lean2/hott/algebra/category/adjoint.hlean

/-
Copyright (c) 2015 Floris van Doorn. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Floris van Doorn
-/

import algebra.category.constructions function arity

open category functor nat_trans eq is_trunc iso equiv prod trunc function pi is_equiv

namespace category
  variables {C D : Precategory} {F : C ⇒ D} {G : D ⇒ C}

  -- TODO: define a structure "adjoint" and then define
  -- structure is_left_adjoint (F : C ⇒ D) :=
  --   (G : D ⇒ C) -- G
  --   (is_adjoint : adjoint F G)

  structure is_left_adjoint [class] (F : C ⇒ D) :=
    (G : D ⇒ C)
    (η : 1 ⟹ G ∘f F)
    (ε : F ∘f G ⟹ 1)
    (H : Π(c : C), ε (F c) ∘ F (η c) = ID (F c))
    (K : Π(d : D), G (ε d) ∘ η (G d) = ID (G d))

  abbreviation right_adjoint := @is_left_adjoint.G
  abbreviation unit          := @is_left_adjoint.η
  abbreviation counit        := @is_left_adjoint.ε

  structure is_equivalence [class] (F : C ⇒ D) extends is_left_adjoint F :=
    mk' ::
    (is_iso_unit : is_iso η)
    (is_iso_counit : is_iso ε)

  abbreviation inverse := @is_equivalence.G
  postfix ⁻¹ := inverse
  --a second notation for the inverse, which is not overloaded
  postfix [parsing_only] `⁻¹F`:std.prec.max_plus := inverse

  --TODO: review and change
  definition faithful [class] (F : C ⇒ D) := Π⦃c c' : C⦄ ⦃f f' : c ⟶ c'⦄, F f = F f' → f = f'
  definition full [class] (F : C ⇒ D) := Π⦃c c' : C⦄, is_surjective (@(to_fun_hom F) c c')
  definition fully_faithful [class] (F : C ⇒ D) := Π(c c' : C), is_equiv (@(to_fun_hom F) c c')
  definition split_essentially_surjective [class] (F : C ⇒ D) := Π(d : D), Σ(c : C), F c ≅ d
  definition essentially_surjective [class] (F : C ⇒ D) := Π(d : D), ∃(c : C), F c ≅ d
  definition is_weak_equivalence [class] (F : C ⇒ D) := fully_faithful F × essentially_surjective F
  definition is_isomorphism [class] (F : C ⇒ D) := fully_faithful F × is_equiv (to_fun_ob F)

  structure equivalence (C D : Precategory) :=
    (to_functor : C ⇒ D)
    (struct : is_equivalence to_functor)

  structure isomorphism (C D : Precategory) :=
    (to_functor : C ⇒ D)
    (struct : is_isomorphism to_functor)
  --   infix `⊣`:55 := adjoint

  infix ` ⋍ `:25 := equivalence -- \backsimeq or \equiv
  infix ` ≌ `:25 := isomorphism -- \backcong or \iso

  definition is_equiv_of_fully_faithful [instance] [reducible] (F : C ⇒ D) [H : fully_faithful F]
    (c c' : C) : is_equiv (@(to_fun_hom F) c c') :=
  !H

  definition hom_inv [reducible] (F : C ⇒ D) [H : fully_faithful F] (c c' : C) (f : F c ⟶ F c')
    : c ⟶ c' :=
  (to_fun_hom F)⁻¹ᶠ f

  definition hom_equiv_F_hom_F [constructor] (F : C ⇒ D)
    [H : fully_faithful F] (c c' : C) : (c ⟶ c') ≃ (F c ⟶ F c') :=
  equiv.mk _ !H

  definition iso_of_F_iso_F (F : C ⇒ D)
    [H : fully_faithful F] (c c' : C) (g : F c ≅ F c') : c ≅ c' :=
  begin
    induction g with g G, induction G with h p q, fapply iso.MK,
      { rexact (to_fun_hom F)⁻¹ᶠ g},
      { rexact (to_fun_hom F)⁻¹ᶠ h},
      { exact abstract begin
        apply eq_of_fn_eq_fn' (to_fun_hom F),
        rewrite [respect_comp, respect_id,
                 right_inv (to_fun_hom F), right_inv (to_fun_hom F), p],
        end end},
      { exact abstract begin
        apply eq_of_fn_eq_fn' (to_fun_hom F),
        rewrite [respect_comp, respect_id,
                 right_inv (to_fun_hom F), right_inv (@(to_fun_hom F) c' c), q],
        end end}
  end

  definition iso_equiv_F_iso_F [constructor] (F : C ⇒ D)
    [H : fully_faithful F] (c c' : C) : (c ≅ c') ≃ (F c ≅ F c') :=
  begin
    fapply equiv.MK,
    { exact to_fun_iso F},
    { apply iso_of_F_iso_F},
    { exact abstract begin
      intro f, induction f with f F', induction F' with g p q, apply iso_eq,
      esimp [iso_of_F_iso_F], apply right_inv end end},
    { exact abstract begin
      intro f, induction f with f F', induction F' with g p q, apply iso_eq,
      esimp [iso_of_F_iso_F], apply right_inv end end},
  end

  definition is_iso_unit [instance] (F : C ⇒ D) [H : is_equivalence F] : is_iso (unit F) :=
  !is_equivalence.is_iso_unit

  definition is_iso_counit [instance] (F : C ⇒ D) [H : is_equivalence F] : is_iso (counit F) :=
  !is_equivalence.is_iso_counit

  theorem is_hprop_is_left_adjoint {C : Category} {D : Precategory} (F : C ⇒ D)
    : is_hprop (is_left_adjoint F) :=
  begin
    apply is_hprop.mk,
    intro G G', cases G with G η ε H K, cases G' with G' η' ε' H' K',
    assert lem₁ : Π(p : G = G'), p ▸ η = η' → p ▸ ε = ε'
      → is_left_adjoint.mk G η ε H K = is_left_adjoint.mk G' η' ε' H' K',
    { intros p q r, induction p, induction q, induction r, esimp,
      apply apd011 (is_left_adjoint.mk G η ε) !is_hprop.elim !is_hprop.elim},
    assert lem₂ : Π (d : carrier D),
                    (to_fun_hom G (natural_map ε' d) ∘
                    natural_map η (to_fun_ob G' d)) ∘
                    to_fun_hom G' (natural_map ε d) ∘
                    natural_map η' (to_fun_ob G d) = id,
    { intro d, esimp,
      rewrite [assoc],
      rewrite [-assoc (G (ε' d))],
      esimp, rewrite [nf_fn_eq_fn_nf_pt' G' ε η d],
      esimp, rewrite [assoc],
      esimp, rewrite [-assoc],
      rewrite [↑functor.compose, -respect_comp G],
      rewrite [nf_fn_eq_fn_nf_pt ε ε' d,nf_fn_eq_fn_nf_pt η' η (G d),▸*],
      rewrite [respect_comp G],
      rewrite [assoc,▸*,-assoc (G (ε d))],
      rewrite [↑functor.compose, -respect_comp G],
      rewrite [H' (G d)],
      rewrite [respect_id,▸*,id_right],
      apply K},
    assert lem₃ : Π (d : carrier D),
                    (to_fun_hom G' (natural_map ε d) ∘
                    natural_map η' (to_fun_ob G d)) ∘
                    to_fun_hom G (natural_map ε' d) ∘
                    natural_map η (to_fun_ob G' d) = id,
    { intro d, esimp,
      rewrite [assoc, -assoc (G' (ε d))],
      esimp, rewrite [nf_fn_eq_fn_nf_pt' G ε' η' d],
      esimp, rewrite [assoc], esimp, rewrite [-assoc],
      rewrite [↑functor.compose, -respect_comp G'],
      rewrite [nf_fn_eq_fn_nf_pt ε' ε d,nf_fn_eq_fn_nf_pt η η' (G' d)],
      esimp,
      rewrite [respect_comp G'],
      rewrite [assoc,▸*,-assoc (G' (ε' d))],
      rewrite [↑functor.compose, -respect_comp G'],
      rewrite [H (G' d)],
      rewrite [respect_id,▸*,id_right],
      apply K'},
    fapply lem₁,
    { fapply functor.eq_of_pointwise_iso,
      { fapply change_natural_map,
        { exact (G' ∘fn1 ε) ∘n !assoc_natural_rev ∘n (η' ∘1nf G)},
        { intro d, exact (G' (ε d) ∘ η' (G d))},
        { intro d, exact ap (λx, _ ∘ x) !id_left}},
      { intro d, fconstructor,
        { exact (G (ε' d) ∘ η (G' d))},
        { exact lem₂ d },
        { exact lem₃ d }}},
    { clear lem₁, refine transport_hom_of_eq_right _ η ⬝ _,
      krewrite hom_of_eq_compose_right,
      rewrite functor.hom_of_eq_eq_of_pointwise_iso,
      apply nat_trans_eq, intro c, esimp,
      refine !assoc⁻¹ ⬝ ap (λx, _ ∘ x) (nf_fn_eq_fn_nf_pt η η' c) ⬝ !assoc ⬝ _,
      esimp, rewrite [-respect_comp G',H c,respect_id G',▸*,id_left]},
   { clear lem₁, refine transport_hom_of_eq_left _ ε ⬝ _,
     krewrite inv_of_eq_compose_left,
     rewrite functor.inv_of_eq_eq_of_pointwise_iso,
     apply nat_trans_eq, intro d, esimp,
     krewrite [respect_comp],
     rewrite [assoc,nf_fn_eq_fn_nf_pt ε' ε d,-assoc,▸*,H (G' d),id_right]}
   end

  definition full_of_fully_faithful (H : fully_faithful F) : full F :=
  λc c' g, tr (fiber.mk ((@(to_fun_hom F) c c')⁻¹ᶠ g) !right_inv)

  definition faithful_of_fully_faithful (H : fully_faithful F) : faithful F :=
  λc c' f f' p, is_injective_of_is_embedding p

  definition fully_faithful_of_full_of_faithful (H : faithful F) (K : full F) : fully_faithful F :=
  begin
    intro c c',
    apply is_equiv_of_is_surjective_of_is_embedding,
    { apply is_embedding_of_is_injective,
      intros f f' p, exact H p},
    { apply K}
  end

  definition split_essentially_surjective_of_is_equivalence (F : C ⇒ D) [H : is_equivalence F]
    : split_essentially_surjective F :=
  begin
   intro d, fconstructor,
   { exact F⁻¹ d},
   { exact componentwise_iso (@(iso.mk (counit F)) !is_iso_counit) d}
  end

  definition reflect_is_iso [constructor] (F : C ⇒ D) [H : fully_faithful F] {c c' : C} (f : c ⟶ c')
    [H : is_iso (F f)] : is_iso f :=
  begin
    fconstructor,
    { exact (to_fun_hom F)⁻¹ᶠ (F f)⁻¹},
    { apply eq_of_fn_eq_fn' (to_fun_hom F),
      rewrite [respect_comp,right_inv (to_fun_hom F),respect_id,left_inverse]},
    { apply eq_of_fn_eq_fn' (to_fun_hom F),
      rewrite [respect_comp,right_inv (to_fun_hom F),respect_id,right_inverse]},
  end

  definition reflect_iso [constructor] (F : C ⇒ D) [H : fully_faithful F] {c c' : C}
    (f : F c ≅ F c') : c ≅ c' :=
  begin
    fconstructor,
    { exact (to_fun_hom F)⁻¹ᶠ f},
    { assert H : is_iso (F ((to_fun_hom F)⁻¹ᶠ f)),
      { have H' : is_iso (to_hom f), from _, exact (right_inv (to_fun_hom F) (to_hom f))⁻¹ ▸ H'},
      exact reflect_is_iso F _},
  end

  theorem reflect_inverse (F : C ⇒ D) [H : fully_faithful F] {c c' : C} (f : c ⟶ c')
    [H : is_iso f] : (to_fun_hom F)⁻¹ᶠ (F f)⁻¹ = f⁻¹ :=
  inverse_eq_inverse (idp : to_hom (@(iso.mk f) (reflect_is_iso F f)) = f)

/-
  section
    variables (η : Πc, G (F c) ≅ c) (ε : Πd, F (G d) ≅ d) -- we need some kind of naturality
    include η ε
    --definition inverse_of_unit_counit

     private definition adj_η (c : C) : G (F c) ≅ c :=
     to_fun_iso G (to_fun_iso F (η c)⁻¹ⁱ) ⬝i to_fun_iso G (ε (F c)) ⬝i η c
     open iso

     private theorem adjointify_adjH (c : C) :
       to_hom (ε (F c)) ∘ F (to_hom (adj_η η ε c)⁻¹ⁱ) = id  :=
     begin
       exact sorry
     end

     private theorem adjointify_adjK (d : D) :
       G (to_hom (ε d)) ∘ to_hom (adj_η η ε (G d))⁻¹ⁱ = id :=
     begin
       exact sorry
     end

    variables (F G)
    definition is_equivalence.mk : is_equivalence F :=
    begin
      fconstructor,
      { exact G},
      { }
    end

  end

  definition fully_faithful_of_is_equivalence (F : C ⇒ D) [H : is_equivalence F]
    : fully_faithful F :=
  begin
    intro c c',
    fapply adjointify,
    { intro g, exact natural_map (@(iso.inverse (unit F)) !is_iso_unit) c' ∘ F⁻¹ g ∘ unit F c},
    { intro g, rewrite [+respect_comp,▸*],
      krewrite [natural_map_inverse], xrewrite [respect_inv'],
      apply inverse_comp_eq_of_eq_comp,
      exact sorry /-this is basically the naturality of the counit-/ },
    { exact sorry},
  end

  definition fully_faithful_equiv (F : C ⇒ D) : fully_faithful F ≃ (faithful F × full F) :=
  sorry

  definition is_equivalence_equiv (F : C ⇒ D)
    : is_equivalence F ≃ (fully_faithful F × split_essentially_surjective F) :=
  sorry

  definition is_hprop_is_weak_equivalence (F : C ⇒ D) : is_hprop (is_weak_equivalence F) :=
  sorry

  definition is_hprop_is_equivalence {C D : Category} (F : C ⇒ D) : is_hprop (is_equivalence F) :=
  sorry

  definition is_equivalence_equiv_is_weak_equivalence {C D : Category} (F : C ⇒ D)
    : is_equivalence F ≃ is_weak_equivalence F :=
  sorry

  definition is_hprop_is_isomorphism (F : C ⇒ D) : is_hprop (is_isomorphism F) :=
  sorry

  definition is_isomorphism_equiv1 (F : C ⇒ D) : is_equivalence F
    ≃ Σ(G : D ⇒ C) (η : 1 = G ∘f F) (ε : F ∘f G = 1),
        sorry ▸ ap (λ(H : C ⇒ C), F ∘f H) η = ap (λ(H : D ⇒ D), H ∘f F) ε⁻¹ :=
  sorry

  definition is_isomorphism_equiv2 (F : C ⇒ D) : is_equivalence F
    ≃ ∃(G : D ⇒ C), 1 = G ∘f F × F ∘f G = 1 :=
  sorry

  definition is_equivalence_of_isomorphism (H : is_isomorphism F) : is_equivalence F :=
  sorry

  definition is_isomorphism_of_is_equivalence {C D : Category} {F : C ⇒ D} (H : is_equivalence F)
    : is_isomorphism F :=
  sorry

  definition isomorphism_of_eq {C D : Precategory} (p : C = D) : C ≌ D :=
  sorry

  definition is_equiv_isomorphism_of_eq (C D : Precategory) : is_equiv (@isomorphism_of_eq C D) :=
  sorry

  definition equivalence_of_eq {C D : Precategory} (p : C = D) : C ⋍ D :=
  sorry

  definition is_equiv_equivalence_of_eq (C D : Category) : is_equiv (@equivalence_of_eq C D) :=
  sorry
-/
end category