lean2/hott/algebra/category/nat_trans.hlean
Floris van Doorn 7e52c49dce feat(hott): many changes is the HoTT library
Prove that 'is_left_adjoint F' is a mere proposition, although this proof is commented out because it takes ~10 seconds
2015-09-01 15:17:46 -07:00

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/-
Copyright (c) 2015 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 .iso
open eq category functor is_trunc equiv sigma.ops sigma is_equiv function pi funext iso
structure nat_trans {C D : Precategory} (F G : C ⇒ D) :=
(natural_map : Π (a : C), hom (F a) (G a))
(naturality : Π {a b : C} (f : hom a b), G f ∘ natural_map a = natural_map b ∘ F f)
namespace nat_trans
infixl `⟹`:25 := nat_trans -- \==>
variables {B C D E : Precategory} {F G H I : C ⇒ D} {F' G' : D ⇒ E} {F'' G'' : E ⇒ B} {J : C ⇒ C}
attribute natural_map [coercion]
protected definition compose [constructor] (η : G ⟹ H) (θ : F ⟹ G) : F ⟹ H :=
nat_trans.mk
(λ a, η a ∘ θ a)
(λ a b f,
abstract calc
H f ∘ (η a ∘ θ a) = (H f ∘ η a) ∘ θ a : by rewrite assoc
... = (η b ∘ G f) ∘ θ a : by rewrite naturality
... = η b ∘ (G f ∘ θ a) : by rewrite assoc
... = η b ∘ (θ b ∘ F f) : by rewrite naturality
... = (η b ∘ θ b) ∘ F f : by rewrite assoc
end)
infixr `∘n`:60 := nat_trans.compose
protected definition id [reducible] {F : C ⇒ D} : nat_trans F F :=
mk (λa, id) (λa b f, !id_right ⬝ !id_left⁻¹)
protected definition ID [reducible] (F : C ⇒ D) : nat_trans F F :=
(@nat_trans.id C D F)
notation 1 := nat_trans.id
definition nat_trans_mk_eq {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)
(p : η₁ ~ η₂)
: nat_trans.mk η₁ nat₁ = nat_trans.mk η₂ nat₂ :=
apd011 nat_trans.mk (eq_of_homotopy p) !is_hprop.elim
definition nat_trans_eq {η₁ η₂ : F ⟹ G} : natural_map η₁ ~ natural_map η₂ → η₁ = η₂ :=
by induction η₁; induction η₂; apply nat_trans_mk_eq
protected definition assoc (η₃ : H ⟹ I) (η₂ : G ⟹ H) (η₁ : F ⟹ G) :
η₃ ∘n (η₂ ∘n η₁) = (η₃ ∘n η₂) ∘n η₁ :=
nat_trans_eq (λa, !assoc)
protected definition id_left (η : F ⟹ G) : 1 ∘n η = η :=
nat_trans_eq (λa, !id_left)
protected definition id_right (η : F ⟹ G) : η ∘n 1 = η :=
nat_trans_eq (λa, !id_right)
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,
-- TODO(Leo): investigate why we need to use rexact in the following line
{intro S, apply nat_trans.mk, rexact (S.2)},
fapply adjointify,
intro H,
fapply sigma.mk,
intro a, exact (H a),
intro a b f, exact (naturality H f),
intro η, apply nat_trans_eq, intro a, apply idp,
intro S,
fapply sigma_eq,
{ apply eq_of_homotopy, intro a, apply idp},
{ apply is_hprop.elimo}
end
definition is_hset_nat_trans [instance] : is_hset (F ⟹ G) :=
by apply is_trunc_is_equiv_closed; apply (equiv.to_is_equiv !nat_trans.sigma_char)
definition change_natural_map [constructor] (η : F ⟹ G) (f : Π (a : C), F a ⟶ G a)
(p : Πa, η a = f a) : F ⟹ G :=
nat_trans.mk f (λa b g, p a ▸ p b ▸ naturality η g)
definition nat_trans_functor_compose [constructor] (η : G ⟹ H) (F : E ⇒ C)
: G ∘f F ⟹ H ∘f F :=
nat_trans.mk
(λ a, η (F a))
(λ a b f, naturality η (F f))
definition functor_nat_trans_compose [constructor] (F : D ⇒ E) (η : G ⟹ H)
: F ∘f G ⟹ F ∘f H :=
nat_trans.mk
(λ a, F (η a))
(λ a b f, calc
F (H f) ∘ F (η a) = F (H f ∘ η a) : by rewrite respect_comp
... = F (η b ∘ G f) : by rewrite (naturality η f)
... = F (η b) ∘ F (G f) : by rewrite respect_comp)
definition nat_trans_id_functor_compose [constructor] (η : J ⟹ 1) (F : E ⇒ C)
: J ∘f F ⟹ F :=
nat_trans.mk
(λ a, η (F a))
(λ a b f, naturality η (F f))
definition id_nat_trans_functor_compose [constructor] (η : 1 ⟹ J) (F : E ⇒ C)
: F ⟹ J ∘f F :=
nat_trans.mk
(λ a, η (F a))
(λ a b f, naturality η (F f))
definition functor_nat_trans_id_compose [constructor] (F : C ⇒ D) (η : J ⟹ 1)
: F ∘f J ⟹ F :=
nat_trans.mk
(λ a, F (η a))
(λ a b f, calc
F f ∘ F (η a) = F (f ∘ η a) : by rewrite respect_comp
... = F (η b ∘ J f) : by rewrite (naturality η f)
... = F (η b) ∘ F (J f) : by rewrite respect_comp)
definition functor_id_nat_trans_compose [constructor] (F : C ⇒ D) (η : 1 ⟹ J)
: F ⟹ F ∘f J :=
nat_trans.mk
(λ a, F (η a))
(λ a b f, calc
F (J f) ∘ F (η a) = F (J f ∘ η a) : by rewrite respect_comp
... = F (η b ∘ f) : by rewrite (naturality η f)
... = F (η b) ∘ F f : by rewrite respect_comp)
infixr `∘nf`:62 := nat_trans_functor_compose
infixr `∘fn`:62 := functor_nat_trans_compose
infixr `∘n1f`:62 := nat_trans_id_functor_compose
infixr `∘1nf`:62 := id_nat_trans_functor_compose
infixr `∘f1n`:62 := functor_id_nat_trans_compose
infixr `∘fn1`:62 := functor_nat_trans_id_compose
definition nf_fn_eq_fn_nf_pt (η : F ⟹ G) (θ : F' ⟹ G') (c : C)
: (θ (G c)) ∘ (F' (η c)) = (G' (η c)) ∘ (θ (F c)) :=
(naturality θ (η c))⁻¹
variable (F')
definition nf_fn_eq_fn_nf_pt' (η : F ⟹ G) (θ : F'' ⟹ G'') (c : C)
: (θ (F' (G c))) ∘ (F'' (F' (η c))) = (G'' (F' (η c))) ∘ (θ (F' (F c))) :=
(naturality θ (F' (η c)))⁻¹
variable {F'}
definition nf_fn_eq_fn_nf (η : F ⟹ G) (θ : F' ⟹ G')
: (θ ∘nf G) ∘n (F' ∘fn η) = (G' ∘fn η) ∘n (θ ∘nf F) :=
nat_trans_eq (λ c, nf_fn_eq_fn_nf_pt η θ c)
definition fn_n_distrib (F' : D ⇒ E) (η : G ⟹ H) (θ : F ⟹ G)
: F' ∘fn (η ∘n θ) = (F' ∘fn η) ∘n (F' ∘fn θ) :=
nat_trans_eq (λc, by apply respect_comp)
definition n_nf_distrib (η : G ⟹ H) (θ : F ⟹ G) (F' : B ⇒ C)
: (η ∘n θ) ∘nf F' = (η ∘nf F') ∘n (θ ∘nf F') :=
nat_trans_eq (λc, idp)
definition fn_id (F' : D ⇒ E) : F' ∘fn nat_trans.ID F = 1 :=
nat_trans_eq (λc, by apply respect_id)
definition id_nf (F' : B ⇒ C) : nat_trans.ID F ∘nf F' = 1 :=
nat_trans_eq (λc, idp)
definition id_fn (η : G ⟹ H) (c : C) : (1 ∘fn η) c = η c :=
idp
definition nf_id (η : G ⟹ H) (c : C) : (η ∘nf 1) c = η c :=
idp
definition nat_trans_of_eq [reducible] (p : F = G) : F ⟹ G :=
nat_trans.mk (λc, hom_of_eq (ap010 to_fun_ob p c))
(λa b f, eq.rec_on p (!id_right ⬝ !id_left⁻¹))
end nat_trans