Theory Residues

(*  Title:      HOL/Number_Theory/Residues.thy
    Author:     Jeremy Avigad

An algebraic treatment of residue rings, and resulting proofs of
Euler's theorem and Wilson's theorem.
*)

section ‹Residue rings›

theory Residues
imports
  Cong
  "HOL-Algebra.Multiplicative_Group"
  Totient
begin

lemma (in ring_1) CHAR_dvd_CARD: "CHAR('a) dvd card (UNIV :: 'a set)"
proof (cases "card (UNIV :: 'a set) = 0")
  case False
  hence [intro]: "CHAR('a) > 0"
    by (simp add: card_eq_0_iff finite_imp_CHAR_pos)    
  define G where "G = ⦇ carrier = (UNIV :: 'a set), monoid.mult = (+), one = (0 :: 'a) ⦈"
  define H where "H = (of_nat ` {..<CHAR('a)} :: 'a set)"
  interpret group G
  proof (rule groupI)
    fix x assume x: "x ∈ carrier G"
    show "∃y∈carrier G. y ⊗⇘G⇙ x = 𝟭⇘G⇙"
      by (intro bexI[of _ "-x"]) (auto simp: G_def)
  qed (auto simp: G_def add_ac)

  interpret subgroup H G
  proof
    show "𝟭⇘G⇙ ∈ H"
      using False unfolding G_def H_def by force
  next
    fix x y :: 'a
    assume "x ∈ H" "y ∈ H"
    then obtain x' y' where [simp]: "x = of_nat x'" "y = of_nat y'"
      by (auto simp: H_def)
    have "x + y = of_nat ((x' + y') mod CHAR('a))"
      by (auto simp flip: of_nat_add simp: of_nat_eq_iff_cong_CHAR)
    moreover have "(x' + y') mod CHAR('a) < CHAR('a)"
      using H_def ‹y ∈ H› by fastforce
    ultimately show "x ⊗⇘G⇙ y ∈ H"
      by (auto simp: H_def G_def intro!: imageI)
  next
    fix x :: 'a
    assume x: "x ∈ H"
    then obtain x' where [simp]: "x = of_nat x'" and x': "x' < CHAR('a)"
      by (auto simp: H_def)
    have "CHAR('a) dvd x' + (CHAR('a) - x') mod CHAR('a)"
      using mod_eq_0_iff_dvd mod_if x' by fastforce
    hence "x + of_nat ((CHAR('a) - x') mod CHAR('a)) = 0"
      by (auto simp flip: of_nat_add simp: of_nat_eq_0_iff_char_dvd)
    moreover from this have "inv⇘G⇙ x = of_nat ((CHAR('a) - x') mod CHAR('a))"
      by (intro inv_equality) (auto simp: G_def add_ac)
    moreover have "of_nat ((CHAR('a) - x') mod CHAR('a)) ∈ H"
      unfolding H_def using ‹CHAR('a) > 0› by (intro imageI) auto
    ultimately show "inv⇘G⇙ x ∈ H" by force
  qed (auto simp: G_def H_def)

  have "card H dvd card (rcosets⇘G⇙ H) * card H"
    by simp
  also have "card (rcosets⇘G⇙ H) * card H = Coset.order G"
  proof (rule lagrange_finite)
    show "finite (carrier G)"
      using False card_ge_0_finite by (auto simp: G_def)
  qed (fact is_subgroup)
  finally have "card H dvd card (UNIV :: 'a set)"
    by (simp add: Coset.order_def G_def)
  also have "card H = card {..<CHAR('a)}"
    unfolding H_def by (intro card_image inj_onI) (auto simp: of_nat_eq_iff_cong_CHAR cong_def)
  finally show "CHAR('a) dvd card (UNIV :: 'a set)"
    by simp
qed auto

definition QuadRes :: "int ⇒ int ⇒ bool"
  where "QuadRes p a = (∃y. ([y^2 = a] (mod p)))"

definition Legendre :: "int ⇒ int ⇒ int"
  where "Legendre a p =
    (if ([a = 0] (mod p)) then 0
     else if QuadRes p a then 1
     else -1)"


subsection ‹A locale for residue rings›

definition residue_ring :: "int ⇒ int ring"
  where
    "residue_ring m =
      ⦇carrier = {0..m - 1},
       monoid.mult = λx y. (x * y) mod m,
       one = 1,
       zero = 0,
       add = λx y. (x + y) mod m⦈"

locale residues =
  fixes m :: int and R (structure)
  assumes m_gt_one: "m > 1"
  defines R_m_def: "R ≡ residue_ring m"
begin

lemma abelian_group: "abelian_group R"
proof -
  have "∃y∈{0..m - 1}. (x + y) mod m = 0" if "0 ≤ x" "x < m" for x
  proof (cases "x = 0")
    case True
    with m_gt_one show ?thesis by simp
  next
    case False
    then have "(x + (m - x)) mod m = 0"
      by simp
    with m_gt_one that show ?thesis
      by (metis False atLeastAtMost_iff diff_ge_0_iff_ge diff_left_mono int_one_le_iff_zero_less less_le)
  qed
  with m_gt_one show ?thesis
    by (fastforce simp add: R_m_def residue_ring_def mod_add_right_eq ac_simps  intro!: abelian_groupI)
qed

lemma comm_monoid: "comm_monoid R"
proof -
  have "⋀x y z. ⟦x ∈ carrier R; y ∈ carrier R; z ∈ carrier R⟧ ⟹ x ⊗ y ⊗ z = x ⊗ (y ⊗ z)"
    "⋀x y. ⟦x ∈ carrier R; y ∈ carrier R⟧ ⟹ x ⊗ y = y ⊗ x"
    unfolding R_m_def residue_ring_def
    by (simp_all add: algebra_simps mod_mult_right_eq)
  then show ?thesis
    unfolding R_m_def residue_ring_def
    by unfold_locales (use m_gt_one in simp_all)
qed

interpretation comm_monoid R
  using comm_monoid by blast

lemma cring: "cring R"
  apply (intro cringI abelian_group comm_monoid)
  unfolding R_m_def residue_ring_def
  apply (auto simp add: comm_semiring_class.distrib mod_add_eq mod_mult_left_eq)
  done

end

sublocale residues < cring
  by (rule cring)


context residues
begin

text ‹
  These lemmas translate back and forth between internal and
  external concepts.
›

lemma res_carrier_eq: "carrier R = {0..m - 1}"
  by (auto simp: R_m_def residue_ring_def)

lemma res_add_eq: "x ⊕ y = (x + y) mod m"
  by (auto simp: R_m_def residue_ring_def)

lemma res_mult_eq: "x ⊗ y = (x * y) mod m"
  by (auto simp: R_m_def residue_ring_def)

lemma res_zero_eq: "𝟬 = 0"
  by (auto simp: R_m_def residue_ring_def)

lemma res_one_eq: "𝟭 = 1"
  by (auto simp: R_m_def residue_ring_def units_of_def)

lemma res_units_eq: "Units R = {x. 0 < x ∧ x < m ∧ coprime x m}" (is "_ = ?rhs")
proof
  show "Units R ⊆ ?rhs"
    using zero_less_mult_iff invertible_coprime 
    by (fastforce simp: Units_def R_m_def residue_ring_def)
next
  show "?rhs ⊆ Units R"
    unfolding Units_def R_m_def residue_ring_def 
    by (force simp add: cong_def coprime_iff_invertible'_int mult.commute)
qed

lemma res_neg_eq: "⊖ x = (- x) mod m"
proof -
  have "⊖ x = (THE y. 0 ≤ y ∧ y < m ∧ (x + y) mod m = 0 ∧ (y + x) mod m = 0)"
    by (simp add: R_m_def a_inv_def m_inv_def residue_ring_def)
  also have "… = (- x) mod m"
  proof -
    have "⋀y. 0 ≤ y ∧ y < m ∧ (x + y) mod m = 0 ∧ (y + x) mod m = 0 ⟹
         y = - x mod m"
      by (metis minus_add_cancel mod_add_eq plus_int_code(1) zmod_trivial_iff)
    then show ?thesis
      by (intro the_equality) (use m_gt_one  in ‹simp add: add.commute mod_add_right_eq›)
  qed
  finally show ?thesis .
qed

lemma finite [iff]: "finite (carrier R)"
  by (simp add: res_carrier_eq)

lemma finite_Units [iff]: "finite (Units R)"
  by (simp add: finite_ring_finite_units)

text ‹
  The function ‹a ↦ a mod m› maps the integers to the
  residue classes. The following lemmas show that this mapping
  respects addition and multiplication on the integers.
›

lemma mod_in_carrier [iff]: "a mod m ∈ carrier R"
  unfolding res_carrier_eq
  using insert m_gt_one by auto

lemma add_cong: "(x mod m) ⊕ (y mod m) = (x + y) mod m"
  by (auto simp: R_m_def residue_ring_def mod_simps)

lemma mult_cong: "(x mod m) ⊗ (y mod m) = (x * y) mod m"
  by (auto simp: R_m_def residue_ring_def mod_simps)

lemma zero_cong: "𝟬 = 0"
  by (auto simp: R_m_def residue_ring_def)

lemma one_cong: "𝟭 = 1 mod m"
  using m_gt_one by (auto simp: R_m_def residue_ring_def)

(* FIXME revise algebra library to use 1? *)
lemma pow_cong: "(x mod m) [^] n = x^n mod m"
  using m_gt_one
proof (induct n)
  case 0
  then show ?case
    by (simp add: one_cong) 
next
  case (Suc n)
  then show ?case
    by (simp add: mult_cong power_commutes) 
qed

lemma neg_cong: "⊖ (x mod m) = (- x) mod m"
  by (metis mod_minus_eq res_neg_eq)

lemma (in residues) prod_cong: "finite A ⟹ (⨂i∈A. (f i) mod m) = (∏i∈A. f i) mod m"
  by (induct set: finite) (auto simp: one_cong mult_cong)

lemma (in residues) sum_cong: "finite A ⟹ (⨁i∈A. (f i) mod m) = (∑i∈A. f i) mod m"
  by (induct set: finite) (auto simp: zero_cong add_cong)

lemma mod_in_res_units [simp]:
  assumes "1 < m" and "coprime a m"
  shows "a mod m ∈ Units R"
proof (cases "a mod m = 0")
  case True
  with assms show ?thesis
    by (auto simp add: res_units_eq gcd_red_int [symmetric])
next
  case False
  from assms have "0 < m" by simp
  then have "0 ≤ a mod m" by (rule pos_mod_sign [of m a])
  with False have "0 < a mod m" by simp
  with assms show ?thesis
    by (auto simp add: res_units_eq gcd_red_int [symmetric] ac_simps)
qed

lemma res_eq_to_cong: "(a mod m) = (b mod m) ⟷ [a = b] (mod m)"
  by (auto simp: cong_def)


text ‹Simplifying with these will translate a ring equation in R to a congruence.›
lemmas res_to_cong_simps =
  add_cong mult_cong pow_cong one_cong
  prod_cong sum_cong neg_cong res_eq_to_cong

text ‹Other useful facts about the residue ring.›
lemma one_eq_neg_one: "𝟭 = ⊖ 𝟭 ⟹ m = 2"
  using one_cong res_neg_eq res_one_eq zmod_zminus1_eq_if by fastforce

end


subsection ‹Prime residues›

locale residues_prime =
  fixes p :: nat and R (structure)
  assumes p_prime [intro]: "prime p"
  defines "R ≡ residue_ring (int p)"

sublocale residues_prime < residues p
proof
  show "1 < int p"
    using prime_gt_1_nat by auto
qed

context residues_prime
begin

lemma p_coprime_left:
  "coprime p a ⟷ ¬ p dvd a"
  using p_prime by (auto intro: prime_imp_coprime dest: coprime_common_divisor)

lemma p_coprime_right:
  "coprime a p  ⟷ ¬ p dvd a"
  using p_coprime_left [of a] by (simp add: ac_simps)

lemma p_coprime_left_int:
  "coprime (int p) a ⟷ ¬ int p dvd a"
  using p_prime by (auto intro: prime_imp_coprime dest: coprime_common_divisor)

lemma p_coprime_right_int:
  "coprime a (int p) ⟷ ¬ int p dvd a"
  using coprime_commute p_coprime_left_int by blast

lemma is_field: "field R"
proof -
  have "0 < x ⟹ x < int p ⟹ coprime (int p) x" for x
    by (rule prime_imp_coprime) (auto simp add: zdvd_not_zless)
  then show ?thesis
    by (intro cring.field_intro2 cring)
      (auto simp add: res_carrier_eq res_one_eq res_zero_eq res_units_eq ac_simps)
qed

lemma res_prime_units_eq: "Units R = {1..p - 1}"
  by (auto simp add: res_units_eq p_coprime_right_int zdvd_not_zless)

end

sublocale residues_prime < field
  by (rule is_field)


section ‹Test cases: Euler's theorem and Wilson's theorem›

subsection ‹Euler's theorem›

lemma (in residues) totatives_eq:
  "totatives (nat m) = nat ` Units R"
proof -
  from m_gt_one have "¦m¦ > 1"
    by simp
  then have "totatives (nat ¦m¦) = nat ` abs ` Units R"
    by (auto simp add: totatives_def res_units_eq image_iff le_less)
      (use m_gt_one zless_nat_eq_int_zless in force)
  moreover have "¦m¦ = m" "abs ` Units R = Units R"
    using m_gt_one by (auto simp add: res_units_eq image_iff)
  ultimately show ?thesis
    by simp
qed

lemma (in residues) totient_eq:
  "totient (nat m) = card (Units R)"
proof  -
  have *: "inj_on nat (Units R)"
    by (rule inj_onI) (auto simp add: res_units_eq)
  then show ?thesis
    by (simp add: totient_def totatives_eq card_image)
qed

lemma (in residues_prime) prime_totient_eq: "totient p = p - 1"
  using p_prime totient_prime by blast

lemma (in residues) euler_theorem:
  assumes "coprime a m"
  shows "[a ^ totient (nat m) = 1] (mod m)"
proof -
  have "a ^ totient (nat m) mod m = 1 mod m"
    by (metis assms finite_Units m_gt_one mod_in_res_units one_cong totient_eq pow_cong units_power_order_eq_one)
  then show ?thesis
    using res_eq_to_cong by blast
qed

lemma euler_theorem:
  fixes a m :: nat
  assumes "coprime a m"
  shows "[a ^ totient m = 1] (mod m)"
proof (cases "m = 0 ∨ m = 1")
  case True
  then show ?thesis by auto
next
  case False
  with assms show ?thesis
    using residues.euler_theorem [of "int m" "int a"] cong_int_iff
    by (auto simp add: residues_def gcd_int_def) fastforce
qed

lemma fermat_theorem:
  fixes p a :: nat
  assumes "prime p" and "¬ p dvd a"
  shows "[a ^ (p - 1) = 1] (mod p)"
proof -
  from assms prime_imp_coprime [of p a] have "coprime a p"
    by (auto simp add: ac_simps)
  then have "[a ^ totient p = 1] (mod p)"
     by (rule euler_theorem)
  also have "totient p = p - 1"
    by (rule totient_prime) (rule assms)
  finally show ?thesis .
qed


subsection ‹Wilson's theorem›

lemma (in field) inv_pair_lemma: "x ∈ Units R ⟹ y ∈ Units R ⟹
    {x, inv x} ≠ {y, inv y} ⟹ {x, inv x} ∩ {y, inv y} = {}"
  by auto


lemma (in residues_prime) wilson_theorem1:
  assumes a: "p > 2"
  shows "[fact (p - 1) = (-1::int)] (mod p)"
proof -
  let ?Inverse_Pairs = "{{x, inv x}| x. x ∈ Units R - {𝟭, ⊖ 𝟭}}"
  have UR: "Units R = {𝟭, ⊖ 𝟭} ∪ ⋃?Inverse_Pairs"
    by auto
  have 11: "𝟭 ≠ ⊖ 𝟭"
    using a one_eq_neg_one by force
  have "(⨂i∈Units R. i) = (⨂i∈{𝟭, ⊖ 𝟭}. i) ⊗ (⨂i∈⋃?Inverse_Pairs. i)"
    apply (subst UR)
    apply (subst finprod_Un_disjoint)
    using inv_one inv_eq_neg_one_eq apply (auto intro!: funcsetI)+
    done
  also have "(⨂i∈{𝟭, ⊖ 𝟭}. i) = ⊖ 𝟭"
    by (simp add: 11)
  also have "(⨂i∈(⋃?Inverse_Pairs). i) = (⨂A∈?Inverse_Pairs. (⨂y∈A. y))"
    by (rule finprod_Union_disjoint) (auto simp: pairwise_def disjnt_def dest!: inv_eq_imp_eq)
  also have "… = 𝟭"
    apply (rule finprod_one_eqI)
    apply clarsimp
    apply (subst finprod_insert)
        apply auto
    apply (metis inv_eq_self)
    done
  finally have "(⨂i∈Units R. i) = ⊖ 𝟭"
    by simp
  also have "(⨂i∈Units R. i) = (⨂i∈Units R. i mod p)"
    by (rule finprod_cong') (auto simp: res_units_eq)
  also have "… = (∏i∈Units R. i) mod p"
    by (rule prod_cong) auto
  also have "… = fact (p - 1) mod p"
    using assms
    by (simp add: res_prime_units_eq int_prod zmod_int prod_int_eq fact_prod)
  finally have "fact (p - 1) mod p = ⊖ 𝟭" .
  then show ?thesis
    by (simp add: cong_def res_neg_eq res_one_eq zmod_int)
qed

lemma wilson_theorem:
  assumes "prime p"
  shows "[fact (p - 1) = - 1] (mod p)"
proof (cases "p = 2")
  case True
  then show ?thesis
    by (simp add: cong_def fact_prod)
next
  case False
  then show ?thesis
    using assms prime_ge_2_nat
    by (metis residues_prime.wilson_theorem1 residues_prime.intro le_eq_less_or_eq)
qed

text ‹
  This result can be transferred to the multiplicative group of
  ‹ℤ/pℤ› for ‹p› prime.›

lemma mod_nat_int_pow_eq:
  fixes n :: nat and p a :: int
  shows "a ≥ 0 ⟹ p ≥ 0 ⟹ (nat a ^ n) mod (nat p) = nat ((a ^ n) mod p)"
  by (simp add: nat_mod_as_int)

theorem residue_prime_mult_group_has_gen:
 fixes p :: nat
 assumes prime_p : "prime p"
 shows "∃a ∈ {1 .. p - 1}. {1 .. p - 1} = {a^i mod p|i . i ∈ UNIV}"
proof -
  have "p ≥ 2"
    using prime_gt_1_nat[OF prime_p] by simp
  interpret R: residues_prime p "residue_ring p"
    by (simp add: residues_prime_def prime_p)
  have car: "carrier (residue_ring (int p)) - {𝟬⇘residue_ring (int p)⇙} = {1 .. int p - 1}"
    by (auto simp add: R.zero_cong R.res_carrier_eq)

  have "x [^]⇘residue_ring (int p)⇙ i = x ^ i mod (int p)"
    if "x ∈ {1 .. int p - 1}" for x and i :: nat
    using that R.pow_cong[of x i] by auto
  moreover
  obtain a where a: "a ∈ {1 .. int p - 1}"
    and a_gen: "{1 .. int p - 1} = {a[^]⇘residue_ring (int p)⇙i|i::nat . i ∈ UNIV}"
    using field.finite_field_mult_group_has_gen[OF R.is_field]
    by (auto simp add: car[symmetric] carrier_mult_of)
  moreover
  have "nat ` {1 .. int p - 1} = {1 .. p - 1}" (is "?L = ?R")
  proof
    have "n ∈ ?R" if "n ∈ ?L" for n
      using that ‹p≥2› by force
    then show "?L ⊆ ?R" by blast
    have "n ∈ ?L" if "n ∈ ?R" for n
      using that ‹p≥2› by (auto intro: rev_image_eqI [of "int n"])
    then show "?R ⊆ ?L" by blast
  qed
  moreover
  have "nat ` {a^i mod (int p) | i::nat. i ∈ UNIV} = {nat a^i mod p | i . i ∈ UNIV}" (is "?L = ?R")
  proof
    have "x ∈ ?R" if "x ∈ ?L" for x
    proof -
      from that obtain i where i: "x = nat (a^i mod (int p))"
        by blast
      then have "x = nat a ^ i mod p"
        using mod_nat_int_pow_eq[of a "int p" i] a ‹p≥2› by auto
      with i show ?thesis by blast
    qed
    then show "?L ⊆ ?R" by blast
    have "x ∈ ?L" if "x ∈ ?R" for x
    proof -
      from that obtain i where i: "x = nat a^i mod p"
        by blast
      with mod_nat_int_pow_eq[of a "int p" i] a ‹p≥2› show ?thesis
        by auto
    qed
    then show "?R ⊆ ?L" by blast
  qed
  ultimately have "{1 .. p - 1} = {nat a^i mod p | i. i ∈ UNIV}"
    by presburger
  moreover from a have "nat a ∈ {1 .. p - 1}" by force
  ultimately show ?thesis ..
qed


subsection ‹Upper bound for the number of $n$-th roots›

lemma roots_mod_prime_bound:
  fixes n c p :: nat
  assumes "prime p" "n > 0"
  defines "A ≡ {x∈{..<p}. [x ^ n = c] (mod p)}"
  shows   "card A ≤ n"
proof -
  define R where "R = residue_ring (int p)"
  from assms(1) interpret residues_prime p R
    by unfold_locales (simp_all add: R_def)
  interpret R: UP_domain R "UP R" by (unfold_locales)

  let ?f = "UnivPoly.monom (UP R) 𝟭⇘R⇙ n ⊖⇘(UP R)⇙ UnivPoly.monom (UP R) (int (c mod p)) 0"
  have in_carrier: "int (c mod p) ∈ carrier R"
    using prime_gt_1_nat[OF assms(1)] by (simp add: R_def residue_ring_def)
  
  have "deg R ?f = n"
    using assms in_carrier by (simp add: R.deg_minus_eq)
  hence f_not_zero: "?f ≠ 𝟬⇘UP R⇙" using assms by (auto simp add : R.deg_nzero_nzero)
  have roots_bound: "finite {a ∈ carrier R. UnivPoly.eval R R id a ?f = 𝟬⇘R⇙} ∧
                     card {a ∈ carrier R. UnivPoly.eval R R id a ?f = 𝟬⇘R⇙} ≤ deg R ?f"
                    using finite in_carrier by (intro R.roots_bound[OF _ f_not_zero]) simp
  have subs: "{x ∈ carrier R. x [^]⇘R⇙ n = int (c mod p)} ⊆
                {a ∈ carrier R. UnivPoly.eval R R id a ?f = 𝟬⇘R⇙}"
    using in_carrier by (auto simp: R.evalRR_simps)
  then have "card {x ∈ carrier R. x [^]⇘R⇙ n = int (c mod p)} ≤
               card {a ∈ carrier R. UnivPoly.eval R R id a ?f = 𝟬⇘R⇙}"
    using finite by (intro card_mono) auto
  also have "… ≤ n"
    using ‹deg R ?f = n› roots_bound by linarith
  also {
    fix x assume "x ∈ carrier R"
    hence "x [^]⇘R⇙ n = (x ^ n) mod (int p)"
      by (subst pow_cong [symmetric]) (auto simp: R_def residue_ring_def)
  }
  hence "{x ∈ carrier R. x [^]⇘R⇙ n = int (c mod p)} = {x ∈ carrier R. [x ^ n = int c] (mod p)}"
    by (fastforce simp: cong_def zmod_int)
  also have "bij_betw int A {x ∈ carrier R. [x ^ n = int c] (mod p)}"
    by (rule bij_betwI[of int _ _ nat])
       (use cong_int_iff in ‹force simp: R_def residue_ring_def A_def›)+
  from bij_betw_same_card[OF this] have "card {x ∈ carrier R. [x ^ n = int c] (mod p)} = card A" ..
  finally show ?thesis .
qed


end