lean2/library/algebra/function.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.
Author: Leonardo de Moura, Jeremy Avigad, Haitao Zhang
General operations on functions.
-/
import logic.cast
namespace function
variables {A : Type} {B : Type} {C : Type} {D : Type} {E : Type}
definition compose [reducible] [unfold-f] (f : B → C) (g : A → B) : A → C :=
λx, f (g x)
definition compose_right [reducible] [unfold-f] (f : B → B → B) (g : A → B) : B → A → B :=
λ b a, f b (g a)
definition compose_left [reducible] [unfold-f] (f : B → B → B) (g : A → B) : A → B → B :=
λ a b, f (g a) b
definition id [reducible] [unfold-f] (a : A) : A :=
a
definition on_fun [reducible] [unfold-f] (f : B → B → C) (g : A → B) : A → A → C :=
λx y, f (g x) (g y)
definition combine [reducible] [unfold-f] (f : A → B → C) (op : C → D → E) (g : A → B → D)
: A → B → E :=
λx y, op (f x y) (g x y)
definition const [reducible] [unfold-f] (B : Type) (a : A) : B → A :=
λx, a
definition dcompose [reducible] [unfold-f] {B : A → Type} {C : Π {x : A}, B x → Type}
(f : Π {x : A} (y : B x), C y) (g : Πx, B x) : Πx, C (g x) :=
λx, f (g x)
definition swap [reducible] [unfold-f] {C : A → B → Type} (f : Πx y, C x y) : Πy x, C x y :=
λy x, f x y
definition app [reducible] {B : A → Type} (f : Πx, B x) (x : A) : B x :=
f x
definition curry [reducible] [unfold-f] : (A × B → C) → A → B → C :=
λ f a b, f (a, b)
definition uncurry [reducible] [unfold-c 5] : (A → B → C) → (A × B → C) :=
λ f p, match p with (a, b) := f a b end
theorem curry_uncurry (f : A → B → C) : curry (uncurry f) = f :=
rfl
theorem uncurry_curry (f : A × B → C) : uncurry (curry f) = f :=
funext (λ p, match p with (a, b) := rfl end)
precedence `∘'`:60
precedence `on`:1
precedence `$`:1
infixr ∘ := compose
infixr ∘' := dcompose
infixl on := on_fun
infixr $ := app
notation f `-[` op `]-` g := combine f op g
lemma left_id (f : A → B) : id ∘ f = f := rfl
lemma right_id (f : A → B) : f ∘ id = f := rfl
theorem compose.assoc (f : C → D) (g : B → C) (h : A → B) : (f ∘ g) ∘ h = f ∘ (g ∘ h) := rfl
theorem compose.left_id (f : A → B) : id ∘ f = f := rfl
theorem compose.right_id (f : A → B) : f ∘ id = f := rfl
theorem compose_const_right (f : B → C) (b : B) : f ∘ (const A b) = const A (f b) := rfl
definition injective (f : A → B) : Prop := ∀ ⦃a₁ a₂⦄, f a₁ = f a₂ → a₁ = a₂
theorem injective_compose {g : B → C} {f : A → B} (Hg : injective g) (Hf : injective f) :
injective (g ∘ f) :=
take a₁ a₂, assume Heq, Hf (Hg Heq)
definition surjective (f : A → B) : Prop := ∀ b, ∃ a, f a = b
theorem surjective_compose {g : B → C} {f : A → B} (Hg : surjective g) (Hf : surjective f) :
surjective (g ∘ f) :=
take c,
obtain b (Hb : g b = c), from Hg c,
obtain a (Ha : f a = b), from Hf b,
exists.intro a (eq.trans (congr_arg g Ha) Hb)
definition bijective (f : A → B) := injective f ∧ surjective f
theorem bijective_compose {g : B → C} {f : A → B} (Hg : bijective g) (Hf : bijective f) :
bijective (g ∘ f) :=
obtain Hginj Hgsurj, from Hg,
obtain Hfinj Hfsurj, from Hf,
and.intro (injective_compose Hginj Hfinj) (surjective_compose Hgsurj Hfsurj)
-- g is a left inverse to f
definition left_inverse (g : B → A) (f : A → B) : Prop := ∀x, g (f x) = x
definition has_left_inverse (f : A → B) : Prop := ∃ finv : B → A, left_inverse finv f
-- g is a right inverse to f
definition right_inverse (g : B → A) (f : A → B) : Prop := left_inverse f g
definition has_right_inverse (f : A → B) : Prop := ∃ finv : B → A, right_inverse finv f
theorem injective_of_has_left_inverse {f : A → B} : has_left_inverse f → injective f :=
assume h, take a b, assume faeqfb,
obtain (finv : B → A) (inv : left_inverse finv f), from h,
calc a = finv (f a) : by rewrite inv
... = finv (f b) : faeqfb
... = b : by rewrite inv
theorem right_inverse_of_injective_of_left_inverse {f : A → B} {g : B → A}
(injf : injective f) (lfg : left_inverse f g) :
right_inverse f g :=
take x,
have H : f (g (f x)) = f x, from lfg (f x),
injf H
theorem surjective_of_has_right_inverse {f : A → B} : has_right_inverse f → surjective f :=
assume h, take b,
obtain (finv : B → A) (inv : right_inverse finv f), from h,
let a : A := finv b in
have h : f a = b, from calc
f a = (f ∘ finv) b : rfl
... = id b : by rewrite inv
... = b : rfl,
exists.intro a h
theorem left_inverse_of_surjective_of_right_inverse {f : A → B} {g : B → A}
(surjf : surjective f) (rfg : right_inverse f g) :
left_inverse f g :=
take y,
obtain x (Hx : f x = y), from surjf y,
calc
f (g y) = f (g (f x)) : Hx
... = f x : rfg
... = y : Hx
theorem injective_id : injective (@id A) := take a₁ a₂ H, H
theorem surjective_id : surjective (@id A) := take a, exists.intro a rfl
theorem bijective_id : bijective (@id A) := and.intro injective_id surjective_id
end function
-- copy reducible annotations to top-level
export [reduce-hints] [unfold-hints] function