lean2/library/logic/relation.lean

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-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Authors: Leonardo de Moura
import logic.eq
-- TODO(Leo): remove duplication between this file and algebra/relation.lean
-- We need some of the following definitions asap when "initializing" Lean.
variables {A B : Type} (R : B → B → Prop)
infix [local] `≺`:50 := R
definition reflexive := ∀x, x ≺ x
definition symmetric := ∀⦃x y⦄, x ≺ y → y ≺ x
definition transitive := ∀⦃x y z⦄, x ≺ y → y ≺ z → x ≺ z
definition irreflexive := ∀x, ¬ x ≺ x
definition anti_symmetric := ∀⦃x y⦄, x ≺ y → y ≺ x → x = y
definition empty_relation := λa₁ a₂ : A, false
definition subrelation (Q R : B → B → Prop) := ∀⦃x y⦄, Q x y → R x y
definition inv_image (f : A → B) : A → A → Prop :=
λa₁ a₂, f a₁ ≺ f a₂
theorem inv_image.trans (f : A → B) (H : transitive R) : transitive (inv_image R f) :=
λ (a₁ a₂ a₃ : A) (H₁ : inv_image R f a₁ a₂) (H₂ : inv_image R f a₂ a₃), H H₁ H₂
theorem inv_image.irreflexive (f : A → B) (H : irreflexive R) : irreflexive (inv_image R f) :=
λ (a : A) (H₁ : inv_image R f a a), H (f a) H₁
inductive tc {A : Type} (R : A → A → Prop) : A → A → Prop :=
base : ∀a b, R a b → tc R a b,
trans : ∀a b c, tc R a b → tc R b c → tc R a c