lean2/library/hott/equiv.lean

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-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Author: Jeremy Avigad, Jakob von Raumer
-- Ported from Coq HoTT
import .path .trunc
open path function
-- Equivalences
-- ------------
definition Sect {A B : Type} (s : A → B) (r : B → A) := Πx : A, r (s x) ≈ x
-- -- TODO: need better means of declaring structures
-- -- TODO: note that Coq allows projections to be declared to be coercions on the fly
-- Structure IsEquiv
inductive IsEquiv [class] {A B : Type} (f : A → B) :=
IsEquiv_mk : Π
(inv : B → A)
(retr : Sect inv f)
(sect : Sect f inv)
(adj : Πx, retr (f x) ≈ ap f (sect x)),
IsEquiv f
namespace IsEquiv
definition inv [coercion] {A B : Type} (f : A → B) [H : IsEquiv f] : B → A :=
IsEquiv.rec (λinv retr sect adj, inv) H
-- TODO: note: does not type check without giving the type
definition retr [coercion] {A B : Type} (f : A → B) [H : IsEquiv f] : Sect (inv f) f :=
IsEquiv.rec (λinv retr sect adj, retr) H
definition sect [coercion] {A B : Type} (f : A → B) [H : IsEquiv f] : Sect f (inv f) :=
IsEquiv.rec (λinv retr sect adj, sect) H
definition adj [coercion] {A B : Type} (f : A → B) [H : IsEquiv f] :
Πx, retr f (f x) ≈ ap f (sect f x) :=
IsEquiv.rec (λinv retr sect adj, adj) H
notation e `⁻¹` := inv e
end IsEquiv
-- Structure Equiv
inductive Equiv (A B : Type) : Type :=
Equiv_mk : Π
(equiv_fun : A → B)
(equiv_isequiv : IsEquiv equiv_fun),
Equiv A B
namespace Equiv
definition equiv_fun [coercion] {A B : Type} (e : Equiv A B) : A → B :=
Equiv.rec (λequiv_fun equiv_isequiv, equiv_fun) e
definition equiv_isequiv [coercion] {A B : Type} (e : Equiv A B) : IsEquiv (equiv_fun e) :=
Equiv.rec (λequiv_fun equiv_isequiv, equiv_isequiv) e
infix `≃`:25 := Equiv
end Equiv
-- Some instances and closure properties of equivalences
namespace IsEquiv
variables {A B C : Type} {f : A → B} {g : B → C} {f' : A → B}
-- The identity function is an equivalence.
definition id_closed [instance] : (@IsEquiv A A id) := IsEquiv_mk id (λa, idp) (λa, idp) (λa, idp)
-- The composition of two equivalences is, again, an equivalence.
definition comp_closed (Hf : IsEquiv f) (Hg : IsEquiv g) : (IsEquiv (g ∘ f)) :=
IsEquiv_mk ((inv f) ∘ (inv g))
(λc, ap g (retr f (g⁻¹ c)) ⬝ retr g c)
(λa, ap (inv f) (sect g (f a)) ⬝ sect f a)
(λa, (whiskerL _ (adj g (f a))) ⬝
(ap_pp g _ _)⁻¹ ⬝
ap02 g (concat_A1p (retr f) (sect g (f a))⁻¹ ⬝
(ap_compose (inv f) f _ ◾ adj f a) ⬝
(ap_pp f _ _)⁻¹
) ⬝
(ap_compose f g _)⁻¹
)
-- Any function equal to an equivalence is an equivlance as well.
definition path_closed (Hf : IsEquiv f) (Heq : f ≈ f') : (IsEquiv f') :=
path.rec_on Heq Hf
-- Any function pointwise equal to an equivalence is an equivalence as well.
definition homotopic (Hf : IsEquiv f) (Heq : f f') : (IsEquiv f') :=
let sect' := (λ b, (Heq (inv f b))⁻¹ ⬝ retr f b) in
let retr' := (λ a, (ap (inv f) (Heq a))⁻¹ ⬝ sect f a) in
let adj' := (λ (a : A),
let ff'a := Heq a in
let invf := inv f in
let secta := sect f a in
let retrfa := retr f (f a) in
let retrf'a := retr f (f' a) in
have eq1 : _ ≈ _,
from calc ap f secta ⬝ ff'a
≈ retrfa ⬝ ff'a : (ap _ (adj f _ ))⁻¹
... ≈ ap (f ∘ invf) ff'a ⬝ retrf'a : !concat_A1p⁻¹
... ≈ ap f (ap invf ff'a) ⬝ retr f (f' a) : {ap_compose invf f _},
have eq2 : _ ≈ _,
from calc retrf'a
≈ (ap f (ap invf ff'a))⁻¹ ⬝ (ap f secta ⬝ ff'a) : moveL_Vp _ _ _ (eq1⁻¹)
... ≈ ap f (ap invf ff'a)⁻¹ ⬝ (ap f secta ⬝ Heq a) : {ap_V invf ff'a}
... ≈ ap f (ap invf ff'a)⁻¹ ⬝ (Heq (invf (f a)) ⬝ ap f' secta) : {!concat_Ap}
... ≈ (ap f (ap invf ff'a)⁻¹ ⬝ Heq (invf (f a))) ⬝ ap f' secta : {!concat_pp_p⁻¹}
... ≈ (ap f ((ap invf ff'a)⁻¹) ⬝ Heq (invf (f a))) ⬝ ap f' secta : {!ap_V⁻¹}
... ≈ (Heq (invf (f' a)) ⬝ ap f' ((ap invf ff'a)⁻¹)) ⬝ ap f' secta : {!concat_Ap}
... ≈ (Heq (invf (f' a)) ⬝ (ap f' (ap invf ff'a))⁻¹) ⬝ ap f' secta : {!ap_V}
... ≈ Heq (invf (f' a)) ⬝ ((ap f' (ap invf ff'a))⁻¹ ⬝ ap f' secta) : !concat_pp_p,
have eq3 : _ ≈ _,
from calc (Heq (invf (f' a)))⁻¹ ⬝ retr f (f' a)
≈ (ap f' (ap invf ff'a))⁻¹ ⬝ ap f' secta : moveR_Vp _ _ _ eq2
... ≈ (ap f' ((ap invf ff'a)⁻¹)) ⬝ ap f' secta : {!ap_V⁻¹}
... ≈ ap f' ((ap invf ff'a)⁻¹ ⬝ secta) : !ap_pp⁻¹,
eq3) in
IsEquiv_mk (inv f) sect' retr' adj'
end IsEquiv
namespace IsEquiv
variables {A B : Type} (f : A → B) (g : B → A)
(ret : Sect g f) (sec : Sect f g)
context
set_option unifier.max_steps 30000
--To construct an equivalence it suffices to state the proof that the inverse is a quasi-inverse.
definition adjointify : IsEquiv f :=
let sect' := (λx, ap g (ap f (inverse (sec x))) ⬝ ap g (ret (f x)) ⬝ sec x) in
let adj' := (λ (a : A),
let fgretrfa := ap f (ap g (ret (f a))) in
let fgfinvsect := ap f (ap g (ap f ((sec a)⁻¹))) in
let fgfa := f (g (f a)) in
let retrfa := ret (f a) in
have eq1 : ap f (sec a) ≈ _,
from calc ap f (sec a)
≈ idp ⬝ ap f (sec a) : !concat_1p⁻¹
... ≈ (ret (f a) ⬝ (ret (f a)⁻¹)) ⬝ ap f (sec a) : {!concat_pV⁻¹}
... ≈ ((ret (fgfa))⁻¹ ⬝ ap (f ∘ g) (ret (f a))) ⬝ ap f (sec a) : {!concat_pA1⁻¹}
... ≈ ((ret (fgfa))⁻¹ ⬝ fgretrfa) ⬝ ap f (sec a) : {ap_compose g f _}
... ≈ (ret (fgfa))⁻¹ ⬝ (fgretrfa ⬝ ap f (sec a)) : !concat_pp_p,
have eq2 : ap f (sec a) ⬝ idp ≈ (ret fgfa)⁻¹ ⬝ (fgretrfa ⬝ ap f (sec a)),
from !concat_p1 ⬝ eq1,
have eq3 : idp ≈ _,
from calc idp
≈ (ap f (sec a))⁻¹ ⬝ ((ret fgfa)⁻¹ ⬝ (fgretrfa ⬝ ap f (sec a))) : moveL_Vp _ _ _ eq2
... ≈ (ap f (sec a)⁻¹ ⬝ (ret fgfa)⁻¹) ⬝ (fgretrfa ⬝ ap f (sec a)) : !concat_p_pp
... ≈ (ap f ((sec a)⁻¹) ⬝ (ret fgfa)⁻¹) ⬝ (fgretrfa ⬝ ap f (sec a)) : {!ap_V⁻¹}
... ≈ ((ap f ((sec a)⁻¹) ⬝ (ret fgfa)⁻¹) ⬝ fgretrfa) ⬝ ap f (sec a) : !concat_p_pp
... ≈ ((retrfa⁻¹ ⬝ ap (f ∘ g) (ap f ((sec a)⁻¹))) ⬝ fgretrfa) ⬝ ap f (sec a) : {!concat_pA1⁻¹}
... ≈ ((retrfa⁻¹ ⬝ fgfinvsect) ⬝ fgretrfa) ⬝ ap f (sec a) : {ap_compose g f _}
... ≈ (retrfa⁻¹ ⬝ (fgfinvsect ⬝ fgretrfa)) ⬝ ap f (sec a) : {!concat_p_pp⁻¹}
... ≈ retrfa⁻¹ ⬝ ap f (ap g (ap f ((sec a)⁻¹)) ⬝ ap g (ret (f a))) ⬝ ap f (sec a) : {!ap_pp⁻¹}
... ≈ retrfa⁻¹ ⬝ (ap f (ap g (ap f ((sec a)⁻¹)) ⬝ ap g (ret (f a))) ⬝ ap f (sec a)) : !concat_p_pp⁻¹
... ≈ retrfa⁻¹ ⬝ ap f ((ap g (ap f ((sec a)⁻¹)) ⬝ ap g (ret (f a))) ⬝ sec a) : {!ap_pp⁻¹},
have eq4 : ret (f a) ≈ ap f ((ap g (ap f ((sec a)⁻¹)) ⬝ ap g (ret (f a))) ⬝ sec a),
from moveR_M1 _ _ eq3,
eq4) in
IsEquiv_mk g ret sect' adj'
end
end IsEquiv
namespace IsEquiv
variables {A B: Type} {f : A → B}
--The inverse of an equivalence is, again, an equivalence.
definition inv_closed (Hf : IsEquiv f) : (IsEquiv (inv f)) :=
adjointify (inv f) f (sect f) (retr f)
end IsEquiv
namespace IsEquiv
variables {A B C : Type} {f : A → B} {g : B → C} {f' : A → B}
definition cancel_R (Hf : IsEquiv f) (Hgf : IsEquiv (g ∘ f)) : (IsEquiv g) :=
homotopic (comp_closed (inv_closed Hf) Hgf) (λb, ap g (retr f b))
definition cancel_L (Hg : IsEquiv g) (Hgf : IsEquiv (g ∘ f)) : (IsEquiv f) :=
homotopic (comp_closed Hgf (inv_closed Hg)) (λa, sect g (f a))
--Transporting is an equivalence
definition transport [instance] (P : A → Type) {x y : A} (p : x ≈ y) : (IsEquiv (transport P p)) :=
IsEquiv_mk (transport P (p⁻¹)) (transport_pV P p) (transport_Vp P p) (transport_pVp P p)
--Rewrite rules
section
variables (Hf : IsEquiv f)
definition moveR_M (Hf : IsEquiv f) {x : A} {y : B} (p : x ≈ (inv f) y) : (f x ≈ y) :=
(ap f p) ⬝ (retr f y)
definition moveL_M (Hf : IsEquiv f) {x : A} {y : B} (p : (inv f) y ≈ x) : (y ≈ f x) :=
(moveR_M Hf (p⁻¹))⁻¹
definition moveR_V (Hf : IsEquiv f) {x : B} {y : A} (p : x ≈ f y) : (inv f) x ≈ y :=
ap (inv f) p ⬝ sect f y
definition moveL_V (Hf : IsEquiv f) {x : B} {y : A} (p : f y ≈ x) : y ≈ (inv f) x :=
(moveR_V Hf (p⁻¹))⁻¹
definition contr (Hf : IsEquiv f) (HA: Contr A) : (Contr B) :=
Contr.Contr_mk (f (center HA)) (λb, moveR_M Hf (contr HA (inv f b)))
definition ap (Hf : IsEquiv f) (x y : A) : IsEquiv (@ap A B f x y) :=
adjointify (ap f)
(λq, (inverse (sect f x)) ⬝ ap (f⁻¹) q ⬝ sect f y)
(λq, !ap_pp
⬝ whiskerR !ap_pp _
⬝ ((!ap_V ⬝ inverse2 ((adj f _)⁻¹))
◾ (inverse (ap_compose (f⁻¹) f _))
◾ (adj f _)⁻¹)
⬝ concat_pA1_p (retr f) _ _
⬝ whiskerR !concat_Vp _
⬝ !concat_1p)
(λp, whiskerR (whiskerL _ ((ap_compose f (f⁻¹) _)⁻¹)) _
⬝ concat_pA1_p (sect f) _ _
⬝ whiskerR !concat_Vp _
⬝ !concat_1p)
end
end IsEquiv
namespace Equiv
variables {A B C : Type} (eqf : A ≃ B)
definition f : A → B := equiv_fun eqf
definition id : A ≃ A := Equiv_mk id IsEquiv.id_closed
theorem compose (eqg: B ≃ C) : A ≃ C :=
Equiv_mk ((equiv_fun eqg) ∘ (equiv_fun eqf))
(IsEquiv.comp_closed (equiv_isequiv eqf) (equiv_isequiv eqg))
theorem path_closed (f' : A → B) (Heq : equiv_fun eqf ≈ f') : A ≃ B :=
Equiv_mk f' (IsEquiv.path_closed (equiv_isequiv eqf) Heq)
theorem inv_closed : B ≃ A :=
Equiv_mk (@IsEquiv.inv _ _ (equiv_fun eqf) (equiv_isequiv eqf))
(IsEquiv.inv_closed (equiv_isequiv eqf))
theorem cancel_L {f : A → B} {g : B → C}
(Hf : IsEquiv f) (Hgf : IsEquiv (g ∘ f)) : B ≃ C :=
Equiv_mk g (IsEquiv.cancel_R _ _)
theorem cancel_R {f : A → B} {g : B → C}
(Hg : IsEquiv g) (Hgf : IsEquiv (g ∘ f)) : A ≃ B :=
Equiv_mk f (!IsEquiv.cancel_L _ _)
theorem transport (P : A → Type) {x y : A} {p : x ≈ y} : (P x) ≃ (P y) :=
Equiv_mk (transport P p) (IsEquiv.transport P p)
theorem contr_closed (HA: Contr A) : (Contr B) :=
@IsEquiv.contr A B (equiv_fun eqf) (equiv_isequiv eqf) HA
end Equiv
namespace Equiv
variables {A B : Type} {HA : Contr A} {HB : Contr B}
--Each two contractible types are equivalent.
definition contr_contr : A ≃ B :=
Equiv_mk
(λa, center HB)
(IsEquiv.adjointify (λa, center HB) (λb, center HA)
(contr HB) (contr HA))
end Equiv