350 lines
14 KiB
Text
350 lines
14 KiB
Text
/-
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Copyright (c) 2014 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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Module: algebra.precategory.iso
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Author: Floris van Doorn, Jakob von Raumer
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-/
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import algebra.precategory.basic types.sigma arity
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open eq category prod equiv is_equiv sigma sigma.ops is_trunc
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namespace iso
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structure split_mono [class] {ob : Type} [C : precategory ob] {a b : ob} (f : a ⟶ b) :=
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{retraction_of : b ⟶ a}
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(retraction_comp : retraction_of ∘ f = id)
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structure split_epi [class] {ob : Type} [C : precategory ob] {a b : ob} (f : a ⟶ b) :=
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{section_of : b ⟶ a}
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(comp_section : f ∘ section_of = id)
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structure is_iso [class] {ob : Type} [C : precategory ob] {a b : ob} (f : a ⟶ b) :=
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{inverse : b ⟶ a}
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(left_inverse : inverse ∘ f = id)
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(right_inverse : f ∘ inverse = id)
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attribute is_iso.inverse [quasireducible]
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attribute is_iso [multiple-instances]
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open split_mono split_epi is_iso
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definition retraction_of [reducible] := @split_mono.retraction_of
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definition retraction_comp [reducible] := @split_mono.retraction_comp
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definition section_of [reducible] := @split_epi.section_of
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definition comp_section [reducible] := @split_epi.comp_section
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definition inverse [reducible] := @is_iso.inverse
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definition left_inverse [reducible] := @is_iso.left_inverse
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definition right_inverse [reducible] := @is_iso.right_inverse
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postfix `⁻¹` := inverse
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--a second notation for the inverse, which is not overloaded
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postfix [parsing-only] `⁻¹ʰ`:std.prec.max_plus := inverse -- input using \-1h
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variables {ob : Type} [C : precategory ob]
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variables {a b c : ob} {g : b ⟶ c} {f : a ⟶ b} {h : b ⟶ a}
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include C
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definition split_mono_of_is_iso [instance] [priority 300] [reducible]
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(f : a ⟶ b) [H : is_iso f] : split_mono f :=
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split_mono.mk !left_inverse
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definition split_epi_of_is_iso [instance] [priority 300] [reducible]
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(f : a ⟶ b) [H : is_iso f] : split_epi f :=
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split_epi.mk !right_inverse
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definition is_iso_id [instance] [priority 500] (a : ob) : is_iso (ID a) :=
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is_iso.mk !id_comp !id_comp
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definition is_iso_inverse [instance] [priority 200] (f : a ⟶ b) [H : is_iso f] : is_iso f⁻¹ :=
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is_iso.mk !right_inverse !left_inverse
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definition left_inverse_eq_right_inverse {f : a ⟶ b} {g g' : hom b a}
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(Hl : g ∘ f = id) (Hr : f ∘ g' = id) : g = g' :=
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by rewrite [-(id_right g), -Hr, assoc, Hl, id_left]
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definition retraction_eq [H : split_mono f] (H2 : f ∘ h = id) : retraction_of f = h :=
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left_inverse_eq_right_inverse !retraction_comp H2
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definition section_eq [H : split_epi f] (H2 : h ∘ f = id) : section_of f = h :=
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(left_inverse_eq_right_inverse H2 !comp_section)⁻¹
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definition inverse_eq_right [H : is_iso f] (H2 : f ∘ h = id) : f⁻¹ = h :=
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left_inverse_eq_right_inverse !left_inverse H2
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definition inverse_eq_left [H : is_iso f] (H2 : h ∘ f = id) : f⁻¹ = h :=
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(left_inverse_eq_right_inverse H2 !right_inverse)⁻¹
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definition retraction_eq_section (f : a ⟶ b) [Hl : split_mono f] [Hr : split_epi f] :
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retraction_of f = section_of f :=
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retraction_eq !comp_section
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definition is_iso_of_split_epi_of_split_mono (f : a ⟶ b) [Hl : split_mono f] [Hr : split_epi f]
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: is_iso f :=
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is_iso.mk ((retraction_eq_section f) ▹ (retraction_comp f)) (comp_section f)
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definition inverse_unique (H H' : is_iso f) : @inverse _ _ _ _ f H = @inverse _ _ _ _ f H' :=
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inverse_eq_left !left_inverse
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definition inverse_involutive (f : a ⟶ b) [H : is_iso f] [H : is_iso (f⁻¹)]
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: (f⁻¹)⁻¹ = f :=
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inverse_eq_right !left_inverse
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definition retraction_id (a : ob) : retraction_of (ID a) = id :=
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retraction_eq !id_comp
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definition section_id (a : ob) : section_of (ID a) = id :=
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section_eq !id_comp
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definition id_inverse (a : ob) [H : is_iso (ID a)] : (ID a)⁻¹ = id :=
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inverse_eq_left !id_comp
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definition split_mono_comp [instance] [priority 150] (g : b ⟶ c) (f : a ⟶ b)
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[Hf : split_mono f] [Hg : split_mono g] : split_mono (g ∘ f) :=
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split_mono.mk
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(show (retraction_of f ∘ retraction_of g) ∘ g ∘ f = id,
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by rewrite [-assoc, assoc _ g f, retraction_comp, id_left, retraction_comp])
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definition split_epi_comp [instance] [priority 150] (g : b ⟶ c) (f : a ⟶ b)
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[Hf : split_epi f] [Hg : split_epi g] : split_epi (g ∘ f) :=
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split_epi.mk
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(show (g ∘ f) ∘ section_of f ∘ section_of g = id,
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by rewrite [-assoc, {f ∘ _}assoc, comp_section, id_left, comp_section])
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definition is_iso_comp [instance] [priority 150] (g : b ⟶ c) (f : a ⟶ b)
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[Hf : is_iso f] [Hg : is_iso g] : is_iso (g ∘ f) :=
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!is_iso_of_split_epi_of_split_mono
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-- "is_iso f" is equivalent to a certain sigma type
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-- definition is_iso.sigma_char (f : hom a b) :
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-- (Σ (g : hom b a), (g ∘ f = id) × (f ∘ g = id)) ≃ is_iso f :=
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-- begin
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-- fapply equiv.MK,
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-- {intro S, apply is_iso.mk,
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-- exact (pr₁ S.2),
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-- exact (pr₂ S.2)},
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-- {intro H, cases H with (g, η, ε),
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-- exact (sigma.mk g (pair η ε))},
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-- {intro H, cases H, apply idp},
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-- {intro S, cases S with (g, ηε), cases ηε, apply idp},
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-- end
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definition is_hprop_is_iso [instance] (f : hom a b) : is_hprop (is_iso f) :=
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begin
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apply is_hprop.mk, intros (H, H'),
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cases H with (g, li, ri), cases H' with (g', li', ri'),
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fapply (apD0111 (@is_iso.mk ob C a b f)),
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apply left_inverse_eq_right_inverse,
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apply li,
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apply ri',
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apply is_hprop.elim,
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apply is_hprop.elim,
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end
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/- iso objects -/
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structure iso (a b : ob) :=
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(to_hom : hom a b)
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[struct : is_iso to_hom]
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infix `≅`:50 := iso.iso
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attribute iso.struct [instance] [priority 400]
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namespace iso
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attribute to_hom [coercion]
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definition MK (f : a ⟶ b) (g : b ⟶ a) (H1 : g ∘ f = id) (H2 : f ∘ g = id) :=
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@mk _ _ _ _ f (is_iso.mk H1 H2)
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definition to_inv (f : a ≅ b) : b ⟶ a :=
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(to_hom f)⁻¹
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protected definition refl (a : ob) : a ≅ a :=
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mk (ID a)
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protected definition symm ⦃a b : ob⦄ (H : a ≅ b) : b ≅ a :=
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mk (to_hom H)⁻¹
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protected definition trans ⦃a b c : ob⦄ (H1 : a ≅ b) (H2 : b ≅ c) : a ≅ c :=
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mk (to_hom H2 ∘ to_hom H1)
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protected definition eq_mk' {f f' : a ⟶ b} [H : is_iso f] [H' : is_iso f'] (p : f = f')
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: iso.mk f = iso.mk f' :=
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apD011 iso.mk p !is_hprop.elim
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protected definition eq_mk {f f' : a ≅ b} (p : to_hom f = to_hom f') : f = f' :=
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by (cases f; cases f'; apply (iso.eq_mk' p))
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-- The structure for isomorphism can be characterized up to equivalence by a sigma type.
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definition sigma_char ⦃a b : ob⦄ : (Σ (f : hom a b), is_iso f) ≃ (a ≅ b) :=
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begin
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fapply (equiv.mk),
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{intro S, apply iso.mk, apply (S.2)},
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{fapply adjointify,
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{intro p, cases p with (f, H), exact (sigma.mk f H)},
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{intro p, cases p, apply idp},
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{intro S, cases S, apply idp}},
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end
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end iso
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-- The type of isomorphisms between two objects is a set
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definition is_hset_iso [instance] : is_hset (a ≅ b) :=
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begin
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apply is_trunc_is_equiv_closed,
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apply (equiv.to_is_equiv (!iso.sigma_char)),
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end
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definition iso_of_eq (p : a = b) : a ≅ b :=
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eq.rec_on p (iso.refl a)
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definition hom_of_eq [reducible] (p : a = b) : a ⟶ b :=
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iso.to_hom (iso_of_eq p)
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definition inv_of_eq [reducible] (p : a = b) : b ⟶ a :=
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iso.to_inv (iso_of_eq p)
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definition iso_of_eq_inv (p : a = b) : iso_of_eq p⁻¹ = iso.symm (iso_of_eq p) :=
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eq.rec_on p idp
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definition iso_of_eq_con (p : a = b) (q : b = c)
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: iso_of_eq (p ⬝ q) = iso.trans (iso_of_eq p) (iso_of_eq q) :=
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eq.rec_on q (eq.rec_on p (iso.eq_mk !id_comp⁻¹))
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section
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open funext
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variables {X : Type} {x y : X} {F G : X → ob}
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definition transport_hom_of_eq (p : F = G) (f : hom (F x) (F y))
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: p ▹ f = hom_of_eq (apD10 p y) ∘ f ∘ inv_of_eq (apD10 p x) :=
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eq.rec_on p !id_leftright⁻¹
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definition transport_hom (p : F ∼ G) (f : hom (F x) (F y))
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: eq_of_homotopy p ▹ f = hom_of_eq (p y) ∘ f ∘ inv_of_eq (p x) :=
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calc
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eq_of_homotopy p ▹ f =
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hom_of_eq (apD10 (eq_of_homotopy p) y) ∘ f ∘ inv_of_eq (apD10 (eq_of_homotopy p) x)
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: transport_hom_of_eq
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... = hom_of_eq (p y) ∘ f ∘ inv_of_eq (p x) : {retr apD10 p}
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end
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structure mono [class] (f : a ⟶ b) :=
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(elim : ∀c (g h : hom c a), f ∘ g = f ∘ h → g = h)
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structure epi [class] (f : a ⟶ b) :=
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(elim : ∀c (g h : hom b c), g ∘ f = h ∘ f → g = h)
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definition mono_of_split_mono [instance] (f : a ⟶ b) [H : split_mono f] : mono f :=
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mono.mk
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(λ c g h H,
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calc
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g = id ∘ g : by rewrite id_left
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... = (retraction_of f ∘ f) ∘ g : by rewrite retraction_comp
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... = (retraction_of f ∘ f) ∘ h : by rewrite [-assoc, H, -assoc]
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... = id ∘ h : by rewrite retraction_comp
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... = h : by rewrite id_left)
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definition epi_of_split_epi [instance] (f : a ⟶ b) [H : split_epi f] : epi f :=
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epi.mk
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(λ c g h H,
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calc
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g = g ∘ id : by rewrite id_right
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... = g ∘ f ∘ section_of f : by rewrite -comp_section
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... = h ∘ f ∘ section_of f : by rewrite [assoc, H, -assoc]
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... = h ∘ id : by rewrite comp_section
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... = h : by rewrite id_right)
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definition mono_comp [instance] (g : b ⟶ c) (f : a ⟶ b) [Hf : mono f] [Hg : mono g]
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: mono (g ∘ f) :=
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mono.mk
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(λ d h₁ h₂ H,
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have H2 : g ∘ (f ∘ h₁) = g ∘ (f ∘ h₂),
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begin
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rewrite *assoc, exact H
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end,
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!mono.elim (!mono.elim H2))
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definition epi_comp [instance] (g : b ⟶ c) (f : a ⟶ b) [Hf : epi f] [Hg : epi g]
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: epi (g ∘ f) :=
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epi.mk
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(λ d h₁ h₂ H,
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have H2 : (h₁ ∘ g) ∘ f = (h₂ ∘ g) ∘ f,
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begin
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rewrite -*assoc, exact H
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end,
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!epi.elim (!epi.elim H2))
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end iso
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namespace iso
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/-
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rewrite lemmas for inverses, modified from
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https://github.com/JasonGross/HoTT-categories/blob/master/theories/Categories/Category/Morphisms.v
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-/
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section
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variables {ob : Type} [C : precategory ob] include C
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variables {a b c d : ob} (f : b ⟶ a)
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(r : c ⟶ d) (q : b ⟶ c) (p : a ⟶ b)
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(g : d ⟶ c)
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variable [Hq : is_iso q] include Hq
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definition comp.right_inverse : q ∘ q⁻¹ = id := !right_inverse
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definition comp.left_inverse : q⁻¹ ∘ q = id := !left_inverse
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definition inverse_comp_cancel_left : q⁻¹ ∘ (q ∘ p) = p :=
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by rewrite [assoc, left_inverse, id_left]
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definition comp_inverse_cancel_left : q ∘ (q⁻¹ ∘ g) = g :=
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by rewrite [assoc, right_inverse, id_left]
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definition comp_inverse_cancel_right : (r ∘ q) ∘ q⁻¹ = r :=
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by rewrite [-assoc, right_inverse, id_right]
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definition inverse_comp_cancel_right : (f ∘ q⁻¹) ∘ q = f :=
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by rewrite [-assoc, left_inverse, id_right]
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definition comp_inverse [Hp : is_iso p] [Hpq : is_iso (q ∘ p)] : (q ∘ p)⁻¹ʰ = p⁻¹ʰ ∘ q⁻¹ʰ :=
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inverse_eq_left
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(show (p⁻¹ʰ ∘ q⁻¹ʰ) ∘ q ∘ p = id, from
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by rewrite [-assoc, inverse_comp_cancel_left, left_inverse])
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definition inverse_comp_inverse_left [H' : is_iso g] : (q⁻¹ ∘ g)⁻¹ = g⁻¹ ∘ q :=
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inverse_involutive q ▹ comp_inverse q⁻¹ g
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definition inverse_comp_inverse_right [H' : is_iso f] : (q ∘ f⁻¹)⁻¹ = f ∘ q⁻¹ :=
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inverse_involutive f ▹ comp_inverse q f⁻¹
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definition inverse_comp_inverse_inverse [H' : is_iso r] : (q⁻¹ ∘ r⁻¹)⁻¹ = r ∘ q :=
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inverse_involutive r ▹ inverse_comp_inverse_left q r⁻¹
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end
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section
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variables {ob : Type} {C : precategory ob} include C
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variables {d c b a : ob}
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{i : b ⟶ c} {f : b ⟶ a}
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{r : c ⟶ d} {q : b ⟶ c} {p : a ⟶ b}
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{g : d ⟶ c} {h : c ⟶ b}
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{x : b ⟶ d} {z : a ⟶ c}
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{y : d ⟶ b} {w : c ⟶ a}
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variable [Hq : is_iso q] include Hq
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definition comp_eq_of_eq_inverse_comp (H : y = q⁻¹ ∘ g) : q ∘ y = g :=
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H⁻¹ ▹ comp_inverse_cancel_left q g
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definition comp_eq_of_eq_comp_inverse (H : w = f ∘ q⁻¹) : w ∘ q = f :=
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H⁻¹ ▹ inverse_comp_cancel_right f q
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definition inverse_comp_eq_of_eq_comp (H : z = q ∘ p) : q⁻¹ ∘ z = p :=
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H⁻¹ ▹ inverse_comp_cancel_left q p
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definition comp_inverse_eq_of_eq_comp (H : x = r ∘ q) : x ∘ q⁻¹ = r :=
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H⁻¹ ▹ comp_inverse_cancel_right r q
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definition eq_comp_of_inverse_comp_eq (H : q⁻¹ ∘ g = y) : g = q ∘ y :=
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(comp_eq_of_eq_inverse_comp H⁻¹)⁻¹
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definition eq_comp_of_comp_inverse_eq (H : f ∘ q⁻¹ = w) : f = w ∘ q :=
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(comp_eq_of_eq_comp_inverse H⁻¹)⁻¹
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definition eq_inverse_comp_of_comp_eq (H : q ∘ p = z) : p = q⁻¹ ∘ z :=
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(inverse_comp_eq_of_eq_comp H⁻¹)⁻¹
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definition eq_comp_inverse_of_comp_eq (H : r ∘ q = x) : r = x ∘ q⁻¹ :=
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(comp_inverse_eq_of_eq_comp H⁻¹)⁻¹
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definition eq_inverse_of_comp_eq_id' (H : h ∘ q = id) : h = q⁻¹ := (inverse_eq_left H)⁻¹
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definition eq_inverse_of_comp_eq_id (H : q ∘ h = id) : h = q⁻¹ := (inverse_eq_right H)⁻¹
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definition eq_of_comp_inverse_eq_id (H : i ∘ q⁻¹ = id) : i = q :=
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eq_inverse_of_comp_eq_id' H ⬝ inverse_involutive q
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definition eq_of_inverse_comp_eq_id (H : q⁻¹ ∘ i = id) : i = q :=
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eq_inverse_of_comp_eq_id H ⬝ inverse_involutive q
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definition eq_of_id_eq_comp_inverse (H : id = i ∘ q⁻¹) : q = i := (eq_of_comp_inverse_eq_id H⁻¹)⁻¹
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definition eq_of_id_eq_inverse_comp (H : id = q⁻¹ ∘ i) : q = i := (eq_of_inverse_comp_eq_id H⁻¹)⁻¹
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definition inverse_eq_of_id_eq_comp (H : id = h ∘ q) : q⁻¹ = h :=
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(eq_inverse_of_comp_eq_id' H⁻¹)⁻¹
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definition inverse_eq_of_id_eq_comp' (H : id = q ∘ h) : q⁻¹ = h :=
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(eq_inverse_of_comp_eq_id H⁻¹)⁻¹
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end
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end iso
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