_int p) x = 0"
using of_real_eq_0_iff by blast
finally have 2: "poly (map_poly real_of_int p) x = 0" .
from 1 and 2 show "algebraic x"
by (intro algebraicI[of "map_poly of_int p"]) (auto simp: coeff_map_poly)
qed auto
subsection ‹Algebraic integers›
inductive algebraic_int :: "'a :: field ⇒ bool" where
"⟦lead_coeff p = 1; ∀i. coeff p i ∈ ℤ; poly p x = 0⟧ ⟹ algebraic_int x"
lemma algebraic_int_altdef_ipoly:
fixes x :: "'a :: field_char_0"
shows "algebraic_int x ⟷ (∃p. poly (map_poly of_int p) x = 0 ∧ lead_coeff p = 1)"
proof
assume "algebraic_int x"
then obtain p where p: "lead_coeff p = 1" "∀i. coeff p i ∈ ℤ" "poly p x = 0"
by (auto elim: algebraic_int.cases)
define the_int where "the_int = (λx::'a. THE r. x = of_int r)"
define p' where "p' = map_poly the_int p"
have of_int_the_int: "of_int (the_int x) = x" if "x ∈ ℤ" for x
unfolding the_int_def by (rule sym, rule theI') (insert that, auto simp: Ints_def)
have the_int_0_iff: "the_int x = 0 ⟷ x = 0" if "x ∈ ℤ" for x
using of_int_the_int[OF that] by auto
have [simp]: "the_int 0 = 0"
by (subst the_int_0_iff) auto
have "map_poly of_int p' = map_poly (of_int ∘ the_int) p"
by (simp add: p'_def map_poly_map_poly)
also from p of_int_the_int have "… = p"
by (subst poly_eq_iff) (auto simp: coeff_map_poly)
finally have p_p': "map_poly of_int p' = p" .
show "(∃p. poly (map_poly of_int p) x = 0 ∧ lead_coeff p = 1)"
proof (intro exI conjI notI)
from p show "poly (map_poly of_int p') x = 0" by (simp add: p_p')
next
show "lead_coeff p' = 1"
using p by (simp flip: p_p' add: degree_map_poly coeff_map_poly)
qed
next
assume "∃p. poly (map_poly of_int p) x = 0 ∧ lead_coeff p = 1"
then obtain p where p: "poly (map_poly of_int p) x = 0" "lead_coeff p = 1"
by auto
define p' where "p' = (map_poly of_int p :: 'a poly)"
from p have "lead_coeff p' = 1" "poly p' x = 0" "∀i. coeff p' i ∈ ℤ"
by (auto simp: p'_def coeff_map_poly degree_map_poly)
thus "algebraic_int x"
by (intro algebraic_int.intros)
qed
theorem rational_algebraic_int_is_int:
assumes "algebraic_int x" and "x ∈ ℚ"
shows "x ∈ ℤ"
proof -
from assms(2) obtain a b where ab: "b > 0" "Rings.coprime a b" and x_eq: "x = of_int a / of_int b"
by (auto elim: Rats_cases')
from ‹b > 0› have [simp]: "b ≠ 0"
by auto
from assms(1) obtain p
where p: "lead_coeff p = 1" "∀i. coeff p i ∈ ℤ" "poly p x = 0"
by (auto simp: algebraic_int.simps)
define q :: "'a poly" where "q = [:-of_int a, of_int b:]"
have "poly q x = 0" "q ≠ 0" "∀i. coeff q i ∈ ℤ"
by (auto simp: x_eq q_def coeff_pCons split: nat.splits)
define n where "n = degree p"
have "n > 0"
using p by (intro Nat.gr0I) (auto simp: n_def elim!: degree_eq_zeroE)
have "(∑i<n. coeff p i * of_int (a ^ i * b ^ (n - i - 1))) ∈ ℤ"
using p by auto
then obtain R where R: "of_int R = (∑i<n. coeff p i * of_int (a ^ i * b ^ (n - i - 1)))"
by (auto simp: Ints_def)
have [simp]: "coeff p n = 1"
using p by (auto simp: n_def)
have "0 = poly p x * of_int b ^ n"
using p by simp
also have "… = (∑i≤n. coeff p i * x ^ i * of_int b ^ n)"
by (simp add: poly_altdef n_def sum_distrib_right)
also have "… = (∑i≤n. coeff p i * of_int (a ^ i * b ^ (n - i)))"
by (intro sum.cong) (auto simp: x_eq field_simps simp flip: power_add)
also have "{..n} = insert n {..<n}"
using ‹n > 0› by auto
also have "(∑i∈…. coeff p i * of_int (a ^ i * b ^ (n - i))) =
coeff p n * of_int (a ^ n) + (∑i<n. coeff p i * of_int (a ^ i * b ^ (n - i)))"
by (subst sum.insert) auto
also have "(∑i<n. coeff p i * of_int (a ^ i * b ^ (n - i))) =
(∑i<n. coeff p i * of_int (a ^ i * b * b ^ (n - i - 1)))"
by (intro sum.cong) (auto simp flip: power_add power_Suc simp: Suc_diff_Suc)
also have "… = of_int (b * R)"
by (simp add: R sum_distrib_left sum_distrib_right mult_ac)
finally have "of_int (a ^ n) = (-of_int (b * R) :: 'a)"
by (auto simp: add_eq_0_iff)
hence "a ^ n = -b * R"
by (simp flip: of_int_mult of_int_power of_int_minus)
hence "b dvd a ^ n"
by simp
with ‹Rings.coprime a b› have "b dvd 1"
by (meson coprime_power_left_iff dvd_refl not_coprimeI)
with x_eq and ‹b > 0› show ?thesis
by auto
qed
lemma algebraic_int_imp_algebraic [dest]: "algebraic_int x ⟹ algebraic x"
by (auto simp: algebraic_int.simps algebraic_def)
lemma int_imp_algebraic_int:
assumes "x ∈ ℤ"
shows "algebraic_int x"
proof
show "∀i. coeff [:-x, 1:] i ∈ ℤ"
using assms by (auto simp: coeff_pCons split: nat.splits)
qed auto
lemma algebraic_int_0 [simp, intro]: "algebraic_int 0"
and algebraic_int_1 [simp, intro]: "algebraic_int 1"
and algebraic_int_numeral [simp, intro]: "algebraic_int (numeral n)"
and algebraic_int_of_nat [simp, intro]: "algebraic_int (of_nat k)"
and algebraic_int_of_int [simp, intro]: "algebraic_int (of_int m)"
by (simp_all add: int_imp_algebraic_int)
lemma algebraic_int_ii [simp, intro]: "algebraic_int 𝗂"
proof
show "poly [:1, 0, 1:] 𝗂 = 0"
by simp
qed (auto simp: coeff_pCons split: nat.splits)
lemma algebraic_int_minus [intro]:
assumes "algebraic_int x"
shows "algebraic_int (-x)"
proof -
from assms obtain p where p: "lead_coeff p = 1" "∀i. coeff p i ∈ ℤ" "poly p x = 0"
by (auto simp: algebraic_int.simps)
define s where "s = (if even (degree p) then 1 else -1 :: 'a)"
define q where "q = Polynomial.smult s (pcompose p [:0, -1:])"
have "lead_coeff q = s * lead_coeff (pcompose p [:0, -1:])"
by (simp add: q_def)
also have "lead_coeff (pcompose p [:0, -1:]) = lead_coeff p * (- 1) ^ degree p"
by (subst lead_coeff_comp) auto
finally have "poly q (-x) = 0" and "lead_coeff q = 1"
using p by (auto simp: q_def poly_pcompose s_def)
moreover have "coeff q i ∈ ℤ" for i
proof -
have "coeff (pcompose p [:0, -1:]) i ∈ ℤ"
using p by (intro coeff_pcompose_semiring_closed) (auto simp: coeff_pCons split: nat.splits)
thus ?thesis by (simp add: q_def s_def)
qed
ultimately show ?thesis
by (auto simp: algebraic_int.simps)
qed
lemma algebraic_int_minus_iff [simp]:
"algebraic_int (-x) ⟷ algebraic_int (x :: 'a :: field_char_0)"
using algebraic_int_minus[of x] algebraic_int_minus[of "-x"] by auto
lemma algebraic_int_inverse [intro]:
assumes "poly p x = 0" and "∀i. coeff p i ∈ ℤ" and "coeff p 0 = 1"
shows "algebraic_int (inverse x)"
proof
from assms have [simp]: "x ≠ 0"
by (auto simp: poly_0_coeff_0)
show "poly (reflect_poly p) (inverse x) = 0"
using assms by (simp add: poly_reflect_poly_nz)
qed (use assms in ‹auto simp: coeff_reflect_poly›)
lemma algebraic_int_root:
assumes "algebraic_int y"
and "poly p x = y" and "∀i. coeff p i ∈ ℤ" and "lead_coeff p = 1" and "degree p > 0"
shows "algebraic_int x"
proof -
from assms obtain q where q: "poly q y = 0" "∀i. coeff q i ∈ ℤ" "lead_coeff q = 1"
by (auto simp: algebraic_int.simps)
show ?thesis
proof
from assms q show "lead_coeff (pcompose q p) = 1"
by (subst lead_coeff_comp) auto
from assms q show "∀i. coeff (pcompose q p) i ∈ ℤ"
by (intro allI coeff_pcompose_semiring_closed) auto
show "poly (pcompose q p) x = 0"
using assms q by (simp add: poly_pcompose)
qed
qed
lemma algebraic_int_abs_real [simp]:
"algebraic_int ¦x :: real¦ ⟷ algebraic_int x"
by (auto simp: abs_if)
lemma algebraic_int_nth_root_real [intro]:
assumes "algebraic_int x"
shows "algebraic_int (root n x)"
proof (cases "n = 0")
case False
show ?thesis
proof (rule algebraic_int_root)
show "poly (monom 1 n) (root n x) = (if even n then ¦x¦ else x)"
using sgn_power_root[of n x] False
by (auto simp add: poly_monom sgn_if split: if_splits)
qed (use False assms in ‹auto simp: degree_monom_eq›)
qed auto
lemma algebraic_int_sqrt [intro]: "algebraic_int x ⟹ algebraic_int (sqrt x)"
by (auto simp: sqrt_def)
lemma algebraic_int_csqrt [intro]: "algebraic_int x ⟹ algebraic_int (csqrt x)"
by (rule algebraic_int_root[where p = "monom 1 2"])
(auto simp: poly_monom degree_monom_eq)
lemma algebraic_int_cnj [intro]:
assumes "algebraic_int x"
shows "algebraic_int (cnj x)"
proof -
from assms obtain p where p: "lead_coeff p = 1" "∀i. coeff p i ∈ ℤ" "poly p x = 0"
by (auto simp: algebraic_int.simps)
show ?thesis
proof
show "poly (map_poly cnj p) (cnj x) = 0"
using p by simp
show "lead_coeff (map_poly cnj p) = 1"
using p by (simp add: coeff_map_poly degree_map_poly)
show "∀i. coeff (map_poly cnj p) i ∈ ℤ"
using p by (auto simp: coeff_map_poly)
qed
qed
lemma algebraic_int_cnj_iff [simp]: "algebraic_int (cnj x) ⟷ algebraic_int x"
using algebraic_int_cnj[of x] algebraic_int_cnj[of "cnj x"] by auto
lemma algebraic_int_of_real [intro]:
assumes "algebraic_int x"
shows "algebraic_int (of_real x)"
proof -
from assms obtain p where p: "poly p x = 0" "∀i. coeff p i ∈ ℤ" "lead_coeff p = 1"
by (auto simp: algebraic_int.simps)
show "algebraic_int (of_real x :: 'a)"
proof
have "poly (map_poly of_real p) (of_real x) = (of_real (poly p x) :: 'a)"
by (induction p) (auto simp: map_poly_pCons)
thus "poly (map_poly of_real p) (of_real x) = (0 :: 'a)"
using p by simp
qed (use p in ‹auto simp: coeff_map_poly degree_map_poly›)
qed
lemma algebraic_int_of_real_iff [simp]:
"algebraic_int (of_real x :: 'a :: {field_char_0, real_algebra_1}) ⟷ algebraic_int x"
proof
assume "algebraic_int (of_real x :: 'a)"
then obtain p
where p: "poly (map_poly of_int p) (of_real x :: 'a) = 0" "lead_coeff p = 1"
by (auto simp: algebraic_int_altdef_ipoly)
show "algebraic_int x"
unfolding algebraic_int_altdef_ipoly
proof (intro exI[of _ p] conjI)
have "of_real (poly (map_poly real_of_int p) x) = poly (map_poly of_int p) (of_real x :: 'a)"
by (induction p) (auto simp: map_poly_pCons)
also note p(1)
finally show "poly (map_poly real_of_int p) x = 0" by simp
qed (use p in auto)
qed auto
subsection ‹Division of polynomials›
subsubsection ‹Division in general›
instantiation poly :: (idom_divide) idom_divide
begin
fun divide_poly_main :: "'a ⇒ 'a poly ⇒ 'a poly ⇒ 'a poly ⇒ nat ⇒ nat ⇒ 'a poly"
where
"divide_poly_main lc q r d dr (Suc n) =
(let cr = coeff r dr; a = cr div lc; mon = monom a n in
if False ∨ a * lc = cr then
divide_poly_main
lc
(q + mon)
(r - mon * d)
d (dr - 1) n else 0)"
| "divide_poly_main lc q r d dr 0 = q"
definition divide_poly :: "'a poly ⇒ 'a poly ⇒ 'a poly"
where "divide_poly f g =
(if g = 0 then 0
else
divide_poly_main (coeff g (degree g)) 0 f g (degree f)
(1 + length (coeffs f) - length (coeffs g)))"
lemma divide_poly_main:
assumes d: "d ≠ 0" "lc = coeff d (degree d)"
and "degree (d * r) ≤ dr" "divide_poly_main lc q (d * r) d dr n = q'"
and "n = 1 + dr - degree d ∨ dr = 0 ∧ n = 0 ∧ d * r = 0"
shows "q' = q + r"
using assms(3-)
proof (induct n arbitrary: q r dr)
case (Suc n)
let ?rr = "d * r"
let ?a = "coeff ?rr dr"
let ?qq = "?a div lc"
define b where [simp]: "b = monom ?qq n"
let ?rrr = "d * (r - b)"
let ?qqq = "q + b"
note res = Suc(3)
from Suc(4) have dr: "dr = n + degree d" by auto
from d have lc: "lc ≠ 0" by auto
have "coeff (b * d) dr = coeff b n * coeff d (degree d)"
proof (cases "?qq = 0")
case True
then show ?thesis by simp
next
case False
then have n: "n = degree b"
by (simp add: degree_monom_eq)
show ?thesis
unfolding n dr by (simp add: coeff_mult_degree_sum)
qed
also have "… = lc * coeff b n"
by (simp add: d)
finally have c2: "coeff (b * d) dr = lc * coeff b n" .
have rrr: "?rrr = ?rr - b * d"
by (simp add: field_simps)
have c1: "coeff (d * r) dr = lc * coeff r n"
proof (cases "degree r = n")
case True
with Suc(2) show ?thesis
unfolding dr using coeff_mult_degree_sum[of d r] d by (auto simp: ac_simps)
next
case False
from dr Suc(2) have "degree r ≤ n"
by auto
(metis add.commute add_le_cancel_left d(1) degree_0 degree_mult_eq
diff_is_0_eq diff_zero le_cases)
with False have r_n: "degree r < n"
by auto
then have right: "lc * coeff r n = 0"
by (simp add: coeff_eq_0)
have "coeff (d * r) dr = coeff (d * r) (degree d + n)"
by (simp add: dr ac_simps)
also from r_n have "… = 0"
by (metis False Suc.prems(1) add.commute add_left_imp_eq coeff_degree_mult coeff_eq_0
coeff_mult_degree_sum degree_mult_le dr le_eq_less_or_eq)
finally show ?thesis
by (simp only: right)
qed
have c0: "coeff ?rrr dr = 0"
and id: "lc * (coeff (d * r) dr div lc) = coeff (d * r) dr"
unfolding rrr coeff_diff c2
unfolding b_def coeff_monom coeff_smult c1 using lc by auto
from res[unfolded divide_poly_main.simps[of lc q] Let_def] id
have res: "divide_poly_main lc ?qqq ?rrr d (dr - 1) n = q'"
by (simp del: divide_poly_main.simps add: field_simps)
note IH = Suc(1)[OF _ res]
from Suc(4) have dr: "dr = n + degree d" by auto
from Suc(2) have deg_rr: "degree ?rr ≤ dr" by auto
have deg_bd: "degree (b * d) ≤ dr"
unfolding dr b_def by (rule order.trans[OF degree_mult_le]) (auto simp: degree_monom_le)
have "degree ?rrr ≤ dr"
unfolding rrr by (rule degree_diff_le[OF deg_rr deg_bd])
with c0 have deg_rrr: "degree ?rrr ≤ (dr - 1)"
by (rule coeff_0_degree_minus_1)
have "n = 1 + (dr - 1) - degree d ∨ dr - 1 = 0 ∧ n = 0 ∧ ?rrr = 0"
proof (cases dr)
case 0
with Suc(4) have 0: "dr = 0" "n = 0" "degree d = 0"
by auto
with deg_rrr have "degree ?rrr = 0"
by simp
from degree_eq_zeroE[OF this] obtain a where rrr: "?rrr = [:a:]"
by metis
show ?thesis
unfolding 0 using c0 unfolding rrr 0 by simp
next
case _: Suc
with Suc(4) show ?thesis by auto
qed
from IH[OF deg_rrr this] show ?case
by simp
next
case 0
show ?case
proof (cases "r = 0")
case True
with 0 show ?thesis by auto
next
case False
from d False have "degree (d * r) = degree d + degree r"
by (subst degree_mult_eq) auto
with 0 d show ?thesis by auto
qed
qed
lemma divide_poly_main_0: "divide_poly_main 0 0 r d dr n = 0"
proof (induct n arbitrary: r d dr)
case 0
then show ?case by simp
next
case Suc
show ?case
unfolding divide_poly_main.simps[of _ _ r] Let_def
by (simp add: Suc del: divide_poly_main.simps)
qed
lemma divide_poly:
assumes g: "g ≠ 0"
shows "(f * g) div g = (f :: 'a poly)"
proof -
have len: "length (coeffs f) = Suc (degree f)" if "f ≠ 0" for f :: "'a poly"
using that unfolding degree_eq_length_coeffs by auto
have "divide_poly_main (coeff g (degree g)) 0 (g * f) g (degree (g * f))
(1 + length (coeffs (g * f)) - length (coeffs g)) = (f * g) div g"
by (simp add: divide_poly_def Let_def ac_simps)
note main = divide_poly_main[OF g refl le_refl this]
have "(f * g) div g = 0 + f"
proof (rule main, goal_cases)
case 1
show ?case
proof (cases "f = 0")
case True
with g show ?thesis
by (auto simp: degree_eq_length_coeffs)
next
case False
with g have fg: "g * f ≠ 0" by auto
show ?thesis
unfolding len[OF fg] len[OF g] by auto
qed
qed
then show ?thesis by simp
qed
lemma divide_poly_0: "f div 0 = 0"
for f :: "'a poly"
by (simp add: divide_poly_def Let_def divide_poly_main_0)
instance
by standard (auto simp: divide_poly divide_poly_0)
end
instance poly :: (idom_divide) algebraic_semidom ..
lemma div_const_poly_conv_map_poly:
assumes "[:c:] dvd p"
shows "p div [:c:] = map_poly (λx. x div c) p"
proof (cases "c = 0")
case True
then show ?thesis
by (auto intro!: poly_eqI simp: coeff_map_poly)
next
case False
from assms obtain q where p: "p = [:c:] * q" by (rule dvdE)
moreover {
have "smult c q = [:c:] * q"
by simp
also have "… div [:c:] = q"
by (rule nonzero_mult_div_cancel_left) (use False in auto)
finally have "smult c q div [:c:] = q" .
}
ultimately show ?thesis by (intro poly_eqI) (auto simp: coeff_map_poly False)
qed
lemma is_unit_monom_0:
fixes a :: "'a::field"
assumes "a ≠ 0"
shows "is_unit (monom a 0)"
proof
from assms show "1 = monom a 0 * monom (inverse a) 0"
by (simp add: mult_monom)
qed
lemma is_unit_triv: "a ≠ 0 ⟹ is_unit [:a:]"
for a :: "'a::field"
by (simp add: is_unit_monom_0 monom_0 [symmetric])
lemma is_unit_iff_degree:
fixes p :: "'a::field poly"
assumes "p ≠ 0"
shows "is_unit p ⟷ degree p = 0"
(is "?lhs ⟷ ?rhs")
proof
assume ?rhs
then obtain a where "p = [:a:]"
by (rule degree_eq_zeroE)
with assms show ?lhs
by (simp add: is_unit_triv)
next
assume ?lhs
then obtain q where "q ≠ 0" "p * q = 1" ..
then have "degree (p * q) = degree 1"
by simp
with ‹p ≠ 0› ‹q ≠ 0› have "degree p + degree q = 0"
by (simp add: degree_mult_eq)
then show ?rhs by simp
qed
lemma is_unit_pCons_iff: "is_unit (pCons a p) ⟷ p = 0 ∧ a ≠ 0"
for p :: "'a::field poly"
by (cases "p = 0") (auto simp: is_unit_triv is_unit_iff_degree)
lemma is_unit_monom_trivial: "is_unit p ⟹ monom (coeff p (degree p)) 0 = p"
for p :: "'a::field poly"
by (cases p) (simp_all add: monom_0 is_unit_pCons_iff)
lemma is_unit_const_poly_iff: "[:c:] dvd 1 ⟷ c dvd 1"
for c :: "'a::{comm_semiring_1,semiring_no_zero_divisors}"
by (auto simp: one_pCons)
lemma is_unit_polyE:
fixes p :: "'a :: {comm_semiring_1,semiring_no_zero_divisors} poly"
assumes "p dvd 1"
obtains c where "p = [:c:]" "c dvd 1"
proof -
from assms obtain q where "1 = p * q"
by (rule dvdE)
then have "p ≠ 0" and "q ≠ 0"
by auto
from ‹1 = p * q› have "degree 1 = degree (p * q)"
by simp
also from ‹p ≠ 0› and ‹q ≠ 0› have "… = degree p + degree q"
by (simp add: degree_mult_eq)
finally have "degree p = 0" by simp
with degree_eq_zeroE obtain c where c: "p = [:c:]" .
with ‹p dvd 1› have "c dvd 1"
by (simp add: is_unit_const_poly_iff)
with c show thesis ..
qed
lemma is_unit_polyE':
fixes p :: "'a::field poly"
assumes "is_unit p"
obtains a where "p = monom a 0" and "a ≠ 0"
proof -
obtain a q where "p = pCons a q"
by (cases p)
with assms have "p = [:a:]" and "a ≠ 0"
by (simp_all add: is_unit_pCons_iff)
with that show thesis by (simp add: monom_0)
qed
lemma is_unit_poly_iff: "p dvd 1 ⟷ (∃c. p = [:c:] ∧ c dvd 1)"
for p :: "'a::{comm_semiring_1,semiring_no_zero_divisors} poly"
by (auto elim: is_unit_polyE simp add: is_unit_const_poly_iff)
lemma root_imp_reducible_poly:
fixes x :: "'a :: field"
assumes "poly p x = 0" and "degree p > 1"
shows "¬irreducible p"
proof -
from assms have "p ≠ 0"
by auto
define q where "q = [:-x, 1:]"
have "q dvd p"
using assms by (simp add: poly_eq_0_iff_dvd q_def)
then obtain r where p_eq: "p = q * r"
by (elim dvdE)
have [simp]: "q ≠ 0" "r ≠ 0"
using ‹p ≠ 0› by (auto simp: p_eq)
have "degree p = Suc (degree r)"
unfolding p_eq by (subst degree_mult_eq) (auto simp: q_def)
with assms(2) have "degree r > 0"
by auto
hence "¬r dvd 1"
by (auto simp: is_unit_poly_iff)
moreover have "¬q dvd 1"
by (auto simp: is_unit_poly_iff q_def)
ultimately show ?thesis using p_eq
by (auto simp: irreducible_def)
qed
lemma reducible_polyI:
fixes p :: "'a :: field poly"
assumes "p = q * r" "degree q > 0" "degree r > 0"
shows "¬irreducible p"
using assms unfolding irreducible_def
by (metis (no_types, opaque_lifting) is_unitE is_unit_iff_degree not_gr0)
subsubsection ‹Pseudo-Division›
text ‹This part is by René Thiemann and Akihisa Yamada.›
fun pseudo_divmod_main ::
"'a :: comm_ring_1 ⇒ 'a poly ⇒ 'a poly ⇒ 'a poly ⇒ nat ⇒ nat ⇒ 'a poly × 'a poly"
where
"pseudo_divmod_main lc q r d dr (Suc n) =
(let
rr = smult lc r;
qq = coeff r dr;
rrr = rr - monom qq n * d;
qqq = smult lc q + monom qq n
in pseudo_divmod_main lc qqq rrr d (dr - 1) n)"
| "pseudo_divmod_main lc q r d dr 0 = (q,r)"
definition pseudo_divmod :: "'a :: comm_ring_1 poly ⇒ 'a poly ⇒ 'a poly × 'a poly"
where "pseudo_divmod p q ≡
if q = 0 then (0, p)
else
pseudo_divmod_main (coeff q (degree q)) 0 p q (degree p)
(1 + length (coeffs p) - length (coeffs q))"
lemma pseudo_divmod_main:
assumes d: "d ≠ 0" "lc = coeff d (degree d)"
and "degree r ≤ dr" "pseudo_divmod_main lc q r d dr n = (q',r')"
and "n = 1 + dr - degree d ∨ dr = 0 ∧ n = 0 ∧ r = 0"
shows "(r' = 0 ∨ degree r' < degree d) ∧ smult (lc^n) (d * q + r) = d * q' + r'"
using assms(3-)
proof (induct n arbitrary: q r dr)
case 0
then show ?case by auto
next
case (Suc n)
let ?rr = "smult lc r"
let ?qq = "coeff r dr"
define b where [simp]: "b = monom ?qq n"
let ?rrr = "?rr - b * d"
let ?qqq = "smult lc q + b"
note res = Suc(3)
from res[unfolded pseudo_divmod_main.simps[of lc q] Let_def]
have res: "pseudo_divmod_main lc ?qqq ?rrr d (dr - 1) n = (q',r')"
by (simp del: pseudo_divmod_main.simps)
from Suc(4) have dr: "dr = n + degree d" by auto
have "coeff (b * d) dr = coeff b n * coeff d (degree d)"
proof (cases "?qq = 0")
case True
then show ?thesis by auto
next
case False
then have n: "n = degree b"
by (simp add: degree_monom_eq)
show ?thesis
unfolding n dr by (simp add: coeff_mult_degree_sum)
qed
also have "… = lc * coeff b n" by (simp add: d)
finally have "coeff (b * d) dr = lc * coeff b n" .
moreover have "coeff ?rr dr = lc * coeff r dr"
by simp
ultimately have c0: "coeff ?rrr dr = 0"
by auto
from Suc(4) have dr: "dr = n + degree d" by auto
have deg_rr: "degree ?rr ≤ dr"
using Suc(2) degree_smult_le dual_order.trans by blast
have deg_bd: "degree (b * d) ≤ dr"
unfolding dr by (rule order.trans[OF degree_mult_le]) (auto simp: degree_monom_le)
have "degree ?rrr ≤ dr"
using degree_diff_le[OF deg_rr deg_bd] by auto
with c0 have deg_rrr: "degree ?rrr ≤ (dr - 1)"
by (rule coeff_0_degree_minus_1)
have "n = 1 + (dr - 1) - degree d ∨ dr - 1 = 0 ∧ n = 0 ∧ ?rrr = 0"
proof (cases dr)
case 0
with Suc(4) have 0: "dr = 0" "n = 0" "degree d = 0" by auto
with deg_rrr have "degree ?rrr = 0" by simp
then have "∃a. ?rrr = [:a:]"
by (metis degree_pCons_eq_if old.nat.distinct(2) pCons_cases)
from this obtain a where rrr: "?rrr = [:a:]"
by auto
show ?thesis
unfolding 0 using c0 unfolding rrr 0 by simp
next
case _: Suc
with Suc(4) show ?thesis by auto
qed
note IH = Suc(1)[OF deg_rrr res this]
show ?case
proof (intro conjI)
from IH show "r' = 0 ∨ degree r' < degree d"
by blast
show "smult (lc ^ Suc n) (d * q + r) = d * q' + r'"
unfolding IH[THEN conjunct2,symmetric]
by (simp add: field_simps smult_add_right)
qed
qed
lemma pseudo_divmod:
assumes g: "g ≠ 0"
and *: "pseudo_divmod f g = (q,r)"
shows "smult (coeff g (degree g) ^ (Suc (degree f) - degree g)) f = g * q + r" (is ?A)
and "r = 0 ∨ degree r < degree g" (is ?B)
proof -
from *[unfolded pseudo_divmod_def Let_def]
have "pseudo_divmod_main (coeff g (degree g)) 0 f g (degree f)
(1 + length (coeffs f) - length (coeffs g)) = (q, r)"
by (auto simp: g)
note main = pseudo_divmod_main[OF _ _ _ this, OF g refl le_refl]
from g have "1 + length (coeffs f) - length (coeffs g) = 1 + degree f - degree g ∨
degree f = 0 ∧ 1 + length (coeffs f) - length (coeffs g) = 0 ∧ f = 0"
by (cases "f = 0"; cases "coeffs g") (auto simp: degree_eq_length_coeffs)
note main' = main[OF this]
then show "r = 0 ∨ degree r < degree g" by auto
show "smult (coeff g (degree g) ^ (Suc (degree f) - degree g)) f = g * q + r"
by (subst main'[THEN conjunct2, symmetric], simp add: degree_eq_length_coeffs,
cases "f = 0"; cases "coeffs g", use g in auto)
qed
definition "pseudo_mod_main lc r d dr n = snd (pseudo_divmod_main lc 0 r d dr n)"
lemma snd_pseudo_divmod_main:
"snd (pseudo_divmod_main lc q r d dr n) = snd (pseudo_divmod_main lc q' r d dr n)"
by (induct n arbitrary: q q' lc r d dr) (simp_all add: Let_def)
definition pseudo_mod :: "'a::{comm_ring_1,semiring_1_no_zero_divisors} poly ⇒ 'a poly ⇒ 'a poly"
where "pseudo_mod f g = snd (pseudo_divmod f g)"
lemma pseudo_mod:
fixes f g :: "'a::{comm_ring_1,semiring_1_no_zero_divisors} poly"
defines "r ≡ pseudo_mod f g"
assumes g: "g ≠ 0"
shows "∃a q. a ≠ 0 ∧ smult a f = g * q + r" "r = 0 ∨ degree r < degree g"
proof -
let ?cg = "coeff g (degree g)"
let ?cge = "?cg ^ (Suc (degree f) - degree g)"
define a where "a = ?cge"
from r_def[unfolded pseudo_mod_def] obtain q where pdm: "pseudo_divmod f g = (q, r)"
by (cases "pseudo_divmod f g") auto
from pseudo_divmod[OF g pdm] have id: "smult a f = g * q + r" and "r = 0 ∨ degree r < degree g"
by (auto simp: a_def)
show "r = 0 ∨ degree r < degree g" by fact
from g have "a ≠ 0"
by (auto simp: a_def)
with id show "∃a q. a ≠ 0 ∧ smult a f = g * q + r"
by auto
qed
lemma fst_pseudo_divmod_main_as_divide_poly_main:
assumes d: "d ≠ 0"
defines lc: "lc ≡ coeff d (degree d)"
shows "fst (pseudo_divmod_main lc q r d dr n) =
divide_poly_main lc (smult (lc^n) q) (smult (lc^n) r) d dr n"
proof (induct n arbitrary: q r dr)
case 0
then show ?case by simp
next
case (Suc n)
note lc0 = leading_coeff_neq_0[OF d, folded lc]
then have "pseudo_divmod_main lc q r d dr (Suc n) =
pseudo_divmod_main lc (smult lc q + monom (coeff r dr) n)
(smult lc r - monom (coeff r dr) n * d) d (dr - 1) n"
by (simp add: Let_def ac_simps)
also have "fst … = divide_poly_main lc
(smult (lc^n) (smult lc q + monom (coeff r dr) n))
(smult (lc^n) (smult lc r - monom (coeff r dr) n * d))
d (dr - 1) n"
by (simp only: Suc[unfolded divide_poly_main.simps Let_def])
also have "… = divide_poly_main lc (smult (lc ^ Suc n) q) (smult (lc ^ Suc n) r) d dr (Suc n)"
unfolding smult_monom smult_distribs mult_smult_left[symmetric]
using lc0 by (simp add: Let_def ac_simps)
finally show ?case .
qed
subsubsection ‹Division in polynomials over fields›
lemma pseudo_divmod_field:
fixes g :: "'a::field poly"
assumes g: "g ≠ 0"
and *: "pseudo_divmod f g = (q,r)"
defines "c ≡ coeff g (degree g) ^ (Suc (degree f) - degree g)"
shows "f = g * smult (1/c) q + smult (1/c) r"
proof -
from leading_coeff_neq_0[OF g] have c0: "c ≠ 0"
by (auto simp: c_def)
from pseudo_divmod(1)[OF g *, folded c_def] have "smult c f = g * q + r"
by auto
also have "smult (1 / c) … = g * smult (1 / c) q + smult (1 / c) r"
by (simp add: smult_add_right)
finally show ?thesis
using c0 by auto
qed
lemma divide_poly_main_field:
fixes d :: "'a::field poly"
assumes d: "d ≠ 0"
defines lc: "lc ≡ coeff d (degree d)"
shows "divide_poly_main lc q r d dr n =
fst (pseudo_divmod_main lc (smult ((1 / lc)^n) q) (smult ((1 / lc)^n) r) d dr n)"
unfolding lc by (subst fst_pseudo_divmod_main_as_divide_poly_main) (auto simp: d power_one_over)
lemma divide_poly_field:
fixes f g :: "'a::field poly"
defines "f' ≡ smult ((1 / coeff g (degree g)) ^ (Suc (degree f) - degree g)) f"
shows "f div g = fst (pseudo_divmod f' g)"
proof (cases "g = 0")
case True
show ?thesis
unfolding divide_poly_def pseudo_divmod_def Let_def f'_def True
by (simp add: divide_poly_main_0)
next
case False
from leading_coeff_neq_0[OF False] have "degree f' = degree f"
by (auto simp: f'_def)
then show ?thesis
using length_coeffs_degree[of f'] length_coeffs_degree[of f]
unfolding divide_poly_def pseudo_divmod_def Let_def
divide_poly_main_field[OF False]
length_coeffs_degree[OF False]
f'_def
by force
qed
instantiation poly :: ("{semidom_divide_unit_factor,idom_divide}") normalization_semidom
begin
definition unit_factor_poly :: "'a poly ⇒ 'a poly"
where "unit_factor_poly p = [:unit_factor (lead_coeff p):]"
definition normalize_poly :: "'a poly ⇒ 'a poly"
where "normalize p = p div [:unit_factor (lead_coeff p):]"
instance
proof
fix p :: "'a poly"
show "unit_factor p * normalize p = p"
proof (cases "p = 0")
case True
then show ?thesis
by (simp add: unit_factor_poly_def normalize_poly_def)
next
case False
then have "lead_coeff p ≠ 0"
by simp
then have *: "unit_factor (lead_coeff p) ≠ 0"
using unit_factor_is_unit [of "lead_coeff p"] by auto
then have "unit_factor (lead_coeff p) dvd 1"
by (auto intro: unit_factor_is_unit)
then have **: "unit_factor (lead_coeff p) dvd c" for c
by (rule dvd_trans) simp
have ***: "unit_factor (lead_coeff p) * (c div unit_factor (lead_coeff p)) = c" for c
proof -
from ** obtain b where "c = unit_factor (lead_coeff p) * b" ..
with False * show ?thesis by simp
qed
have "p div [:unit_factor (lead_coeff p):] =
map_poly (λc. c div unit_factor (lead_coeff p)) p"
by (simp add: const_poly_dvd_iff div_const_poly_conv_map_poly **)
then show ?thesis
by (simp add: normalize_poly_def unit_factor_poly_def
smult_conv_map_poly map_poly_map_poly o_def ***)
qed
next
fix p :: "'a poly"
assume "is_unit p"
then obtain c where p: "p = [:c:]" "c dvd 1"
by (auto simp: is_unit_poly_iff)
then show "unit_factor p = p"
by (simp add: unit_factor_poly_def monom_0 is_unit_unit_factor)
next
fix p :: "'a poly"
assume "p ≠ 0"
then show "is_unit (unit_factor p)"
by (simp add: unit_factor_poly_def monom_0 is_unit_poly_iff unit_factor_is_unit)
next
fix a b :: "'a poly" assume "is_unit a"
thus "unit_factor (a * b) = a * unit_factor b"
by (auto simp: unit_factor_poly_def lead_coeff_mult unit_factor_mult elim!: is_unit_polyE)
qed (simp_all add: normalize_poly_def unit_factor_poly_def monom_0 lead_coeff_mult unit_factor_mult)
end
instance poly :: ("{semidom_divide_unit_factor,idom_divide,normalization_semidom_multiplicative}")
normalization_semidom_multiplicative
by intro_classes (auto simp: unit_factor_poly_def lead_coeff_mult unit_factor_mult)
lemma normalize_poly_eq_map_poly: "normalize p = map_poly (λx. x div unit_factor (lead_coeff p)) p"
proof -
have "[:unit_factor (lead_coeff p):] dvd p"
by (metis unit_factor_poly_def unit_factor_self)
then show ?thesis
by (simp add: normalize_poly_def div_const_poly_conv_map_poly)
qed
lemma coeff_normalize [simp]:
"coeff (normalize p) n = coeff p n div unit_factor (lead_coeff p)"
by (simp add: normalize_poly_eq_map_poly coeff_map_poly)
class field_unit_factor = field + unit_factor +
assumes unit_factor_field [simp]: "unit_factor = id"
begin
subclass semidom_divide_unit_factor
proof
fix a
assume "a ≠ 0"
then have "1 = a * inverse a" by simp
then have "a dvd 1" ..
then show "unit_factor a dvd 1" by simp
qed simp_all
end
lemma unit_factor_pCons:
"unit_factor (pCons a p) = (if p = 0 then [:unit_factor a:] else unit_factor p)"
by (simp add: unit_factor_poly_def)
lemma normalize_monom [simp]: "normalize (monom a n) = monom (normalize a) n"
by (cases "a = 0") (simp_all add: map_poly_monom normalize_poly_eq_map_poly degree_monom_eq)
lemma unit_factor_monom [simp]: "unit_factor (monom a n) = [:unit_factor a:]"
by (cases "a = 0") (simp_all add: unit_factor_poly_def degree_monom_eq)
lemma normalize_const_poly: "normalize [:c:] = [:normalize c:]"
by (simp add: normalize_poly_eq_map_poly map_poly_pCons)
lemma normalize_smult:
fixes c :: "'a :: {normalization_semidom_multiplicative, idom_divide}"
shows "normalize (smult c p) = smult (normalize c) (normalize p)"
proof -
have "smult c p = [:c:] * p" by simp
also have "normalize … = smult (normalize c) (normalize p)"
by (subst normalize_mult) (simp add: normalize_const_poly)
finally show ?thesis .
qed
instantiation poly :: (field) idom_modulo
begin
definition modulo_poly :: "'a poly ⇒ 'a poly ⇒ 'a poly"
where mod_poly_def: "f mod g =
(if g = 0 then f else pseudo_mod (smult ((1 / lead_coeff g) ^ (Suc (degree f) - degree g)) f) g)"
instance
proof
fix x y :: "'a poly"
show "x div y * y + x mod y = x"
proof (cases "y = 0")
case True
then show ?thesis
by (simp add: divide_poly_0 mod_poly_def)
next
case False
then have "pseudo_divmod (smult ((1 / lead_coeff y) ^ (Suc (degree x) - degree y)) x) y =
(x div y, x mod y)"
by (simp add: divide_poly_field mod_poly_def pseudo_mod_def)
with False pseudo_divmod [OF False this] show ?thesis
by (simp add: power_mult_distrib [symmetric] ac_simps)
qed
qed
end
lemma pseudo_divmod_eq_div_mod:
‹pseudo_divmod f g = (f div g, f mod g)› if ‹lead_coeff g = 1›
using that by (auto simp add: divide_poly_field mod_poly_def pseudo_mod_def)
lemma degree_mod_less_degree:
‹degree (x mod y) < degree y› if ‹y ≠ 0› ‹¬ y dvd x›
proof -
from pseudo_mod(2) [of y] ‹y ≠ 0›
have *: ‹pseudo_mod f y ≠ 0 ⟹ degree (pseudo_mod f y) < degree y› for f
by blast
from ‹¬ y dvd x› have ‹x mod y ≠ 0›
by blast
with ‹y ≠ 0› show ?thesis
by (auto simp add: mod_poly_def intro: *)
qed
instantiation poly :: (field) unique_euclidean_ring
begin
definition euclidean_size_poly :: "'a poly ⇒ nat"
where "euclidean_size_poly p = (if p = 0 then 0 else 2 ^ degree p)"
definition division_segment_poly :: "'a poly ⇒ 'a poly"
where [simp]: "division_segment_poly p = 1"
instance proof
show ‹(q * p + r) div p = q› if ‹p ≠ 0›
and ‹euclidean_size r < euclidean_size p› for q p r :: ‹'a poly›
proof (cases ‹r = 0›)
case True
with that show ?thesis
by simp
next
case False
with ‹p ≠ 0› ‹euclidean_size r < euclidean_size p›
have ‹degree r < degree p›
by (simp add: euclidean_size_poly_def)
with ‹r ≠ 0› have ‹¬ p dvd r›
by (auto dest: dvd_imp_degree)
have ‹(q * p + r) div p = q ∧ (q * p + r) mod p = r›
proof (rule ccontr)
assume ‹¬ ?thesis›
moreover have *: ‹((q * p + r) div p - q) * p = r - (q * p + r) mod p›
by (simp add: algebra_simps)
ultimately have ‹(q * p + r) div p ≠ q› and ‹(q * p + r) mod p ≠ r›
using ‹p ≠ 0› by auto
from ‹¬ p dvd r› have ‹¬ p dvd (q * p + r)›
by simp
with ‹p ≠ 0› have ‹degree ((q * p + r) mod p) < degree p›
by (rule degree_mod_less_degree)
with ‹degree r < degree p› ‹(q * p + r) mod p ≠ r›
have ‹degree (r - (q * p + r) mod p) < degree p›
by (auto intro: degree_diff_less)
also have ‹degree p ≤ degree ((q * p + r) div p - q) + degree p›
by simp
also from ‹(q * p + r) div p ≠ q› ‹p ≠ 0›
have ‹… = degree (((q * p + r) div p - q) * p)›
by (simp add: degree_mult_eq)
also from * have ‹… = degree (r - (q * p + r) mod p)›
by simp
finally have ‹degree (r - (q * p + r) mod p) < degree (r - (q * p + r) mod p)› .
then show False
by simp
qed
then show ‹(q * p + r) div p = q› ..
qed
qed (auto simp: euclidean_size_poly_def degree_mult_eq power_add intro: degree_mod_less_degree)
end
lemma euclidean_relation_polyI [case_names by0 divides euclidean_relation]:
‹(x div y, x mod y) = (q, r)›
if by0: ‹y = 0 ⟹ q = 0 ∧ r = x›
and divides: ‹y ≠ 0 ⟹ y dvd x ⟹ r = 0 ∧ x = q * y›
and euclidean_relation: ‹y ≠ 0 ⟹ ¬ y dvd x ⟹ degree r < degree y ∧ x = q * y + r›
by (rule euclidean_relationI)
(use that in ‹simp_all add: euclidean_size_poly_def›)
lemma div_poly_eq_0_iff:
‹x div y = 0 ⟷ x = 0 ∨ y = 0 ∨ degree x < degree y› for x y :: ‹'a::field poly›
by (simp add: unique_euclidean_semiring_class.div_eq_0_iff euclidean_size_poly_def)
lemma div_poly_less:
‹x div y = 0› if ‹degree x < degree y› for x y :: ‹'a::field poly›
using that by (simp add: div_poly_eq_0_iff)
lemma mod_poly_less:
‹x mod y = x› if ‹degree x < degree y›
using that by (simp add: mod_eq_self_iff_div_eq_0 div_poly_eq_0_iff)
lemma degree_div_less:
‹degree (x div y) < degree x›
if ‹degree x > 0› ‹degree y > 0›
for x y :: ‹'a::field poly›
proof (cases ‹x div y = 0›)
case True
with ‹degree x > 0› show ?thesis
by simp
next
case False
from that have ‹x ≠ 0› ‹y ≠ 0›
and *: ‹degree (x div y * y + x mod y) > 0›
by auto
show ?thesis
proof (cases ‹y dvd x›)
case True
then obtain z where ‹x = y * z› ..
then have ‹degree (x div y) < degree (x div y * y)›
using ‹y ≠ 0› ‹x ≠ 0› ‹degree y > 0› by (simp add: degree_mult_eq)
with ‹y dvd x› show ?thesis
by simp
next
case False
with ‹y ≠ 0› have ‹degree (x mod y) < degree y›
by (rule degree_mod_less_degree)
with ‹y ≠ 0› ‹x div y ≠ 0› have ‹degree (x mod y) < degree (x div y * y)›
by (simp add: degree_mult_eq)
then have ‹degree (x div y * y + x mod y) = degree (x div y * y)›
by (rule degree_add_eq_left)
with ‹y ≠ 0› ‹x div y ≠ 0› ‹degree y > 0› show ?thesis
by (simp add: degree_mult_eq)
qed
qed
lemma degree_mod_less': "b ≠ 0 ⟹ a mod b ≠ 0 ⟹ degree (a mod b) < degree b"
by (rule degree_mod_less_degree) auto
lemma degree_mod_less: "y ≠ 0 ⟹ x mod y = 0 ∨ degree (x mod y) < degree y"
using degree_mod_less' by blast
lemma div_smult_left: ‹smult a x div y = smult a (x div y)› (is ?Q)
and mod_smult_left: ‹smult a x mod y = smult a (x mod y)› (is ?R)
for x y :: ‹'a::field poly›
proof -
have ‹(smult a x div y, smult a x mod y) = (smult a (x div y), smult a (x mod y))›
proof (cases ‹a = 0›)
case True
then show ?thesis
by simp
next
case False
show ?thesis
by (rule euclidean_relation_polyI)
(use False in ‹simp_all add: dvd_smult_iff degree_mod_less_degree flip: smult_add_right›)
qed
then show ?Q and ?R
by simp_all
qed
lemma poly_div_minus_left [simp]: "(- x) div y = - (x div y)"
for x y :: "'a::field poly"
using div_smult_left [of "- 1::'a"] by simp
lemma poly_mod_minus_left [simp]: "(- x) mod y = - (x mod y)"
for x y :: "'a::field poly"
using mod_smult_left [of "- 1::'a"] by simp
lemma poly_div_add_left: ‹(x + y) div z = x div z + y div z› (is ?Q)
and poly_mod_add_left: ‹(x + y) mod z = x mod z + y mod z› (is ?R)
for x y z :: ‹'a::field poly›
proof -
have ‹((x + y) div z, (x + y) mod z) = (x div z + y div z, x mod z + y mod z)›
proof (induction rule: euclidean_relation_polyI)
case by0
then show ?case by simp
next
case divides
then obtain w where ‹x + y = z * w›
by blast
then have y: ‹y = z * w - x›
by (simp add: algebra_simps)
from ‹z ≠ 0› show ?case
using mod_mult_self4 [of z w ‹- x›] div_mult_self4 [of z w ‹- x›]
by (simp add: algebra_simps y)
next
case euclidean_relation
then have ‹degree (x mod z + y mod z) < degree z›
using degree_mod_less_degree [of z x] degree_mod_less_degree [of z y]
dvd_add_right_iff [of z x y] dvd_add_left_iff [of z y x]
by (cases ‹z dvd x ∨ z dvd y›) (auto intro: degree_add_less)
moreover have ‹x + y = (x div z + y div z) * z + (x mod z + y mod z)›
by (simp add: algebra_simps)
ultimately show ?case
by simp
qed
then show ?Q and ?R
by simp_all
qed
lemma poly_div_diff_left: "(x - y) div z = x div z - y div z"
for x y z :: "'a::field poly"
by (simp only: diff_conv_add_uminus poly_div_add_left poly_div_minus_left)
lemma poly_mod_diff_left: "(x - y) mod z = x mod z - y mod z"
for x y z :: "'a::field poly"
by (simp only: diff_conv_add_uminus poly_mod_add_left poly_mod_minus_left)
lemma div_smult_right: ‹x div smult a y = smult (inverse a) (x div y)› (is ?Q)
and mod_smult_right: ‹x mod smult a y = (if a = 0 then x else x mod y)› (is ?R)
proof -
have ‹(x div smult a y, x mod smult a y) = (smult (inverse a) (x div y), (if a = 0 then x else x mod y))›
proof (induction rule: euclidean_relation_polyI)
case by0
then show ?case by auto
next
case divides
moreover define w where ‹w = x div y›
ultimately have ‹x = y * w›
by (simp add: smult_dvd_iff)
with divides show ?case
by simp
next
case euclidean_relation
then show ?case
by (simp add: smult_dvd_iff degree_mod_less_degree)
qed
then show ?Q and ?R
by simp_all
qed
lemma mod_mult_unit_eq:
‹x mod (z * y) = x mod y›
if ‹is_unit z›
for x y z :: ‹'a::field poly›
proof (cases ‹y = 0›)
case True
then show ?thesis
by simp
next
case False
moreover have ‹z ≠ 0›
using that by auto
moreover define a where ‹a = lead_coeff z›
ultimately have ‹z = [:a:]› ‹a ≠ 0›
using that monom_0 [of a] by (simp_all add: is_unit_monom_trivial)
then show ?thesis
by (simp add: mod_smult_right)
qed
lemma poly_div_minus_right [simp]: "x div (- y) = - (x div y)"
for x y :: "'a::field poly"
using div_smult_right [of _ "- 1::'a"] by (simp add: nonzero_inverse_minus_eq)
lemma poly_mod_minus_right [simp]: "x mod (- y) = x mod y"
for x y :: "'a::field poly"
using mod_smult_right [of _ "- 1::'a"] by simp
lemma poly_div_mult_right: ‹x div (y * z) = (x div y) div z› (is ?Q)
and poly_mod_mult_right: ‹x mod (y * z) = y * (x div y mod z) + x mod y› (is ?R)
for x y z :: ‹'a::field poly›
proof -
have ‹(x div (y * z), x mod (y * z)) = ((x div y) div z, y * (x div y mod z) + x mod y)›
proof (induction rule: euclidean_relation_polyI)
case by0
then show ?case by auto
next
case divides
then show ?case by auto
next
case euclidean_relation
then have ‹y ≠ 0› ‹z ≠ 0›
by simp_all
with ‹¬ y * z dvd x› have ‹degree (y * (x div y mod z) + x mod y) < degree (y * z)›
using degree_mod_less_degree [of y x] degree_mod_less_degree [of z ‹x div y›]
degree_add_eq_left [of ‹x mod y› ‹y * (x div y mod z)›]
by (cases ‹z dvd x div y›; cases ‹y dvd x›)
(auto simp add: degree_mult_eq not_dvd_imp_mod_neq_0 dvd_div_iff_mult)
moreover have ‹x = x div y div z * (y * z) + (y * (x div y mod z) + x mod y)›
by (simp add: field_simps flip: distrib_left)
ultimately show ?case
by simp
qed
then show ?Q and ?R
by simp_all
qed
lemma dvd_pCons_imp_dvd_pCons_mod:
‹y dvd pCons a (x mod y)› if ‹y dvd pCons a x›
proof -
have ‹pCons a x = pCons a (x div y * y + x mod y)›
by simp
also have ‹… = pCons 0 (x div y * y) + pCons a (x mod y)›
by simp
also have ‹pCons 0 (x div y * y) = (x div y * monom 1 (Suc 0)) * y›
by (simp add: monom_Suc)
finally show ‹y dvd pCons a (x mod y)›
using ‹y dvd pCons a x› by simp
qed
lemma degree_less_if_less_eqI:
‹degree x < degree y› if ‹degree x ≤ degree y› ‹coeff x (degree y) = 0› ‹x ≠ 0›
proof (cases ‹degree x = degree y›)
case True
with ‹coeff x (degree y) = 0› have ‹lead_coeff x = 0›
by simp
then have ‹x = 0›
by simp
with ‹x ≠ 0› show ?thesis
by simp
next
case False
with ‹degree x ≤ degree y› show ?thesis
by simp
qed
lemma div_pCons_eq:
‹pCons a p div q = (if q = 0 then 0 else pCons (coeff (pCons a (p mod q)) (degree q) / lead_coeff q) (p div q))› (is ?Q)
and mod_pCons_eq:
‹pCons a p mod q = (if q = 0 then pCons a p else pCons a (p mod q) - smult (coeff (pCons a (p mod q)) (degree q) / lead_coeff q) q)› (is ?R)
for x y :: ‹'a::field poly›
proof -
have ‹?Q› and ‹?R› if ‹q = 0›
using that by simp_all
moreover have ‹?Q› and ‹?R› if ‹q ≠ 0›
proof -
define b where ‹b = coeff (pCons a (p mod q)) (degree q) / lead_coeff q›
have ‹(pCons a p div q, pCons a p mod q) =
(pCons b (p div q), (pCons a (p mod q) - smult b q))› (is ‹_ = (?q, ?r)›)
proof (induction rule: euclidean_relation_polyI)
case by0
with ‹q ≠ 0› show ?case by simp
next
case divides
show ?case
proof (cases ‹pCons a (p mod q) = 0›)
case True
then show ?thesis
by (auto simp add: b_def)
next
case False
have ‹q dvd pCons a (p mod q)›
using ‹q dvd pCons a p› by (rule dvd_pCons_imp_dvd_pCons_mod)
then obtain s where *: ‹pCons a (p mod q) = q * s› ..
with False have ‹s ≠ 0›
by auto
from ‹q ≠ 0› have ‹degree (pCons a (p mod q)) ≤ degree q›
by (auto simp add: Suc_le_eq intro: degree_mod_less_degree)
moreover from ‹s ≠ 0› have ‹degree q ≤ degree (pCons a (p mod q))›
by (simp add: degree_mult_right_le *)
ultimately have ‹degree (pCons a (p mod q)) = degree q›
by (rule order.antisym)
with ‹s ≠ 0› ‹q ≠ 0› have ‹degree s = 0›
by (simp add: * degree_mult_eq)
then obtain c where ‹s = [:c:]›
by (rule degree_eq_zeroE)
also have ‹c = b›
using ‹q ≠ 0› by (simp add: b_def * ‹s = [:c:]›)
finally have ‹smult b q = pCons a (p mod q)›
by (simp add: *)
then show ?thesis
by simp
qed
next
case euclidean_relation
then have ‹degree q > 0›
using is_unit_iff_degree by blast
from ‹q ≠ 0› have ‹degree (pCons a (p mod q)) ≤ degree q›
by (auto simp add: Suc_le_eq intro: degree_mod_less_degree)
moreover have ‹degree (smult b q) ≤ degree q›
by (rule degree_smult_le)
ultimately have ‹degree (pCons a (p mod q) - smult b q) ≤ degree q›
by (rule degree_diff_le)
moreover have ‹coeff (pCons a (p mod q) - smult b q) (degree q) = 0›
using ‹degree q > 0› by (auto simp add: b_def)
ultimately have ‹degree (pCons a (p mod q) - smult b q) < degree q›
using ‹degree q > 0›
by (cases ‹pCons a (p mod q) = smult b q›)
(auto intro: degree_less_if_less_eqI)
then show ?case
by simp
qed
with ‹q ≠ 0› show ?Q and ?R
by (simp_all add: b_def)
qed
ultimately show ?Q and ?R
by simp_all
qed
lemma div_mod_fold_coeffs:
"(p div q, p mod q) =
(if q = 0 then (0, p)
else
fold_coeffs
(λa (s, r).
let b = coeff (pCons a r) (degree q) / coeff q (degree q)
in (pCons b s, pCons a r - smult b q)) p (0, 0))"
by (rule sym, induct p) (auto simp: div_pCons_eq mod_pCons_eq Let_def)
lemma mod_pCons:
fixes a :: "'a::field"
and x y :: "'a::field poly"
assumes y: "y ≠ 0"
defines "b ≡ coeff (pCons a (x mod y)) (degree y) / coeff y (degree y)"
shows "(pCons a x) mod y = pCons a (x mod y) - smult b y"
unfolding b_def
by (simp add: mod_pCons_eq)
subsubsection ‹List-based versions for fast implementation›
fun minus_poly_rev_list :: "'a :: group_add list ⇒ 'a list ⇒ 'a list"
where
"minus_poly_rev_list (x # xs) (y # ys) = (x - y) # (minus_poly_rev_list xs ys)"
| "minus_poly_rev_list xs [] = xs"
| "minus_poly_rev_list [] (y # ys) = []"
fun pseudo_divmod_main_list ::
"'a::comm_ring_1 ⇒ 'a list ⇒ 'a list ⇒ 'a list ⇒ nat ⇒ 'a list × 'a list"
where
"pseudo_divmod_main_list lc q r d (Suc n) =
(let
rr = map ((*) lc) r;
a = hd r;
qqq = cCons a (map ((*) lc) q);
rrr = tl (if a = 0 then rr else minus_poly_rev_list rr (map ((*) a) d))
in pseudo_divmod_main_list lc qqq rrr d n)"
| "pseudo_divmod_main_list lc q r d 0 = (q, r)"
fun pseudo_mod_main_list :: "'a::comm_ring_1 ⇒ 'a list ⇒ 'a list ⇒ nat ⇒ 'a list"
where
"pseudo_mod_main_list lc r d (Suc n) =
(let
rr = map ((*) lc) r;
a = hd r;
rrr = tl (if a = 0 then rr else minus_poly_rev_list rr (map ((*) a) d))
in pseudo_mod_main_list lc rrr d n)"
| "pseudo_mod_main_list lc r d 0 = r"
fun divmod_poly_one_main_list ::
"'a::comm_ring_1 list ⇒ 'a list ⇒ 'a list ⇒ nat ⇒ 'a list × 'a list"
where
"divmod_poly_one_main_list q r d (Suc n) =
(let
a = hd r;
qqq = cCons a q;
rr = tl (if a = 0 then r else minus_poly_rev_list r (map ((*) a) d))
in divmod_poly_one_main_list qqq rr d n)"
| "divmod_poly_one_main_list q r d 0 = (q, r)"
fun mod_poly_one_main_list :: "'a::comm_ring_1 list ⇒ 'a list ⇒ nat ⇒ 'a list"
where
"mod_poly_one_main_list r d (Suc n) =
(let
a = hd r;
rr = tl (if a = 0 then r else minus_poly_rev_list r (map ((*) a) d))
in mod_poly_one_main_list rr d n)"
| "mod_poly_one_main_list r d 0 = r"
definition pseudo_divmod_list :: "'a::comm_ring_1 list ⇒ 'a list ⇒ 'a list × 'a list"
where "pseudo_divmod_list p q =
(if q = [] then ([], p)
else
(let rq = rev q;
(qu,re) = pseudo_divmod_main_list (hd rq) [] (rev p) rq (1 + length p - length q)
in (qu, rev re)))"
definition pseudo_mod_list :: "'a::comm_ring_1 list ⇒ 'a list ⇒ 'a list"
where "pseudo_mod_list p q =
(if q = [] then p
else
(let
rq = rev q;
re = pseudo_mod_main_list (hd rq) (rev p) rq (1 + length p - length q)
in rev re))"
lemma minus_zero_does_nothing: "minus_poly_rev_list x (map ((*) 0) y) = x"
for x :: "'a::ring list"
by (induct x y rule: minus_poly_rev_list.induct) auto
lemma length_minus_poly_rev_list [simp]: "length (minus_poly_rev_list xs ys) = length xs"
by (induct xs ys rule: minus_poly_rev_list.induct) auto
lemma if_0_minus_poly_rev_list:
"(if a = 0 then x else minus_poly_rev_list x (map ((*) a) y)) =
minus_poly_rev_list x (map ((*) a) y)"
for a :: "'a::ring"
by(cases "a = 0") (simp_all add: minus_zero_does_nothing)
lemma Poly_append: "Poly (a @ b) = Poly a + monom 1 (length a) * Poly b"
for a :: "'a::comm_semiring_1 list"
by (induct a) (auto simp: monom_0 monom_Suc)
lemma minus_poly_rev_list: "length p ≥ length q ⟹
Poly (rev (minus_poly_rev_list (rev p) (rev q))) =
Poly p - monom 1 (length p - length q) * Poly q"
for p q :: "'a :: comm_ring_1 list"
proof (induct "rev p" "rev q" arbitrary: p q rule: minus_poly_rev_list.induct)
case (1 x xs y ys)
then have "length (rev q) ≤ length (rev p)"
by simp
from this[folded 1(2,3)] have ys_xs: "length ys ≤ length xs"
by simp
then have *: "Poly (rev (minus_poly_rev_list xs ys)) =
Poly (rev xs) - monom 1 (length xs - length ys) * Poly (rev ys)"
by (subst "1.hyps"(1)[of "rev xs" "rev ys", unfolded rev_rev_ident length_rev]) auto
have "Poly p - monom 1 (length p - length q) * Poly q =
Poly (rev (rev p)) - monom 1 (length (rev (rev p)) - length (rev (rev q))) * Poly (rev (rev q))"
by simp
also have "… =
Poly (rev (x # xs)) - monom 1 (length (x # xs) - length (y # ys)) * Poly (rev (y # ys))"
unfolding 1(2,3) by simp
also from ys_xs have "… =
Poly (rev xs) + monom x (length xs) -
(monom 1 (length xs - length ys) * Poly (rev ys) + monom y (length xs))"
by (simp add: Poly_append distrib_left mult_monom smult_monom)
also have "… = Poly (rev (minus_poly_rev_list xs ys)) + monom (x - y) (length xs)"
unfolding * diff_monom[symmetric] by simp
finally show ?case
by (simp add: 1(2,3)[symmetric] smult_monom Poly_append)
qed auto
lemma smult_monom_mult: "smult a (monom b n * f) = monom (a * b) n * f"
using smult_monom [of a _ n] by (metis mult_smult_left)
lemma head_minus_poly_rev_list:
"length d ≤ length r ⟹ d ≠ [] ⟹
hd (minus_poly_rev_list (map ((*) (last d)) r) (map ((*) (hd r)) (rev d))) = 0"
for d r :: "'a::comm_ring list"
proof (induct r)
case Nil
then show ?case by simp
next
case (Cons a rs)
then show ?case by (cases "rev d") (simp_all add: ac_simps)
qed
lemma Poly_map: "Poly (map ((*) a) p) = smult a (Poly p)"
proof (induct p)
case Nil
then show ?case by simp
next
case (Cons x xs)
then show ?case by (cases "Poly xs = 0") auto
qed
lemma last_coeff_is_hd: "xs ≠ [] ⟹ coeff (Poly xs) (length xs - 1) = hd (rev xs)"
by (simp_all add: hd_conv_nth rev_nth nth_default_nth nth_append)
lemma pseudo_divmod_main_list_invar:
assumes leading_nonzero: "last d ≠ 0"
and lc: "last d = lc"
and "d ≠ []"
and "pseudo_divmod_main_list lc q (rev r) (rev d) n = (q', rev r')"
and "n = 1 + length r - length d"
shows "pseudo_divmod_main lc (monom 1 n * Poly q) (Poly r) (Poly d) (length r - 1) n =
(Poly q', Poly r')"
using assms(4-)
proof (induct n arbitrary: r q)
case (Suc n)
from Suc.prems have *: "¬ Suc (length r) ≤ length d"
by simp
with ‹d ≠ []› have "r ≠ []"
using Suc_leI length_greater_0_conv list.size(3) by fastforce
let ?a = "(hd (rev r))"
let ?rr = "map ((*) lc) (rev r)"
let ?rrr = "rev (tl (minus_poly_rev_list ?rr (map ((*) ?a) (rev d))))"
let ?qq = "cCons ?a (map ((*) lc) q)"
from * Suc(3) have n: "n = (1 + length r - length d - 1)"
by simp
from * have rr_val:"(length ?rrr) = (length r - 1)"
by auto
with ‹r ≠ []› * have rr_smaller: "(1 + length r - length d - 1) = (1 + length ?rrr - length d)"
by auto
from * have id: "Suc (length r) - length d = Suc (length r - length d)"
by auto
from Suc.prems *
have "pseudo_divmod_main_list lc ?qq (rev ?rrr) (rev d) (1 + length r - length d - 1) = (q', rev r')"
by (simp add: Let_def if_0_minus_poly_rev_list id)
with n have v: "pseudo_divmod_main_list lc ?qq (rev ?rrr) (rev d) n = (q', rev r')"
by auto
from * have sucrr:"Suc (length r) - length d = Suc (length r - length d)"
using Suc_diff_le not_less_eq_eq by blast
from Suc(3) ‹r ≠ []› have n_ok : "n = 1 + (length ?rrr) - length d"
by simp
have cong: "⋀x1 x2 x3 x4 y1 y2 y3 y4. x1 = y1 ⟹ x2 = y2 ⟹ x3 = y3 ⟹ x4 = y4 ⟹
pseudo_divmod_main lc x1 x2 x3 x4 n = pseudo_divmod_main lc y1 y2 y3 y4 n"
by simp
have hd_rev: "coeff (Poly r) (length r - Suc 0) = hd (rev r)"
using last_coeff_is_hd[OF ‹r ≠ []›] by simp
show ?case
unfolding Suc.hyps(1)[OF v n_ok, symmetric] pseudo_divmod_main.simps Let_def
proof (rule cong[OF _ _ refl], goal_cases)
case 1
show ?case
by (simp add: monom_Suc hd_rev[symmetric] smult_monom Poly_map)
next
case 2
show ?case
proof (subst Poly_on_rev_starting_with_0, goal_cases)
show "hd (minus_poly_rev_list (map ((*) lc) (rev r)) (map ((*) (hd (rev r))) (rev d))) = 0"
by (fold lc, subst head_minus_poly_rev_list, insert * ‹d ≠ []›, auto)
from * have "length d ≤ length r"
by simp
then show "smult lc (Poly r) - monom (coeff (Poly r) (length r - 1)) n * Poly d =
Poly (rev (minus_poly_rev_list (map ((*) lc) (rev r)) (map ((*) (hd (rev r))) (rev d))))"
by (fold rev_map) (auto simp add: n smult_monom_mult Poly_map hd_rev [symmetric]
minus_poly_rev_list)
qed
qed simp
qed simp
lemma pseudo_divmod_impl [code]:
"pseudo_divmod f g = map_prod poly_of_list poly_of_list (pseudo_divmod_list (coeffs f) (coeffs g))"
for f g :: "'a::comm_ring_1 poly"
proof (cases "g = 0")
case False
then have "last (coeffs g) ≠ 0"
and "last (coeffs g) = lead_coeff g"
and "coeffs g ≠ []"
by (simp_all add: last_coeffs_eq_coeff_degree)
moreover obtain q r where qr: "pseudo_divmod_main_list
(last (coeffs g)) (rev [])
(rev (coeffs f)) (rev (coeffs g))
(1 + length (coeffs f) -
length (coeffs g)) = (q, rev (rev r))"
by force
ultimately have "(Poly q, Poly (rev r)) = pseudo_divmod_main (lead_coeff g) 0 f g
(length (coeffs f) - Suc 0) (Suc (length (coeffs f)) - length (coeffs g))"
by (subst pseudo_divmod_main_list_invar [symmetric]) auto
moreover have "pseudo_divmod_main_list
(hd (rev (coeffs g))) []
(rev (coeffs f)) (rev (coeffs g))
(1 + length (coeffs f) -
length (coeffs g)) = (q, r)"
by (metis hd_rev qr rev.simps(1) rev_swap)
ultimately show ?thesis
by (simp add: degree_eq_length_coeffs pseudo_divmod_def pseudo_divmod_list_def)
next
case True
then show ?thesis
by (auto simp add: pseudo_divmod_def pseudo_divmod_list_def)
qed
lemma pseudo_mod_main_list:
"snd (pseudo_divmod_main_list l q xs ys n) = pseudo_mod_main_list l xs ys n"
by (induct n arbitrary: l q xs ys) (auto simp: Let_def)
lemma pseudo_mod_impl[code]: "pseudo_mod f g = poly_of_list (pseudo_mod_list (coeffs f) (coeffs g))"
proof -
have snd_case: "⋀f g p. snd ((λ(x,y). (f x, g y)) p) = g (snd p)"
by auto
show ?thesis
unfolding pseudo_mod_def pseudo_divmod_impl pseudo_divmod_list_def
pseudo_mod_list_def Let_def
by (simp add: snd_case pseudo_mod_main_list)
qed
subsubsection ‹Improved Code-Equations for Polynomial (Pseudo) Division›
lemma pdivmod_via_pseudo_divmod:
‹(f div g, f mod g) =
(if g = 0 then (0, f)
else
let
ilc = inverse (lead_coeff g);
h = smult ilc g;
(q,r) = pseudo_divmod f h
in (smult ilc q, r))›
(is ‹?l = ?r›)
proof (cases ‹g = 0›)
case True
then show ?thesis by simp
next
case False
define ilc where ‹ilc = inverse (lead_coeff g)›
define h where ‹h = smult ilc g›
from False have ‹lead_coeff h = 1›
and ‹ilc ≠ 0›
by (auto simp: h_def ilc_def)
define q r where ‹q = f div h› and ‹r = f mod h›
with ‹lead_coeff h = 1› have p: ‹pseudo_divmod f h = (q, r)›
by (simp add: pseudo_divmod_eq_div_mod)
from ‹ilc ≠ 0› have ‹(f div g, f mod g) = (smult ilc q, r)›
by (auto simp: h_def div_smult_right mod_smult_right q_def r_def)
also have ‹(smult ilc q, r) = ?r›
using ‹g ≠ 0› by (auto simp: Let_def p simp flip: h_def ilc_def)
finally show ?thesis .
qed
lemma pdivmod_via_pseudo_divmod_list:
"(f div g, f mod g) =
(let cg = coeffs g in
if cg = [] then (0, f)
else
let
cf = coeffs f;
ilc = inverse (last cg);
ch = map ((*) ilc) cg;
(q, r) = pseudo_divmod_main_list 1 [] (rev cf) (rev ch) (1 + length cf - length cg)
in (poly_of_list (map ((*) ilc) q), poly_of_list (rev r)))"
proof -
note d = pdivmod_via_pseudo_divmod pseudo_divmod_impl pseudo_divmod_list_def
show ?thesis
proof (cases "g = 0")
case True
with d show ?thesis by auto
next
case False
define ilc where "ilc = inverse (coeff g (degree g))"
from False have ilc: "ilc ≠ 0"
by (auto simp: ilc_def)
with False have id: "g = 0 ⟷ False" "coeffs g = [] ⟷ False"
"last (coeffs g) = coeff g (degree g)"
"coeffs (smult ilc g) = [] ⟷ False"
by (auto simp: last_coeffs_eq_coeff_degree)
have id2: "hd (rev (coeffs (smult ilc g))) = 1"
by (subst hd_rev, insert id ilc, auto simp: coeffs_smult, subst last_map, auto simp: id ilc_def)
have id3: "length (coeffs (smult ilc g)) = length (coeffs g)"
"rev (coeffs (smult ilc g)) = rev (map ((*) ilc) (coeffs g))"
unfolding coeffs_smult using ilc by auto
obtain q r where pair:
"pseudo_divmod_main_list 1 [] (rev (coeffs f)) (rev (map ((*) ilc) (coeffs g)))
(1 + length (coeffs f) - length (coeffs g)) = (q, r)"
by force
show ?thesis
unfolding d Let_def id if_False ilc_def[symmetric] map_prod_def[symmetric] id2
unfolding id3 pair map_prod_def split
by (auto simp: Poly_map)
qed
qed
lemma pseudo_divmod_main_list_1: "pseudo_divmod_main_list 1 = divmod_poly_one_main_list"
proof (intro ext, goal_cases)
case (1 q r d n)
have *: "map ((*) 1) xs = xs" for xs :: "'a list"
by (induct xs) auto
show ?case
by (induct n arbitrary: q r d) (auto simp: * Let_def)
qed
fun divide_poly_main_list :: "'a::idom_divide ⇒ 'a list ⇒ 'a list ⇒ 'a list ⇒ nat ⇒ 'a list"
where
"divide_poly_main_list lc q r d (Suc n) =
(let
cr = hd r
in if cr = 0 then divide_poly_main_list lc (cCons cr q) (tl r) d n else let
a = cr div lc;
qq = cCons a q;
rr = minus_poly_rev_list r (map ((*) a) d)
in if hd rr = 0 then divide_poly_main_list lc qq (tl rr) d n else [])"
| "divide_poly_main_list lc q r d 0 = q"
lemma divide_poly_main_list_simp [simp]:
"divide_poly_main_list lc q r d (Suc n) =
(let
cr = hd r;
a = cr div lc;
qq = cCons a q;
rr = minus_poly_rev_list r (map ((*) a) d)
in if hd rr = 0 then divide_poly_main_list lc qq (tl rr) d n else [])"
by (simp add: Let_def minus_zero_does_nothing)
declare divide_poly_main_list.simps(1)[simp del]
definition divide_poly_list :: "'a::idom_divide poly ⇒ 'a poly ⇒ 'a poly"
where "divide_poly_list f g =
(let cg = coeffs g in
if cg = [] then g
else
let
cf = coeffs f;
cgr = rev cg
in poly_of_list (divide_poly_main_list (hd cgr) [] (rev cf) cgr (1 + length cf - length cg)))"
lemmas pdivmod_via_divmod_list = pdivmod_via_pseudo_divmod_list[unfolded pseudo_divmod_main_list_1]
lemma mod_poly_one_main_list: "snd (divmod_poly_one_main_list q r d n) = mod_poly_one_main_list r d n"
by (induct n arbitrary: q r d) (auto simp: Let_def)
lemma mod_poly_code [code]:
"f mod g =
(let cg = coeffs g in
if cg = [] then f
else
let
cf = coeffs f;
ilc = inverse (last cg);
ch = map ((*) ilc) cg;
r = mod_poly_one_main_list (rev cf) (rev ch) (1 + length cf - length cg)
in poly_of_list (rev r))"
(is "_ = ?rhs")
proof -
have "snd (f div g, f mod g) = ?rhs"
unfolding pdivmod_via_divmod_list Let_def mod_poly_one_main_list [symmetric, of _ _ _ Nil]
by (auto split: prod.splits)
then show ?thesis by simp
qed
definition div_field_poly_impl :: "'a :: field poly ⇒ 'a poly ⇒ 'a poly"
where "div_field_poly_impl f g =
(let cg = coeffs g in
if cg = [] then 0
else
let
cf = coeffs f;
ilc = inverse (last cg);
ch = map ((*) ilc) cg;
q = fst (divmod_poly_one_main_list [] (rev cf) (rev ch) (1 + length cf - length cg))
in poly_of_list ((map ((*) ilc) q)))"
text ‹We do not declare the following lemma as code equation, since then polynomial division
on non-fields will no longer be executable. However, a code-unfold is possible, since
‹div_field_poly_impl› is a bit more efficient than the generic polynomial division.›
lemma div_field_poly_impl[code_unfold]: "(div) = div_field_poly_impl"
proof (intro ext)
fix f g :: "'a poly"
have "fst (f div g, f mod g) = div_field_poly_impl f g"
unfolding div_field_poly_impl_def pdivmod_via_divmod_list Let_def
by (auto split: prod.splits)
then show "f div g = div_field_poly_impl f g"
by simp
qed
lemma divide_poly_main_list:
assumes lc0: "lc ≠ 0"
and lc: "last d = lc"
and d: "d ≠ []"
and "n = (1 + length r - length d)"
shows "Poly (divide_poly_main_list lc q (rev r) (rev d) n) =
divide_poly_main lc (monom 1 n * Poly q) (Poly r) (Poly d) (length r - 1) n"
using assms(4-)
proof (induct "n" arbitrary: r q)
case (Suc n)
from Suc.prems have ifCond: "¬ Suc (length r) ≤ length d"
by simp
with d have r: "r ≠ []"
using Suc_leI length_greater_0_conv list.size(3) by fastforce
then obtain rr lcr where r: "r = rr @ [lcr]"
by (cases r rule: rev_cases) auto
from d lc obtain dd where d: "d = dd @ [lc]"
by (cases d rule: rev_cases) auto
from Suc(2) ifCond have n: "n = 1 + length rr - length d"
by (auto simp: r)
from ifCond have len: "length dd ≤ length rr"
by (simp add: r d)
show ?case
proof (cases "lcr div lc * lc = lcr")
case False
with r d show ?thesis
unfolding Suc(2)[symmetric]
by (auto simp add: Let_def nth_default_append)
next
case True
with r d have id:
"?thesis ⟷
Poly (divide_poly_main_list lc (cCons (lcr div lc) q)
(rev (rev (minus_poly_rev_list (rev rr) (rev (map ((*) (lcr div lc)) dd))))) (rev d) n) =
divide_poly_main lc
(monom 1 (Suc n) * Poly q + monom (lcr div lc) n)
(Poly r - monom (lcr div lc) n * Poly d)
(Poly d) (length rr - 1) n"
by (cases r rule: rev_cases; cases "d" rule: rev_cases)
(auto simp add: Let_def rev_map nth_default_append)
have cong: "⋀x1 x2 x3 x4 y1 y2 y3 y4. x1 = y1 ⟹ x2 = y2 ⟹ x3 = y3 ⟹ x4 = y4 ⟹
divide_poly_main lc x1 x2 x3 x4 n = divide_poly_main lc y1 y2 y3 y4 n"
by simp
show ?thesis
unfolding id
proof (subst Suc(1), simp add: n,
subst minus_poly_rev_list, force simp: len, rule cong[OF _ _ refl], goal_cases)
case 2
have "monom lcr (length rr) = monom (lcr div lc) (length rr - length dd) * monom lc (length dd)"
by (simp add: mult_monom len True)
then show ?case unfolding r d Poly_append n ring_distribs
by (auto simp: Poly_map smult_monom smult_monom_mult)
qed (auto simp: len monom_Suc smult_monom)
qed
qed simp
lemma divide_poly_list[code]: "f div g = divide_poly_list f g"
proof -
note d = divide_poly_def divide_poly_list_def
show ?thesis
proof (cases "g = 0")
case True
show ?thesis by (auto simp: d True)
next
case False
then obtain cg lcg where cg: "coeffs g = cg @ [lcg]"
by (cases "coeffs g" rule: rev_cases) auto
with False have id: "(g = 0) = False" "(cg @ [lcg] = []) = False"
by auto
from cg False have lcg: "coeff g (degree g) = lcg"
using last_coeffs_eq_coeff_degree last_snoc by force
with False have "lcg ≠ 0" by auto
from cg Poly_coeffs [of g] have ltp: "Poly (cg @ [lcg]) = g"
by auto
show ?thesis
unfolding d cg Let_def id if_False poly_of_list_def
by (subst divide_poly_main_list, insert False cg ‹lcg ≠ 0›)
(auto simp: lcg ltp, simp add: degree_eq_length_coeffs)
qed
qed
subsection ‹Primality and irreducibility in polynomial rings›
lemma prod_mset_const_poly: "(∏x∈#A. [:f x:]) = [:prod_mset (image_mset f A):]"
by (induct A) (simp_all add: ac_simps)
lemma irreducible_const_poly_iff:
fixes c :: "'a :: {comm_semiring_1,semiring_no_zero_divisors}"
shows "irreducible [:c:] ⟷ irreducible c"
proof
assume A: "irreducible c"
show "irreducible [:c:]"
proof (rule irreducibleI)
fix a b assume ab: "[:c:] = a * b"
hence "degree [:c:] = degree (a * b)" by (simp only: )
also from A ab have "a ≠ 0" "b ≠ 0" by auto
hence "degree (a * b) = degree a + degree b" by (simp add: degree_mult_eq)
finally have "degree a = 0" "degree b = 0" by auto
then obtain a' b' where ab': "a = [:a':]" "b = [:b':]" by (auto elim!: degree_eq_zeroE)
from ab have "coeff [:c:] 0 = coeff (a * b) 0" by (simp only: )
hence "c = a' * b'" by (simp add: ab' mult_ac)
from A and this have "a' dvd 1 ∨ b' dvd 1" by (rule irreducibleD)
with ab' show "a dvd 1 ∨ b dvd 1"
by (auto simp add: is_unit_const_poly_iff)
qed (insert A, auto simp: irreducible_def is_unit_poly_iff)
next
assume A: "irreducible [:c:]"
then have "c ≠ 0" and "¬ c dvd 1"
by (auto simp add: irreducible_def is_unit_const_poly_iff)
then show "irreducible c"
proof (rule irreducibleI)
fix a b assume ab: "c = a * b"
hence "[:c:] = [:a:] * [:b:]" by (simp add: mult_ac)
from A and this have "[:a:] dvd 1 ∨ [:b:] dvd 1" by (rule irreducibleD)
then show "a dvd 1 ∨ b dvd 1"
by (auto simp add: is_unit_const_poly_iff)
qed
qed
lemma lift_prime_elem_poly:
assumes "prime_elem (c :: 'a :: semidom)"
shows "prime_elem [:c:]"
proof (rule prime_elemI)
fix a b assume *: "[:c:] dvd a * b"
from * have dvd: "c dvd coeff (a * b) n" for n
by (subst (asm) const_poly_dvd_iff) blast
{
define m where "m = (GREATEST m. ¬c dvd coeff b m)"
assume "¬[:c:] dvd b"
hence A: "∃i. ¬c dvd coeff b i" by (subst (asm) const_poly_dvd_iff) blast
have B: "⋀i. ¬c dvd coeff b i ⟹ i ≤ degree b"
by (auto intro: le_degree)
have coeff_m: "¬c dvd coeff b m" unfolding m_def by (rule GreatestI_ex_nat[OF A B])
have "i ≤ m" if "¬c dvd coeff b i" for i
unfolding m_def by (metis (mono_tags, lifting) B Greatest_le_nat that)
hence dvd_b: "c dvd coeff b i" if "i > m" for i using that by force
have "c dvd coeff a i" for i
proof (induction i rule: nat_descend_induct[of "degree a"])
case (base i)
thus ?case by (simp add: coeff_eq_0)
next
case (descend i)
let ?A = "{..i+m} - {i}"
have "c dvd coeff (a * b) (i + m)" by (rule dvd)
also have "coeff (a * b) (i + m) = (∑k≤i + m. coeff a k * coeff b (i + m - k))"
by (simp add: coeff_mult)
also have "{..i+m} = insert i ?A" by auto
also have "(∑k∈…. coeff a k * coeff b (i + m - k)) =
coeff a i * coeff b m + (∑k∈?A. coeff a k * coeff b (i + m - k))"
(is "_ = _ + ?S")
by (subst sum.insert) simp_all
finally have eq: "c dvd coeff a i * coeff b m + ?S" .
moreover have "c dvd ?S"
proof (rule dvd_sum)
fix k assume k: "k ∈ {..i+m} - {i}"
show "c dvd coeff a k * coeff b (i + m - k)"
proof (cases "k < i")
case False
with k have "c dvd coeff a k" by (intro descend.IH) simp
thus ?thesis by simp
next
case True
hence "c dvd coeff b (i + m - k)" by (intro dvd_b) simp
thus ?thesis by simp
qed
qed
ultimately have "c dvd coeff a i * coeff b m"
by (simp add: dvd_add_left_iff)
with assms coeff_m show "c dvd coeff a i"
by (simp add: prime_elem_dvd_mult_iff)
qed
hence "[:c:] dvd a" by (subst const_poly_dvd_iff) blast
}
then show "[:c:] dvd a ∨ [:c:] dvd b" by blast
next
from assms show "[:c:] ≠ 0" and "¬ [:c:] dvd 1"
by (simp_all add: prime_elem_def is_unit_const_poly_iff)
qed
lemma prime_elem_const_poly_iff:
fixes c :: "'a :: semidom"
shows "prime_elem [:c:] ⟷ prime_elem c"
proof
assume A: "prime_elem [:c:]"
show "prime_elem c"
proof (rule prime_elemI)
fix a b assume "c dvd a * b"
hence "[:c:] dvd [:a:] * [:b:]" by (simp add: mult_ac)
from A and this have "[:c:] dvd [:a:] ∨ [:c:] dvd [:b:]" by (rule prime_elem_dvd_multD)
thus "c dvd a ∨ c dvd b" by simp
qed (insert A, auto simp: prime_elem_def is_unit_poly_iff)
qed (auto intro: lift_prime_elem_poly)
subsection ‹Content and primitive part of a polynomial›
definition content :: "'a::semiring_gcd poly ⇒ 'a"
where "content p = gcd_list (coeffs p)"
lemma content_eq_fold_coeffs [code]: "content p = fold_coeffs gcd p 0"
by (simp add: content_def Gcd_fin.set_eq_fold fold_coeffs_def foldr_fold fun_eq_iff ac_simps)
lemma content_0 [simp]: "content 0 = 0"
by (simp add: content_def)
lemma content_1 [simp]: "content 1 = 1"
by (simp add: content_def)
lemma content_const [simp]: "content [:c:] = normalize c"
by (simp add: content_def cCons_def)
lemma const_poly_dvd_iff_dvd_content: "[:c:] dvd p ⟷ c dvd content p"
for c :: "'a::semiring_gcd"
proof (cases "p = 0")
case True
then show ?thesis by simp
next
case False
have "[:c:] dvd p ⟷ (∀n. c dvd coeff p n)"
by (rule const_poly_dvd_iff)
also have "… ⟷ (∀a∈set (coeffs p). c dvd a)"
proof safe
fix n :: nat
assume "∀a∈set (coeffs p). c dvd a"
then show "c dvd coeff p n"
by (cases "n ≤ degree p") (auto simp: coeff_eq_0 coeffs_def split: if_splits)
qed (auto simp: coeffs_def simp del: upt_Suc split: if_splits)
also have "… ⟷ c dvd content p"
by (simp add: content_def dvd_Gcd_fin_iff dvd_mult_unit_iff)
finally show ?thesis .
qed
lemma content_dvd [simp]: "[:content p:] dvd p"
by (subst const_poly_dvd_iff_dvd_content) simp_all
lemma content_dvd_coeff [simp]: "content p dvd coeff p n"
proof (cases "p = 0")
case True
then show ?thesis
by simp
next
case False
then show ?thesis
by (cases "n ≤ degree p")
(auto simp add: content_def not_le coeff_eq_0 coeff_in_coeffs intro: Gcd_fin_dvd)
qed
lemma content_dvd_coeffs: "c ∈ set (coeffs p) ⟹ content p dvd c"
by (simp add: content_def Gcd_fin_dvd)
lemma normalize_content [simp]: "normalize (content p) = content p"
by (simp add: content_def)
lemma is_unit_content_iff [simp]: "is_unit (content p) ⟷ content p = 1"
proof
assume "is_unit (content p)"
then have "normalize (content p) = 1" by (simp add: is_unit_normalize del: normalize_content)
then show "content p = 1" by simp
qed auto
lemma content_smult [simp]:
fixes c :: "'a :: {normalization_semidom_multiplicative, semiring_gcd}"
shows "content (smult c p) = normalize c * content p"
by (simp add: content_def coeffs_smult Gcd_fin_mult normalize_mult)
lemma content_eq_zero_iff [simp]: "content p = 0 ⟷ p = 0"
by (auto simp: content_def simp: poly_eq_iff coeffs_def)
definition primitive_part :: "'a :: semiring_gcd poly ⇒ 'a poly"
where "primitive_part p = map_poly (λx. x div content p) p"
lemma primitive_part_0 [simp]: "primitive_part 0 = 0"
by (simp add: primitive_part_def)
lemma content_times_primitive_part [simp]: "smult (content p) (primitive_part p) = p"
for p :: "'a :: semiring_gcd poly"
proof (cases "p = 0")
case True
then show ?thesis by simp
next
case False
then show ?thesis
unfolding primitive_part_def
by (auto simp: smult_conv_map_poly map_poly_map_poly o_def content_dvd_coeffs
intro: map_poly_idI)
qed
lemma primitive_part_eq_0_iff [simp]: "primitive_part p = 0 ⟷ p = 0"
proof (cases "p = 0")
case True
then show ?thesis by simp
next
case False
then have "primitive_part p = map_poly (λx. x div content p) p"
by (simp add: primitive_part_def)
also from False have "… = 0 ⟷ p = 0"
by (intro map_poly_eq_0_iff) (auto simp: dvd_div_eq_0_iff content_dvd_coeffs)
finally show ?thesis
using False by simp
qed
lemma content_primitive_part [simp]:
fixes p :: "'a :: {normalization_semidom_multiplicative, semiring_gcd} poly"
assumes "p ≠ 0"
shows "content (primitive_part p) = 1"
proof -
have "p = smult (content p) (primitive_part p)"
by simp
also have "content … = content (primitive_part p) * content p"
by (simp del: content_times_primitive_part add: ac_simps)
finally have "1 * content p = content (primitive_part p) * content p"
by simp
then have "1 * content p div content p = content (primitive_part p) * content p div content p"
by simp
with assms show ?thesis
by simp
qed
lemma content_decompose:
obtains p' :: "'a :: {normalization_semidom_multiplicative, semiring_gcd} poly"
where "p = smult (content p) p'" "content p' = 1"
proof (cases "p = 0")
case True
then have "p = smult (content p) 1" "content 1 = 1"
by simp_all
then show ?thesis ..
next
case False
then have "p = smult (content p) (primitive_part p)" "content (primitive_part p) = 1"
by simp_all
then show ?thesis ..
qed
lemma content_dvd_contentI [intro]: "p dvd q ⟹ content p dvd content q"
using const_poly_dvd_iff_dvd_content content_dvd dvd_trans by blast
lemma primitive_part_const_poly [simp]: "primitive_part [:x:] = [:unit_factor x:]"
by (simp add: primitive_part_def map_poly_pCons)
lemma primitive_part_prim: "content p = 1 ⟹ primitive_part p = p"
by (auto simp: primitive_part_def)
lemma degree_primitive_part [simp]: "degree (primitive_part p) = degree p"
proof (cases "p = 0")
case True
then show ?thesis by simp
next
case False
have "p = smult (content p) (primitive_part p)"
by simp
also from False have "degree … = degree (primitive_part p)"
by (subst degree_smult_eq) simp_all
finally show ?thesis ..
qed
lemma smult_content_normalize_primitive_part [simp]:
fixes p :: "'a :: {normalization_semidom_multiplicative, semiring_gcd, idom_divide} poly"
shows "smult (content p) (normalize (primitive_part p)) = normalize p"
proof -
have "smult (content p) (normalize (primitive_part p)) =
normalize ([:content p:] * primitive_part p)"
by (subst normalize_mult) (simp_all add: normalize_const_poly)
also have "[:content p:] * primitive_part p = p" by simp
finally show ?thesis .
qed
context
begin
private
lemma content_1_mult:
fixes f g :: "'a :: {semiring_gcd, factorial_semiring} poly"
assumes "content f = 1" "content g = 1"
shows "content (f * g) = 1"
proof (cases "f * g = 0")
case False
from assms have "f ≠ 0" "g ≠ 0" by auto
hence "f * g ≠ 0" by auto
{
assume "¬is_unit (content (f * g))"
with False have "∃p. p dvd content (f * g) ∧ prime p"
by (intro prime_divisor_exists) simp_all
then obtain p where "p dvd content (f * g)" "prime p" by blast
from ‹p dvd content (f * g)› have "[:p:] dvd f * g"
by (simp add: const_poly_dvd_iff_dvd_content)
moreover from ‹prime p› have "prime_elem [:p:]" by (simp add: lift_prime_elem_poly)
ultimately have "[:p:] dvd f ∨ [:p:] dvd g"
by (simp add: prime_elem_dvd_mult_iff)
with assms have "is_unit p" by (simp add: const_poly_dvd_iff_dvd_content)
with ‹prime p› have False by simp
}
hence "is_unit (content (f * g))" by blast
hence "normalize (content (f * g)) = 1" by (simp add: is_unit_normalize del: normalize_content)
thus ?thesis by simp
qed (insert assms, auto)
lemma content_mult:
fixes p q :: "'a :: {factorial_semiring, semiring_gcd, normalization_semidom_multiplicative} poly"
shows "content (p * q) = content p * content q"
proof (cases "p * q = 0")
case False
then have "p ≠ 0" and "q ≠ 0"
by simp_all
then have *: "content (primitive_part p * primitive_part q) = 1"
by (auto intro: content_1_mult)
have "p * q = smult (content p) (primitive_part p) * smult (content q) (primitive_part q)"
by simp
also have "… = smult (content p * content q) (primitive_part p * primitive_part q)"
by (metis mult.commute mult_smult_right smult_smult)
with * show ?thesis
by (simp add: normalize_mult)
next
case True
then show ?thesis
by auto
qed
end
lemma primitive_part_mult:
fixes p q :: "'a :: {factorial_semiring, semiring_Gcd, ring_gcd, idom_divide,
normalization_semidom_multiplicative} poly"
shows "primitive_part (p * q) = primitive_part p * primitive_part q"
proof -
have "primitive_part (p * q) = p * q div [:content (p * q):]"
by (simp add: primitive_part_def div_const_poly_conv_map_poly)
also have "… = (p div [:content p:]) * (q div [:content q:])"
by (subst div_mult_div_if_dvd) (simp_all add: content_mult mult_ac)
also have "… = primitive_part p * primitive_part q"
by (simp add: primitive_part_def div_const_poly_conv_map_poly)
finally show ?thesis .
qed
lemma primitive_part_smult:
fixes p :: "'a :: {factorial_semiring, semiring_Gcd, ring_gcd, idom_divide,
normalization_semidom_multiplicative} poly"
shows "primitive_part (smult a p) = smult (unit_factor a) (primitive_part p)"
proof -
have "smult a p = [:a:] * p" by simp
also have "primitive_part … = smult (unit_factor a) (primitive_part p)"
by (subst primitive_part_mult) simp_all
finally show ?thesis .
qed
lemma primitive_part_dvd_primitive_partI [intro]:
fixes p q :: "'a :: {factorial_semiring, semiring_Gcd, ring_gcd, idom_divide,
normalization_semidom_multiplicative} poly"
shows "p dvd q ⟹ primitive_part p dvd primitive_part q"
by (auto elim!: dvdE simp: primitive_part_mult)
lemma content_prod_mset:
fixes A :: "'a :: {factorial_semiring, semiring_Gcd, normalization_semidom_multiplicative}
poly multiset"
shows "content (prod_mset A) = prod_mset (image_mset content A)"
by (induction A) (simp_all add: content_mult mult_ac)
lemma content_prod_eq_1_iff:
fixes p q :: "'a :: {factorial_semiring, semiring_Gcd, normalization_semidom_multiplicative} poly"
shows "content (p * q) = 1 ⟷ content p = 1 ∧ content q = 1"
proof safe
assume A: "content (p * q) = 1"
{
fix p q :: "'a poly" assume "content p * content q = 1"
hence "1 = content p * content q" by simp
hence "content p dvd 1" by (rule dvdI)
hence "content p = 1" by simp
} note B = this
from A B[of p q] B [of q p] show "content p = 1" "content q = 1"
by (simp_all add: content_mult mult_ac)
qed (auto simp: content_mult)
subsection ‹A typeclass for algebraically closed fields›
text ‹
Since the required sort constraints are not available inside the class, we have to resort
to a somewhat awkward way of writing the definition of algebraically closed fields:
›
class alg_closed_field = field +
assumes alg_closed: "n > 0 ⟹ f n ≠ 0 ⟹ ∃x. (∑k≤n. f k * x ^ k) = 0"
text ‹
We can then however easily show the equivalence to the proper definition:
›
lemma alg_closed_imp_poly_has_root:
assumes "degree (p :: 'a :: alg_closed_field poly) > 0"
shows "∃x. poly p x = 0"
proof -
have "∃x. (∑k≤degree p. coeff p k * x ^ k) = 0"
using assms by (intro alg_closed) auto
thus ?thesis
by (simp add: poly_altdef)
qed
lemma alg_closedI [Pure.intro]:
assumes "⋀p :: 'a poly. degree p > 0 ⟹ lead_coeff p = 1 ⟹ ∃x. poly p x = 0"
shows "OFCLASS('a :: field, alg_closed_field_class)"
proof
fix n :: nat and f :: "nat ⇒ 'a"
assume n: "n > 0" "f n ≠ 0"
define p where "p = Abs_poly (λk. if k ≤ n then f k else 0)"
have coeff_p: "coeff p k = (if k ≤ n then f k else 0)" for k
proof -
have "eventually (λk. k > n) cofinite"
by (auto simp: MOST_nat)
hence "eventually (λk. (if k ≤ n then f k else 0) = 0) cofinite"
by eventually_elim auto
thus ?thesis
unfolding p_def by (subst Abs_poly_inverse) auto
qed
from n have "degree p ≥ n"
by (intro le_degree) (auto simp: coeff_p)
moreover have "degree p ≤ n"
by (intro degree_le) (auto simp: coeff_p)
ultimately have deg_p: "degree p = n"
by linarith
from deg_p and n have [simp]: "p ≠ 0"
by auto
define p' where "p' = smult (inverse (lead_coeff p)) p"
have deg_p': "degree p' = degree p"
by (auto simp: p'_def)
have lead_coeff_p' [simp]: "lead_coeff p' = 1"
by (auto simp: p'_def)
from deg_p and deg_p' and n have "degree p' > 0"
by simp
from assms[OF this] obtain x where "poly p' x = 0"
by auto
hence "poly p x = 0"
by (simp add: p'_def)
also have "poly p x = (∑k≤n. f k * x ^ k)"
unfolding poly_altdef by (intro sum.cong) (auto simp: deg_p coeff_p)
finally show "∃x. (∑k≤n. f k * x ^ k) = 0" ..
qed
lemma (in alg_closed_field) nth_root_exists:
assumes "n > 0"
shows "∃y. y ^ n = (x :: 'a)"
proof -
define f where "f = (λi. if i = 0 then -x else if i = n then 1 else 0)"
have "∃x. (∑k≤n. f k * x ^ k) = 0"
by (rule alg_closed) (use assms in ‹auto simp: f_def›)
also have "(λx. ∑k≤n. f k * x ^ k) = (λx. ∑k∈{0,n}. f k * x ^ k)"
by (intro ext sum.mono_neutral_right) (auto simp: f_def)
finally show "∃y. y ^ n = x"
using assms by (simp add: f_def)
qed
text ‹
We can now prove by induction that every polynomial of degree ‹n› splits into a product of
‹n› linear factors:
›
lemma alg_closed_imp_factorization:
fixes p :: "'a :: alg_closed_field poly"
assumes "p ≠ 0"
shows "∃A. size A = degree p ∧ p = smult (lead_coeff p) (∏x∈#A. [:-x, 1:])"
using assms
proof (induction "degree p" arbitrary: p rule: less_induct)
case (less p)
show ?case
proof (cases "degree p = 0")
case True
thus ?thesis
by (intro exI[of _ "{#}"]) (auto elim!: degree_eq_zeroE)
next
case False
then obtain x where x: "poly p x = 0"
using alg_closed_imp_poly_has_root by blast
hence "[:-x, 1:] dvd p"
using poly_eq_0_iff_dvd by blast
then obtain q where p_eq: "p = [:-x, 1:] * q"
by (elim dvdE)
have "q ≠ 0"
using less.prems p_eq by auto
moreover from this have deg: "degree p = Suc (degree q)"
unfolding p_eq by (subst degree_mult_eq) auto
ultimately obtain A where A: "size A = degree q" "q = smult (lead_coeff q) (∏x∈#A. [:-x, 1:])"
using less.hyps[of q] by auto
have "smult (lead_coeff p) (∏y∈#add_mset x A. [:- y, 1:]) =
[:- x, 1:] * smult (lead_coeff q) (∏y∈#A. [:- y, 1:])"
unfolding p_eq lead_coeff_mult by simp
also note A(2) [symmetric]
also note p_eq [symmetric]
finally show ?thesis using A(1)
by (intro exI[of _ "add_mset x A"]) (auto simp: deg)
qed
qed
text ‹
As an alternative characterisation of algebraic closure, one can also say that any
polynomial of degree at least 2 splits into non-constant factors:
›
lemma alg_closed_imp_reducible:
assumes "degree (p :: 'a :: alg_closed_field poly) > 1"
shows "¬irreducible p"
proof -
have "degree p > 0"
using assms by auto
then obtain z where z: "poly p z = 0"
using alg_closed_imp_poly_has_root[of p] by blast
then have dvd: "[:-z, 1:] dvd p"
by (subst dvd_iff_poly_eq_0) auto
then obtain q where q: "p = [:-z, 1:] * q"
by (erule dvdE)
have [simp]: "q ≠ 0"
using assms q by auto
show ?thesis
proof (rule reducible_polyI)
show "p = [:-z, 1:] * q"
by fact
next
have "degree p = degree ([:-z, 1:] * q)"
by (simp only: q)
also have "… = degree q + 1"
by (subst degree_mult_eq) auto
finally show "degree q > 0"
using assms by linarith
qed auto
qed
text ‹
When proving algebraic closure through reducibility, we can assume w.l.o.g. that the polynomial
is monic and has a non-zero constant coefficient:
›
lemma alg_closedI_reducible:
assumes "⋀p :: 'a poly. degree p > 1 ⟹ lead_coeff p = 1 ⟹ coeff p 0 ≠ 0 ⟹
¬irreducible p"
shows "OFCLASS('a :: field, alg_closed_field_class)"
proof
fix p :: "'a poly" assume p: "degree p > 0" "lead_coeff p = 1"
show "∃x. poly p x = 0"
proof (cases "coeff p 0 = 0")
case True
hence "poly p 0 = 0"
by (simp add: poly_0_coeff_0)
thus ?thesis by blast
next
case False
from p and this show ?thesis
proof (induction "degree p" arbitrary: p rule: less_induct)
case (less p)
show ?case
proof (cases "degree p = 1")
case True
then obtain a b where p: "p = [:a, b:]"
by (cases p) (auto split: if_splits elim!: degree_eq_zeroE)
from True have [simp]: "b ≠ 0"
by (auto simp: p)
have "poly p (-a/b) = 0"
by (auto simp: p)
thus ?thesis by blast
next
case False
hence "degree p > 1"
using less.prems by auto
from assms[OF ‹degree p > 1› ‹lead_coeff p = 1› ‹coeff p 0 ≠ 0›]
have "¬irreducible p" by auto
then obtain r s where rs: "degree r > 0" "degree s > 0" "p = r * s"
using less.prems unfolding irreducible_def
by (metis is_unit_iff_degree mult_not_zero zero_less_iff_neq_zero)
hence "coeff r 0 ≠ 0"
using ‹coeff p 0 ≠ 0› by (auto simp: coeff_mult_0)
define r' where "r' = smult (inverse (lead_coeff r)) r"
have [simp]: "degree r' = degree r"
by (simp add: r'_def)
have lc: "lead_coeff r' = 1"
using rs by (auto simp: r'_def)
have nz: "coeff r' 0 ≠ 0"
using ‹coeff r 0 ≠ 0› by (auto simp: r'_def)
have "degree r < degree r + degree s"
using rs by linarith
also have "… = degree (r * s)"
using rs(3) less.prems by (subst degree_mult_eq) auto
also have "r * s = p"
using rs(3) by simp
finally have "∃x. poly r' x = 0"
by (intro less) (use lc rs nz in auto)
thus ?thesis
using rs(3) by (auto simp: r'_def)
qed
qed
qed
qed
text ‹
Using a clever Tschirnhausen transformation mentioned e.g. in the article by
Nowak~\<^cite>‹"nowak2000"›, we can also assume w.l.o.g. that the coefficient $a_{n-1}$ is zero.
›
lemma alg_closedI_reducible_coeff_deg_minus_one_eq_0:
assumes "⋀p :: 'a poly. degree p > 1 ⟹ lead_coeff p = 1 ⟹ coeff p (degree p - 1) = 0 ⟹
coeff p 0 ≠ 0 ⟹ ¬irreducible p"
shows "OFCLASS('a :: field_char_0, alg_closed_field_class)"
proof (rule alg_closedI_reducible, goal_cases)
case (1 p)
define n where [simp]: "n = degree p"
define a where "a = coeff p (n - 1)"
define r where "r = [: -a / of_nat n, 1 :]"
define s where "s = [: a / of_nat n, 1 :]"
define q where "q = pcompose p r"
have "n > 0"
using 1 by simp
have r_altdef: "r = monom 1 1 + [:-a / of_nat n:]"
by (simp add: r_def monom_altdef)
have deg_q: "degree q = n"
by (simp add: q_def r_def degree_pcompose)
have lc_q: "lead_coeff q = 1"
unfolding q_def using 1 by (subst lead_coeff_comp) (simp_all add: r_def)
have "q ≠ 0"
using 1 deg_q by auto
have "coeff q (n - 1) =
(∑i≤n. ∑k≤i. coeff p i * (of_nat (i choose k) *
((-a / of_nat n) ^ (i - k) * (if k = n - 1 then 1 else 0))))"
unfolding q_def pcompose_altdef poly_altdef r_altdef
by (simp_all add: degree_map_poly coeff_map_poly coeff_sum binomial_ring sum_distrib_left poly_const_pow
sum_distrib_right mult_ac monom_power coeff_monom_mult of_nat_poly cong: if_cong)
also have "… = (∑i≤n. ∑k∈(if i ≥ n - 1 then {n-1} else {}).
coeff p i * (of_nat (i choose k) * (-a / of_nat n) ^ (i - k)))"
by (rule sum.cong [OF refl], rule sum.mono_neutral_cong_right) (auto split: if_splits)
also have "… = (∑i∈{n-1,n}. ∑k∈(if i ≥ n - 1 then {n-1} else {}).
coeff p i * (of_nat (i choose k) * (-a / of_nat n) ^ (i - k)))"
by (rule sum.mono_neutral_right) auto
also have "… = a - of_nat (n choose (n - 1)) * a / of_nat n"
using 1 by (simp add: a_def)
also have "n choose (n - 1) = n"
using ‹n > 0› by (subst binomial_symmetric) auto
also have "a - of_nat n * a / of_nat n = 0"
using ‹n > 0› by simp
finally have "coeff q (n - 1) = 0" .
show ?case
proof (cases "coeff q 0 = 0")
case True
hence "poly p (- (a / of_nat (degree p))) = 0"
by (auto simp: q_def r_def)
thus ?thesis
by (rule root_imp_reducible_poly) (use 1 in auto)
next
case False
hence "¬irreducible q"
using assms[of q] and lc_q and 1 and ‹coeff q (n - 1) = 0›
by (auto simp: deg_q)
then obtain u v where uv: "degree u > 0" "degree v > 0" "q = u * v"
using ‹q ≠ 0› 1 deg_q unfolding irreducible_def
by (metis degree_mult_eq_0 is_unit_iff_degree n_def neq0_conv not_one_less_zero)
have "p = pcompose q s"
by (simp add: q_def r_def s_def pcompose_pCons flip: pcompose_assoc)
also have "q = u * v"
by fact
finally have "p = pcompose u s * pcompose v s"
by (simp add: pcompose_mult)
moreover have "degree (pcompose u s) > 0" "degree (pcompose v s) > 0"
using uv by (simp_all add: s_def degree_pcompose)
ultimately show "¬irreducible p"
using 1 by (intro reducible_polyI)
qed
qed
text ‹
As a consequence of the full factorisation lemma proven above, we can also show that any
polynomial with at least two different roots splits into two non-constant coprime factors:
›
lemma alg_closed_imp_poly_splits_coprime:
assumes "degree (p :: 'a :: {alg_closed_field} poly) > 1"
assumes "poly p x = 0" "poly p y = 0" "x ≠ y"
obtains r s where "degree r > 0" "degree s > 0" "coprime r s" "p = r * s"
proof -
define n where "n = order x p"
have "n > 0"
using assms by (metis degree_0 gr0I n_def not_one_less_zero order_root)
have "[:-x, 1:] ^ n dvd p"
unfolding n_def by (simp add: order_1)
then obtain q where p_eq: "p = [:-x, 1:] ^ n * q"
by (elim dvdE)
from assms have [simp]: "q ≠ 0"
by (auto simp: p_eq)
have "order x p = n + Polynomial.order x q"
unfolding p_eq by (subst order_mult) (auto simp: order_power_n_n)
hence "Polynomial.order x q = 0"
by (simp add: n_def)
hence "poly q x ≠ 0"
by (simp add: order_root)
show ?thesis
proof (rule that)
show "coprime ([:-x, 1:] ^ n) q"
proof (rule coprimeI)
fix d
assume d: "d dvd [:-x, 1:] ^ n" "d dvd q"
have "degree d = 0"
proof (rule ccontr)
assume "¬(degree d = 0)"
then obtain z where z: "poly d z = 0"
using alg_closed_imp_poly_has_root by blast
moreover from this and d(1) have "poly ([:-x, 1:] ^ n) z = 0"
using dvd_trans poly_eq_0_iff_dvd by blast
ultimately have "poly d x = 0"
by auto
with d(2) have "poly q x = 0"
using dvd_trans poly_eq_0_iff_dvd by blast
with ‹poly q x ≠ 0› show False by contradiction
qed
thus "is_unit d" using d
by (metis ‹q ≠ 0› dvd_0_left is_unit_iff_degree)
qed
next
have "poly q y = 0"
using ‹poly p y = 0› ‹x ≠ y› by (auto simp: p_eq)
with ‹q ≠ 0› show "degree q > 0"
using order_degree order_gt_0_iff order_less_le_trans by blast
qed (use ‹n > 0› in ‹simp_all add: p_eq degree_power_eq›)
qed
no_notation cCons (infixr ‹##› 65)
end