Theory Conjugate

(*  
    Author:      René Thiemann 
                 Akihisa Yamada
    License:     BSD
*)
theory Conjugate
  imports HOL.Complex "HOL-Library.Complex_Order"
begin

class conjugate =
  fixes conjugate :: "'a ⇒ 'a"
  assumes conjugate_id[simp]: "conjugate (conjugate a) = a"
      and conjugate_cancel_iff[simp]: "conjugate a = conjugate b ⟷ a = b"

class conjugatable_ring = ring + conjugate +
  assumes conjugate_dist_mul: "conjugate (a * b) = conjugate a * conjugate b"
      and conjugate_dist_add: "conjugate (a + b) = conjugate a + conjugate b"
      and conjugate_neg: "conjugate (-a) = - conjugate a"
      and conjugate_zero[simp]: "conjugate 0 = 0"
begin
  lemma conjugate_zero_iff[simp]: "conjugate a = 0 ⟷ a = 0"
    using conjugate_cancel_iff[of _ 0, unfolded conjugate_zero].
end

class conjugatable_field = conjugatable_ring + field

lemma sum_conjugate:
  fixes f :: "'b ⇒ 'a :: conjugatable_ring"
  assumes finX: "finite X"
  shows "conjugate (sum f X) = sum (λx. conjugate (f x)) X"
  using finX by (induct set:finite, auto simp: conjugate_dist_add)

class conjugatable_ordered_ring = conjugatable_ring + ordered_comm_monoid_add +
  assumes conjugate_square_positive: "a * conjugate a ≥ 0"

class conjugatable_ordered_field = conjugatable_ordered_ring + field
begin
  subclass conjugatable_field..
end

lemma conjugate_square_0:
  fixes a :: "'a :: {conjugatable_ordered_ring, semiring_no_zero_divisors}"
  shows "a * conjugate a = 0 ⟹ a = 0" by auto


subsection ‹Instantiations›

instantiation complex :: conjugatable_ordered_field
begin
  definition [simp]: "conjugate ≡ cnj"
  
instance
  by intro_classes (auto simp: less_eq_complex_def)

end

instantiation real :: conjugatable_ordered_field
begin
  definition [simp]: "conjugate (x::real) ≡ x"
  instance by (intro_classes, auto)
end

instantiation rat :: conjugatable_ordered_field
begin
  definition [simp]: "conjugate (x::rat) ≡ x"
  instance by (intro_classes, auto)
end

instantiation int :: conjugatable_ordered_ring
begin
  definition [simp]: "conjugate (x::int) ≡ x"
  instance by (intro_classes, auto)
end

lemma conjugate_square_eq_0 [simp]:
  fixes x :: "'a :: {conjugatable_ring,semiring_no_zero_divisors}"
  shows "x * conjugate x = 0 ⟷ x = 0" "conjugate x * x = 0 ⟷ x = 0"
  by auto

lemma conjugate_square_greater_0 [simp]:
  fixes x :: "'a :: {conjugatable_ordered_ring,ring_no_zero_divisors}"
  shows "x * conjugate x > 0 ⟷ x ≠ 0" 
  using conjugate_square_positive[of x]
  by (auto simp: le_less)

lemma conjugate_square_smaller_0 [simp]:
  fixes x :: "'a :: {conjugatable_ordered_ring,ring_no_zero_divisors}"
  shows "¬ x * conjugate x < 0"
  using conjugate_square_positive[of x] by auto

end