lean2/hott/algebra/category/groupoid.hlean

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/-
Copyright (c) 2014 Jakob von Raumer. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author: Jakob von Raumer
Ported from Coq HoTT
-/
import .iso ..group
open eq is_trunc iso category algebra nat unit
namespace category
structure groupoid [class] (ob : Type) extends parent : precategory ob :=
mk' :: (all_iso : Π ⦃a b : ob⦄ (f : hom a b), @is_iso ob parent a b f)
abbreviation all_iso := @groupoid.all_iso
attribute groupoid.all_iso [instance] [priority 100000]
definition groupoid.mk [reducible] {ob : Type} (C : precategory ob)
(H : Π (a b : ob) (f : a ⟶ b), is_iso f) : groupoid ob :=
precategory.rec_on C groupoid.mk' H
-- We can turn each group into a groupoid on the unit type
definition groupoid_of_group.{l} (A : Type.{l}) [G : group A] : groupoid.{0 l} unit :=
begin
fapply groupoid.mk, fapply precategory.mk,
intros, exact A,
intros, apply (@group.is_hset_carrier A G),
intros [a, b, c, g, h], exact (@group.mul A G g h),
intro a, exact (@group.one A G),
intros, exact (@group.mul_assoc A G h g f)⁻¹,
intros, exact (@group.one_mul A G f),
intros, exact (@group.mul_one A G f),
intros, esimp [precategory.mk], apply is_iso.mk,
apply mul.left_inv,
apply mul.right_inv,
end
definition hom_group {A : Type} [G : groupoid A] (a : A) :
group (hom a a) :=
begin
fapply group.mk,
intro f g, apply (comp f g),
apply is_hset_hom,
intros f g h, apply (assoc f g h)⁻¹,
apply (ID a),
intro f, apply id_left,
intro f, apply id_right,
intro f, exact (iso.inverse f),
intro f, exact (iso.left_inverse f),
end
definition group_of_is_contr_groupoid {ob : Type} [H : is_contr ob]
[G : groupoid ob] : group (hom (center ob) (center ob)) := !hom_group
definition group_of_groupoid_unit [G : groupoid unit] : group (hom ⋆ ⋆) := !hom_group
-- Bundled version of categories
-- we don't use Groupoid.carrier explicitly, but rather use Groupoid.carrier (to_Precategory C)
structure Groupoid : Type :=
(carrier : Type)
(struct : groupoid carrier)
attribute Groupoid.struct [instance] [coercion]
definition Groupoid.to_Precategory [coercion] [reducible] (C : Groupoid) : Precategory :=
Precategory.mk (Groupoid.carrier C) C
definition groupoid.Mk [reducible] := Groupoid.mk
definition groupoid.MK [reducible] (C : Precategory) (H : Π (a b : C) (f : a ⟶ b), is_iso f)
: Groupoid :=
Groupoid.mk C (groupoid.mk C H)
definition Groupoid.eta (C : Groupoid) : Groupoid.mk C C = C :=
Groupoid.rec (λob c, idp) C
end category