lean2/hott/algebra/precategory/natural_transformation.hlean
2015-01-03 22:31:39 -08:00

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-- Copyright (c) 2014 Floris van Doorn. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Author: Floris van Doorn, Jakob von Raumer
import .functor types.pi
open eq precategory functor truncation equiv sigma.ops sigma is_equiv function pi
inductive natural_transformation {C D : Precategory} (F G : C ⇒ D) : Type :=
mk : Π (η : Π (a : C), hom (F a) (G a))
(nat : Π {a b : C} (f : hom a b), G f ∘ η a = η b ∘ F f),
natural_transformation F G
infixl `⟹`:25 := natural_transformation -- \==>
namespace natural_transformation
variables {C D : Precategory} {F G H I : functor C D}
definition natural_map [coercion] (η : F ⟹ G) : Π (a : C), F a ⟶ G a :=
rec (λ x y, x) η
theorem naturality (η : F ⟹ G) : Π⦃a b : C⦄ (f : a ⟶ b), G f ∘ η a = η b ∘ F f :=
rec (λ x y, y) η
protected definition compose (η : G ⟹ H) (θ : F ⟹ G) : F ⟹ H :=
natural_transformation.mk
(λ a, η a ∘ θ a)
(λ a b f,
calc
H f ∘ (η a ∘ θ a) = (H f ∘ η a) ∘ θ a : assoc
... = (η b ∘ G f) ∘ θ a : naturality η f
... = η b ∘ (G f ∘ θ a) : assoc
... = η b ∘ (θ b ∘ F f) : naturality θ f
... = (η b ∘ θ b) ∘ F f : assoc)
infixr `∘n`:60 := compose
protected theorem congr
{C : Precategory} {D : Precategory}
(F G : C ⇒ D)
(η₁ η₂ : Π (a : C), hom (F a) (G a))
(nat₁ : Π (a b : C) (f : hom a b), G f ∘ η₁ a = η₁ b ∘ F f)
(nat₂ : Π (a b : C) (f : hom a b), G f ∘ η₂ a = η₂ b ∘ F f)
(p₁ : η₁ = η₂) (p₂ : p₁ ▹ nat₁ = nat₂)
: @natural_transformation.mk C D F G η₁ nat₁ = @natural_transformation.mk C D F G η₂ nat₂
:=
begin
apply (dcongr_arg2 (@natural_transformation.mk C D F G) p₁ p₂),
end
protected definition assoc (η₃ : H ⟹ I) (η₂ : G ⟹ H) (η₁ : F ⟹ G) :
η₃ ∘n (η₂ ∘n η₁) = (η₃ ∘n η₂) ∘n η₁ :=
begin
apply (rec_on η₃), intros (η₃1, η₃2),
apply (rec_on η₂), intros (η₂1, η₂2),
apply (rec_on η₁), intros (η₁1, η₁2),
fapply natural_transformation.congr,
apply funext.path_pi, intro a,
apply assoc,
apply funext.path_pi, intro a,
apply funext.path_pi, intro b,
apply funext.path_pi, intro f,
apply (@is_hset.elim), apply !homH,
end
protected definition id {C D : Precategory} {F : functor C D} : natural_transformation F F :=
mk (λa, id) (λa b f, !id_right ⬝ (!id_left⁻¹))
protected definition ID {C D : Precategory} (F : functor C D) : natural_transformation F F :=
id
protected definition id_left (η : F ⟹ G) : id ∘n η = η :=
begin
apply (rec_on η), intros (η₁, nat₁),
fapply (natural_transformation.congr F G),
apply funext.path_pi, intro a,
apply id_left,
apply funext.path_pi, intro a,
apply funext.path_pi, intro b,
apply funext.path_pi, intro f,
apply (@is_hset.elim), apply !homH,
end
protected definition id_right (η : F ⟹ G) : η ∘n id = η :=
begin
apply (rec_on η), intros (η₁, nat₁),
fapply (natural_transformation.congr F G),
apply funext.path_pi, intro a,
apply id_right,
apply funext.path_pi, intro a,
apply funext.path_pi, intro b,
apply funext.path_pi, intro f,
apply (@is_hset.elim), apply !homH,
end
--set_option pp.implicit true
protected definition sigma_char (F G : C ⇒ D) :
(Σ (η : Π (a : C), hom (F a) (G a)), Π (a b : C) (f : hom a b), G f ∘ η a = η b ∘ F f) ≃ (F ⟹ G) :=
begin
fapply equiv.mk,
intro S, apply natural_transformation.mk, exact (S.2),
fapply adjointify,
intro H,
fapply sigma.mk,
intro a, exact (H a),
intros (a, b, f), exact (naturality H f),
intro H, apply (natural_transformation.rec_on H),
intros (eta, nat), unfold function.id,
fapply natural_transformation.congr,
apply idp,
repeat ( apply funext.path_pi ; intro a ),
apply (@is_hset.elim), apply !homH,
intro S,
fapply sigma.path,
apply funext.path_pi, intro a,
apply idp,
repeat ( apply funext.path_pi ; intro a ),
apply (@is_hset.elim), apply !homH,
end
protected definition to_hset : is_hset (F ⟹ G) :=
begin
apply trunc_equiv, apply (equiv.to_is_equiv !sigma_char),
apply trunc_sigma,
apply trunc_pi, intro a, exact (@homH (objects D) _ (F a) (G a)),
intro η, apply trunc_pi, intro a,
apply trunc_pi, intro b, apply trunc_pi, intro f,
apply succ_is_trunc, apply trunc_succ, exact (@homH (objects D) _ (F a) (G b)),
end
end natural_transformation