104 lines
4.2 KiB
Text
104 lines
4.2 KiB
Text
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/-
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Copyright (c) 2015 Floris van Doorn. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Floris van Doorn
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Adjoint functors
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-/
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import .attributes
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open category functor nat_trans eq is_trunc iso equiv prod trunc function pi is_equiv
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namespace category
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-- TODO(?): define a structure "adjoint" and then define
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-- structure is_left_adjoint (F : C ⇒ D) :=
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-- (G : D ⇒ C) -- G
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-- (is_adjoint : adjoint F G)
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structure is_left_adjoint [class] {C D : Precategory} (F : C ⇒ D) :=
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(G : D ⇒ C)
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(η : 1 ⟹ G ∘f F)
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(ε : F ∘f G ⟹ 1)
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(H : Π(c : C), ε (F c) ∘ F (η c) = ID (F c))
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(K : Π(d : D), G (ε d) ∘ η (G d) = ID (G d))
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abbreviation right_adjoint [unfold 4] := @is_left_adjoint.G
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abbreviation unit [unfold 4] := @is_left_adjoint.η
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abbreviation counit [unfold 4] := @is_left_adjoint.ε
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abbreviation counit_unit_eq [unfold 4] := @is_left_adjoint.H
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abbreviation unit_counit_eq [unfold 4] := @is_left_adjoint.K
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theorem is_hprop_is_left_adjoint [instance] {C : Category} {D : Precategory} (F : C ⇒ D)
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: is_hprop (is_left_adjoint F) :=
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begin
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apply is_hprop.mk,
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intro G G', cases G with G η ε H K, cases G' with G' η' ε' H' K',
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assert lem₁ : Π(p : G = G'), p ▸ η = η' → p ▸ ε = ε'
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→ is_left_adjoint.mk G η ε H K = is_left_adjoint.mk G' η' ε' H' K',
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{ intros p q r, induction p, induction q, induction r, esimp,
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apply apd011 (is_left_adjoint.mk G η ε) !is_hprop.elim !is_hprop.elim},
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assert lem₂ : Π (d : carrier D),
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(to_fun_hom G (natural_map ε' d) ∘
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natural_map η (to_fun_ob G' d)) ∘
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to_fun_hom G' (natural_map ε d) ∘
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natural_map η' (to_fun_ob G d) = id,
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{ intro d, esimp,
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rewrite [assoc],
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rewrite [-assoc (G (ε' d))],
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esimp, rewrite [nf_fn_eq_fn_nf_pt' G' ε η d],
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esimp, rewrite [assoc],
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esimp, rewrite [-assoc],
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rewrite [↑functor.compose, -respect_comp G],
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rewrite [nf_fn_eq_fn_nf_pt ε ε' d,nf_fn_eq_fn_nf_pt η' η (G d),▸*],
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rewrite [respect_comp G],
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rewrite [assoc,▸*,-assoc (G (ε d))],
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rewrite [↑functor.compose, -respect_comp G],
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rewrite [H' (G d)],
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rewrite [respect_id,▸*,id_right],
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apply K},
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assert lem₃ : Π (d : carrier D),
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(to_fun_hom G' (natural_map ε d) ∘
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natural_map η' (to_fun_ob G d)) ∘
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to_fun_hom G (natural_map ε' d) ∘
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natural_map η (to_fun_ob G' d) = id,
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{ intro d, esimp,
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rewrite [assoc, -assoc (G' (ε d))],
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esimp, rewrite [nf_fn_eq_fn_nf_pt' G ε' η' d],
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esimp, rewrite [assoc], esimp, rewrite [-assoc],
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rewrite [↑functor.compose, -respect_comp G'],
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rewrite [nf_fn_eq_fn_nf_pt ε' ε d,nf_fn_eq_fn_nf_pt η η' (G' d)],
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esimp,
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rewrite [respect_comp G'],
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rewrite [assoc,▸*,-assoc (G' (ε' d))],
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rewrite [↑functor.compose, -respect_comp G'],
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rewrite [H (G' d)],
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rewrite [respect_id,▸*,id_right],
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apply K'},
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fapply lem₁,
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{ fapply functor.eq_of_pointwise_iso,
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{ fapply change_natural_map,
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{ exact (G' ∘fn1 ε) ∘n !assoc_natural_rev ∘n (η' ∘1nf G)},
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{ intro d, exact (G' (ε d) ∘ η' (G d))},
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{ intro d, exact ap (λx, _ ∘ x) !id_left}},
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{ intro d, fconstructor,
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{ exact (G (ε' d) ∘ η (G' d))},
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{ exact lem₂ d },
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{ exact lem₃ d }}},
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{ clear lem₁, refine transport_hom_of_eq_right _ η ⬝ _,
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krewrite hom_of_eq_compose_right,
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rewrite functor.hom_of_eq_eq_of_pointwise_iso,
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apply nat_trans_eq, intro c, esimp,
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refine !assoc⁻¹ ⬝ ap (λx, _ ∘ x) (nf_fn_eq_fn_nf_pt η η' c) ⬝ !assoc ⬝ _,
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esimp, rewrite [-respect_comp G',H c,respect_id G',▸*,id_left]},
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{ clear lem₁, refine transport_hom_of_eq_left _ ε ⬝ _,
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krewrite inv_of_eq_compose_left,
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rewrite functor.inv_of_eq_eq_of_pointwise_iso,
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apply nat_trans_eq, intro d, esimp,
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krewrite [respect_comp],
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rewrite [assoc,nf_fn_eq_fn_nf_pt ε' ε d,-assoc,▸*,H (G' d),id_right]}
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end
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end category
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