/- Copyright (c) 2014 Robert Lewis. All rights reserved. Released under Apache 2.0 license as described in the file LICENSE. Module: algebra.field Authors: Robert Lewis Structures with multiplicative and additive components, including division rings and fields. The development is modeled after Isabelle's library. -/ ---------------------------------------------------------------------------------------------------- import logic.eq logic.connectives data.unit data.sigma data.prod import algebra.function algebra.binary algebra.group algebra.ring open eq eq.ops namespace algebra variable {A : Type} structure division_ring [class] (A : Type) extends ring A, has_inv A, zero_ne_one_class A := (mul_inv_cancel : ∀{a}, a ≠ zero → mul a (inv a) = one) (inv_mul_cancel : ∀{a}, a ≠ zero → mul (inv a) a = one) section division_ring variables [s : division_ring A] {a b c : A} include s definition divide (a b : A) : A := a * b⁻¹ infix `/` := divide -- only in this file local attribute divide [reducible] theorem mul_inv_cancel (H : a ≠ 0) : a * a⁻¹ = 1 := division_ring.mul_inv_cancel H theorem inv_mul_cancel (H : a ≠ 0) : a⁻¹ * a = 1 := division_ring.inv_mul_cancel H theorem inv_eq_one_div : a⁻¹ = 1 / a := !one_mul⁻¹ theorem div_eq_mul_one_div : a / b = a * (1 / b) := by rewrite [↑divide, one_mul] theorem mul_one_div_cancel (H : a ≠ 0) : a * (1 / a) = 1 := by rewrite [-inv_eq_one_div, (mul_inv_cancel H)] theorem one_div_mul_cancel (H : a ≠ 0) : (1 / a) * a = 1 := by rewrite [-inv_eq_one_div, (inv_mul_cancel H)] theorem div_self (H : a ≠ 0) : a / a = 1 := mul_inv_cancel H theorem mul_div_assoc : (a * b) / c = a * (b / c) := !mul.assoc theorem one_div_ne_zero (H : a ≠ 0) : 1 / a ≠ 0 := assume H2 : 1 / a = 0, have C1 : 0 = 1, from symm (by rewrite [-(mul_one_div_cancel H), H2, mul_zero]), absurd C1 zero_ne_one -- the analogue in group is called inv_one theorem inv_one_is_one : 1⁻¹ = 1 := by rewrite [-mul_one, (inv_mul_cancel (ne.symm zero_ne_one))] theorem div_one : a / 1 = a := by rewrite [↑divide, inv_one_is_one, mul_one] theorem zero_div : 0 / a = 0 := !zero_mul -- note: integral domain has a "mul_ne_zero". Discrete fields are int domains. theorem mul_ne_zero' (Ha : a ≠ 0) (Hb : b ≠ 0) : a * b ≠ 0 := assume H : a * b = 0, have C1 : a = 0, by rewrite [-mul_one, -(mul_one_div_cancel Hb), -mul.assoc, H, zero_mul], absurd C1 Ha -- this belongs in ring? theorem mul_ne_zero_imp_ne_zero (H : a * b ≠ 0) : a ≠ 0 ∧ b ≠ 0 := have Ha : a ≠ 0, from (assume Ha1 : a = 0, have H1 : a * b = 0, by rewrite [Ha1, zero_mul], absurd H1 H), have Hb : b ≠ 0, from (assume Hb1 : b = 0, have H1 : a * b = 0, by rewrite [Hb1, mul_zero], absurd H1 H), and.intro Ha Hb theorem mul_ne_zero_comm (H : a * b ≠ 0) : b * a ≠ 0 := have H2 : a ≠ 0 ∧ b ≠ 0, from mul_ne_zero_imp_ne_zero H, mul_ne_zero' (and.right H2) (and.left H2) -- theorem inv_zero_imp_zero (H : a⁻¹ = 0) : a = 0 := -- classical? -- make "left" and "right" versions? theorem eq_one_div_of_mul_eq_one (H : a * b = 1) : b = 1 / a := have H2 : a ≠ 0, from (assume A : a = 0, have B : 0 = 1, by rewrite [-(zero_mul b), -A, H], absurd B zero_ne_one), show b = 1 / a, from symm (calc 1 / a = (1 / a) * 1 : mul_one ... = (1 / a) * (a * b) : H ... = (1 / a) * a * b : mul.assoc ... = 1 * b : one_div_mul_cancel H2 ... = b : one_mul) -- which one is left and which is right? theorem eq_one_div_of_mul_eq_one_left (H : b * a = 1) : b = 1 / a := have H2 : a ≠ 0, from (assume A : a = 0, have B : 0 = 1, from symm (calc 1 = b * a : symm H ... = b * 0 : A ... = 0 : mul_zero), absurd B zero_ne_one), show b = 1 / a, from symm (calc 1 / a = 1 * (1 / a) : one_mul ... = b * a * (1 / a) : H ... = b * (a * (1 / a)) : mul.assoc ... = b * 1 : mul_one_div_cancel H2 ... = b : mul_one) theorem one_div_mul_one_div (Ha : a ≠ 0) (Hb : b ≠ 0) : (1 / a) * (1 / b) = 1 / (b * a) := have H : (b * a) * ((1 / a) * (1 / b)) = 1, by rewrite [mul.assoc, -(mul.assoc a), (mul_one_div_cancel Ha), one_mul, (mul_one_div_cancel Hb)], eq_one_div_of_mul_eq_one H theorem one_div_neg_one_eq_neg_one : 1 / (-1) = -1 := have H : (-1) * (-1) = 1, by rewrite [-neg_eq_neg_one_mul, neg_neg], symm (eq_one_div_of_mul_eq_one H) -- this should be in ring theorem mul_neg_one_eq_neg : a * (-1) = -a := have H : a + a * -1 = 0, from calc a + a * -1 = a * 1 + a * -1 : mul_one ... = a * (1 + -1) : left_distrib ... = a * 0 : add.right_inv ... = 0 : mul_zero, symm (neg_eq_of_add_eq_zero H) theorem one_div_neg_eq_neg_one_div (H : a ≠ 0) : 1 / (- a) = - (1 / a) := have H1 : -1 ≠ 0, from (assume H2 : -1 = 0, absurd (symm (calc 1 = -(-1) : neg_neg ... = -0 : H2 ... = 0 : neg_zero)) zero_ne_one), calc 1 / (- a) = 1 / ((-1) * a) : neg_eq_neg_one_mul ... = (1 / a) * (1 / (- 1)) : one_div_mul_one_div H H1 ... = (1 / a) * (-1) : one_div_neg_one_eq_neg_one ... = - (1 / a) : mul_neg_one_eq_neg theorem div_neg_eq_neg_div (Ha : a ≠ 0) : b / (- a) = - (b / a) := calc b / (- a) = b * (1 / (- a)) : inv_eq_one_div ... = b * -(1 / a) : one_div_neg_eq_neg_one_div Ha ... = -(b * (1 / a)) : neg_mul_eq_mul_neg ... = - (b * a⁻¹) : inv_eq_one_div theorem neg_div (Ha : a ≠ 0) : (-b) / a = - (b / a) := by rewrite [neg_eq_neg_one_mul, mul_div_assoc, -neg_eq_neg_one_mul] theorem neg_div_neg_eq_div (Hb : b ≠ 0) : (-a) / (-b) = a / b := by rewrite [(div_neg_eq_neg_div Hb), (neg_div Hb), neg_neg] theorem div_div (H : a ≠ 0) : 1 / (1 / a) = a := symm (eq_one_div_of_mul_eq_one_left (mul_one_div_cancel H)) theorem eq_of_invs_eq (Ha : a ≠ 0) (Hb : b ≠ 0) (H : 1 / a = 1 / b) : a = b := by rewrite [-(div_div Ha), H, (div_div Hb)] -- oops, the analogous theorem in group is called inv_mul, but it *should* be called -- mul_inv, in which case, we will have to rename this one theorem mul_inv (Ha : a ≠ 0) (Hb : b ≠ 0) : (b * a)⁻¹ = a⁻¹ * b⁻¹ := have H1 : b * a ≠ 0, from mul_ne_zero' Hb Ha, eq.symm (calc a⁻¹ * b⁻¹ = (1 / a) * b⁻¹ : inv_eq_one_div ... = (1 / a) * (1 / b) : inv_eq_one_div ... = (1 / (b * a)) : one_div_mul_one_div Ha Hb ... = (b * a)⁻¹ : inv_eq_one_div) theorem mul_div_cancel (Hb : b ≠ 0) : a * b / b = a := by rewrite [↑divide, mul.assoc, (mul_inv_cancel Hb), mul_one] theorem div_mul_cancel (Hb : b ≠ 0) : a / b * b = a := by rewrite [↑divide, mul.assoc, (inv_mul_cancel Hb), mul_one] theorem div_add_div_same : a / c + b / c = (a + b) / c := !right_distrib⁻¹ theorem inv_mul_add_mul_inv_eq_inv_add_inv (Ha : a ≠ 0) (Hb : b ≠ 0) : (1 / a) * (a + b) * (1 / b) = 1 / a + 1 / b := by rewrite [(left_distrib (1 / a)), (one_div_mul_cancel Ha), right_distrib, one_mul, mul.assoc, (mul_one_div_cancel Hb), mul_one, add.comm] theorem inv_mul_sub_mul_inv_eq_inv_add_inv (Ha : a ≠ 0) (Hb : b ≠ 0) : (1 / a) * (b - a) * (1 / b) = 1 / a - 1 / b := by rewrite [(mul_sub_left_distrib (1 / a)), (one_div_mul_cancel Ha), mul_sub_right_distrib, one_mul, mul.assoc, (mul_one_div_cancel Hb), mul_one, one_mul] theorem div_eq_one_iff_eq (Hb : b ≠ 0) : a / b = 1 ↔ a = b := iff.intro (assume H1 : a / b = 1, symm (calc b = 1 * b : one_mul ... = a / b * b : H1 ... = a : div_mul_cancel Hb)) (assume H2 : a = b, calc a / b = b / b : H2 ... = 1 : div_self Hb) theorem eq_div_iff_mul_eq (Hc : c ≠ 0) : a = b / c ↔ a * c = b := iff.intro (assume H : a = b / c, by rewrite [H, (div_mul_cancel Hc)]) (assume H : a * c = b, by rewrite [-(mul_div_cancel Hc), H]) theorem add_div_eq_mul_add_div (Hc : c ≠ 0) : a + b / c = (a * c + b) / c := have H : (a + b / c) * c = a * c + b, by rewrite [right_distrib, (div_mul_cancel Hc)], (iff.elim_right (eq_div_iff_mul_eq Hc)) H -- There are many similar rules to these last two in the Isabelle library -- that haven't been ported yet. Do as necessary. end division_ring structure field [class] (A : Type) extends division_ring A, comm_ring A section field variables [s : field A] {a b c d: A} include s local attribute divide [reducible] theorem one_div_mul_one_div' (Ha : a ≠ 0) (Hb : b ≠ 0) : (1 / a) * (1 / b) = 1 / (a * b) := by rewrite [(one_div_mul_one_div Ha Hb), mul.comm b] theorem div_mul_right (Hb : b ≠ 0) (H : a * b ≠ 0) : a / (a * b) = 1 / b := let Ha : a ≠ 0 := and.left (mul_ne_zero_imp_ne_zero H) in symm (calc 1 / b = 1 * (1 / b) : one_mul ... = (a * a⁻¹) * (1 / b) : mul_inv_cancel Ha ... = a * (a⁻¹ * (1 / b)) : mul.assoc ... = a * ((1 / a) * (1 / b)) :inv_eq_one_div ... = a * (1 / (b * a)) : one_div_mul_one_div Ha Hb ... = a * (1 / (a * b)) : mul.comm ... = a * (a * b)⁻¹ : inv_eq_one_div) theorem div_mul_left (Ha : a ≠ 0) (H : a * b ≠ 0) : b / (a * b) = 1 / a := let H1 : b * a ≠ 0 := mul_ne_zero_comm H in by rewrite [mul.comm a, (div_mul_right Ha H1)] theorem mul_div_cancel_left (Ha : a ≠ 0) : a * b / a = b := by rewrite [mul.comm a, (mul_div_cancel Ha)] theorem mul_div_cancel' (Hb : b ≠ 0) : b * (a / b) = a := by rewrite [mul.comm, (div_mul_cancel Hb)] theorem one_div_add_one_div (Ha : a ≠ 0) (Hb : b ≠ 0) : 1 / a + 1 / b = (a + b) / (a * b) := have H [visible] : a * b ≠ 0, from (mul_ne_zero' Ha Hb), by rewrite [add.comm, -(div_mul_left Ha H), -(div_mul_right Hb H), ↑divide, -right_distrib] theorem div_mul_div (Hb : b ≠ 0) (Hd : d ≠ 0) : (a / b) * (c / d) = (a * c) / (b * d) := by rewrite [↑divide, 2 mul.assoc, (mul.comm b⁻¹), mul.assoc, (mul_inv Hd Hb)] theorem mul_div_mul_left (Hb : b ≠ 0) (Hc : c ≠ 0) : (c * a) / (c * b) = a / b := have H [visible] : c * b ≠ 0, from mul_ne_zero' Hc Hb, by rewrite [-(div_mul_div Hc Hb), (div_self Hc), one_mul] theorem mul_div_mul_right (Hb : b ≠ 0) (Hc : c ≠ 0) : (a * c) / (b * c) = a / b := by rewrite [(mul.comm a), (mul.comm b), (mul_div_mul_left Hb Hc)] theorem div_mul_eq_mul_div (Hc : c ≠ 0) : (b / c) * a = (b * a) / c := by rewrite [↑divide, mul.assoc, (mul.comm c⁻¹), -mul.assoc] -- this one is odd -- I am not sure what to call it, but again, the prefix is right theorem div_mul_eq_mul_div_comm (Hc : c ≠ 0) : (b / c) * a = b * (a / c) := by rewrite [(div_mul_eq_mul_div Hc), -(one_mul c), -(div_mul_div (ne.symm zero_ne_one) Hc), div_one, one_mul] theorem div_add_div (Hb : b ≠ 0) (Hd : d ≠ 0) : (a / b) + (c / d) = ((a * d) + (b * c)) / (b * d) := have H [visible] : b * d ≠ 0, from mul_ne_zero' Hb Hd, by rewrite [-(mul_div_mul_right Hb Hd), -(mul_div_mul_left Hd Hb), div_add_div_same] theorem div_sub_div (Hb : b ≠ 0) (Hd : d ≠ 0) : (a / b) - (c / d) = ((a * d) - (b * c)) / (b * d) := by rewrite [↑sub, neg_eq_neg_one_mul, -mul_div_assoc, (div_add_div Hb Hd), -mul.assoc, (mul.comm b), mul.assoc, -neg_eq_neg_one_mul] theorem mul_eq_mul_of_div_eq_div (Hb : b ≠ 0) (Hd : d ≠ 0) (H : a / b = c / d) : a * d = c * b := by rewrite [-mul_one, mul.assoc, (mul.comm d), -mul.assoc, -(div_self Hb), -(div_mul_eq_mul_div_comm Hb), H, (div_mul_eq_mul_div Hd), (div_mul_cancel Hd)] theorem one_div_div (Ha : a ≠ 0) (Hb : b ≠ 0) : 1 / (a / b) = b / a := have H : (a / b) * (b / a) = 1, from calc (a / b) * (b / a) = (a * b) / (b * a) : div_mul_div Hb Ha ... = (a * b) / (a * b) : mul.comm ... = 1 : div_self (mul_ne_zero' Ha Hb), symm (eq_one_div_of_mul_eq_one H) theorem div_div_eq_mul_div (Hb : b ≠ 0) (Hc : c ≠ 0) : a / (b / c) = (a * c) / b := by rewrite [div_eq_mul_one_div, (one_div_div Hb Hc), -mul_div_assoc] theorem div_div_eq_div_mul (Hb : b ≠ 0) (Hc : c ≠ 0) : (a / b) / c = a / (b * c) := by rewrite [div_eq_mul_one_div, (div_mul_div Hb Hc), mul_one] theorem div_div_div_div (Hb : b ≠ 0) (Hc : c ≠ 0) (Hd : d ≠ 0) : (a / b) / (c / d) = (a * d) / (b * c) := by rewrite [(div_div_eq_mul_div Hc Hd), (div_mul_eq_mul_div Hb), (div_div_eq_div_mul Hb Hc)] -- remaining to transfer from Isabelle fields: ordered fields end field structure discrete_field [class] (A : Type) extends field A := (decidable_equality : ∀x y : A, decidable (x = y)) section discrete_field variable [s : discrete_field A] include s variables {a b c : A} -- name clash with order definition decidable_eq' [instance] (a b : A) : decidable (a = b) := @discrete_field.decidable_equality A s a b theorem discrete_field.eq_zero_or_eq_zero_of_mul_eq_zero (x y : A) (H : x * y = 0) : x = 0 ∨ y = 0 := decidable.by_cases (assume H : x = 0, or.inl H) (assume H1 : x ≠ 0, or.inr (by rewrite [-one_mul, -(inv_mul_cancel H1), mul.assoc, H, mul_zero])) definition discrete_field.to_integral_domain [instance] [reducible] [coercion] : integral_domain A := ⦃ integral_domain, s, eq_zero_or_eq_zero_of_mul_eq_zero := discrete_field.eq_zero_or_eq_zero_of_mul_eq_zero⦄ example (H1 : a ≠ 0) (H2 : b ≠ 0) : a * b ≠ 0 := @mul_ne_zero A s a b H1 H2 theorem inv_zero_imp_zero (H : 1 / a = 0) : a = 0 := decidable.by_cases (assume Ha : a = 0, Ha) (assume Ha: a ≠ 0, false.elim ((one_div_ne_zero Ha) H)) end discrete_field end algebra