module Algebra.Group.Concrete where

# Concrete groupsπ

In homotopy type theory, we can give an alternative definition of groups: they are the pointed, connected groupoids. The idea is that those groupoids contain exactly the same information as their fundamental group.

Such groups are dubbed concrete, because they represent the groups of symmetries of a given object (the base point); by opposition, βalgebraicβ Groups are called abstract.

record ConcreteGroup β : Type (lsuc β) where
no-eta-equality
constructor concrete-group
field
B                : Typeβ β
has-is-connected : is-connectedβ B
has-is-groupoid  : is-groupoid β B β

pt : β B β
pt = B .snd

Given a concrete group $G$, the underlying pointed type is denoted $\mathbf{B} G$, by analogy with the delooping of an abstract group; note that, here, the delooping is the chosen representation of $G$, so B is just a record field. We write $\bullet_{G}$ for the base point.

Concrete groups are pointed connected types, so they enjoy the following elimination principle, which will be useful later:

B-elim-contr : {P : β B β β Type ββ²}
β is-contr (P pt)
β β x β P x
B-elim-contr {P = P} c = connectedβ-elim-prop {P = P} has-is-connected
(is-contrβis-prop c) (c .centre)

A central example of a concrete group is the circle: the delooping of the integers.

SΒΉ-is-groupoid : is-groupoid SΒΉ
SΒΉ-is-groupoid = connectedβ-elim-prop SΒΉ-is-connected hlevel!
$connectedβ-elim-prop SΒΉ-is-connected hlevel!$ is-hlevelβ 2 Ξ©SΒΉβintegers (hlevel 2)

SΒΉ-concrete : ConcreteGroup lzero
SΒΉ-concrete .B = SΒΉ , base
SΒΉ-concrete .has-is-connected = SΒΉ-is-connected
SΒΉ-concrete .has-is-groupoid = SΒΉ-is-groupoid

## The category of concrete groupsπ

Concrete groups naturally form a category, where the morphisms are given by pointed maps $\mathbf{B} G \to \mathbf{B} H$.

ConcreteGroups : β β β Precategory (lsuc β) β
ConcreteGroups β .Ob = ConcreteGroup β
ConcreteGroups _ .Hom G H = B G ββ B H

We immediately see one reason why the pointedness condition is needed: without it, the morphisms between concrete groups would not form a set.

ConcreteGroups _ .Hom-set G H (f , ptf) (g , ptg) p q =
Ξ£-set-square hlevel! (funext-square (B-elim-contr G square))
where
open ConcreteGroup H using (H-Level-B)
square : is-contr ((Ξ» j β p j .fst (pt G)) β‘ (Ξ» j β q j .fst (pt G)))
square .centre i j = hcomp (β i β¨ β j) Ξ» where
k (k = i0) β pt H
k (i = i0) β p j .snd (~ k)
k (i = i1) β q j .snd (~ k)
k (j = i0) β ptf (~ k)
k (j = i1) β ptg (~ k)
square .paths _ = H .has-is-groupoid _ _ _ _ _ _
The rest of the categorical structure is inherited from functions and paths in a straightforward way.
ConcreteGroups _ .id = (Ξ» x β x) , refl
ConcreteGroups _ ._β_ (f , ptf) (g , ptg) = f β g , ap f ptg β ptf
ConcreteGroups _ .idr f = Ξ£-pathp refl (β-idl _)
ConcreteGroups _ .idl f = Ξ£-pathp refl (β-idr _)
ConcreteGroups _ .assoc (f , ptf) (g , ptg) (h , pth) = Ξ£-pathp refl $β ap f (ap g pth β ptg) β β ptf β‘ (ap (f β g) pth β ap f ptg) β ptf β‘ ap (f β g) pth β ap f ptg β ptf β We can check that ConcreteGroups is a univalent category: this essentially follows from the univalence of the universe of groupoids. ## Concrete vs.Β abstractπ Our goal is now to prove that concrete groups and abstract groups are equivalent, in the sense that there is an equivalence of categories between ConcreteGroups and Groups. To make the following developments easier, we define a version of Οβββ 0 that does not use the set truncation. Indeed, thereβs no need since weβre dealing with groupoids: each loop space is already a set. ΟβB : ConcreteGroup β β Group β ΟβB G = to-group mk where open make-group mk : make-group (pt G β‘ pt G) mk .group-is-set = G .has-is-groupoid _ _ mk .unit = refl mk .mul = _β_ mk .inv = sym mk .assoc = β-assoc mk .invl = β-invl mk .idl = β-idl Οββ‘Οβββ : {G : ConcreteGroup β} β ΟβB G β‘ Οβββ 0 (B G) Οββ‘Οβββ {G = G} = β«-Path Groups-equational (total-hom inc (record { pres-β = Ξ» _ _ β refl })) (β₯-β₯β-idempotent (G .has-is-groupoid _ _)) We define a functor from concrete groups to abstract groups. The object mapping is given by taking the fundamental group. Given a pointed map $f : \mathbf{B} G \to \mathbf{B} H$, we can apply it to a loop on $\bullet_{G}$ to get a loop on $f(\bullet_{G})$; then, we use the fact that $f$ is pointed to get a loop on $\bullet_{H}$. Ξ β : Functor (ConcreteGroups β) (Groups β) Ξ β .Fβ = ΟβB Ξ β .Fβ (f , ptf) .hom x = sym ptf Β·Β· ap f x Β·Β· ptf By some simple path yoga, this preserves multiplication, and the construction is functorial: Ξ β .Fβ (f , ptf) .preserves .pres-β x y = (sym ptf Β·Β· β ap f (x β y) β Β·Β· ptf) β‘ (sym ptf Β·Β· ap f x β ap f y Β·Β· ptf) β‘Λ (sym ptf Β·Β· ap f x Β·Β· ptf) β (sym ptf Β·Β· ap f y Β·Β· ptf) β Ξ β .F-id = Homomorphism-path Ξ» _ β sym (Β·Β·-filler _ _ _) Ξ β .F-β (f , ptf) (g , ptg) = Homomorphism-path Ξ» x β (sym (ap f ptg β ptf) Β·Β· ap (f β g) x Β·Β· (ap f ptg β ptf)) β‘Λ (sym ptf Β·Β· β ap f (sym ptg) Β·Β· ap (f β g) x Β·Β· ap f ptg β Β·Β· ptf) β‘Λ (sym ptf Β·Β· ap f (sym ptg Β·Β· ap g x Β·Β· ptg) Β·Β· ptf) β We start by showing that Ξ β is split essentially surjective. This is the easy part: to build a concrete group out of an abstract group, we simply take its Delooping, and use the fact that the fundamental group of the delooping recovers the original group. Ξ β-is-split-eso : is-split-eso (Ξ β {β}) Ξ β-is-split-eso G .fst = concrete-group (Deloop G , base) Deloop-is-connected squash Ξ β-is-split-eso G .snd = pathβiso (Οββ‘Οβββ β sym (Gβ‘ΟβB G)) We now tackle the hard part: to prove that Ξ β is fully faithful. In order to show that the action on morphisms is an equivalence, we need a way of βdeloopingβ a group homomorphism $f : \pi_1(\mathbf{B} G) \to \pi_1(\mathbf{B} H)$ into a pointed map $\mathbf{B} G \to \mathbf{B} H$. module Deloop-Hom {G H : ConcreteGroup β} (f : Groups β .Hom (ΟβB G) (ΟβB H)) where open ConcreteGroup H using (H-Level-B) How can we build such a map? All we know about $\mathbf{B} G$ is that it is a pointed connected groupoid, and thus that it comes with the elimination principle B-elim-contr. This suggests that we need to define a type family $C : \mathbf{B} G \to \mathrm{Type}$ such that $C(\bullet_{G})$ is contractible, conclude that $\forall x. C(x)$ holds and extract a map $\mathbf{B} G \to \mathbf{B} H$ from that. The following construction is adapted from : record C (x : β G β) : Type β where constructor mk field y : β H β p : pt G β‘ x β pt H β‘ y f-p : (Ο : pt G β‘ pt G) (Ξ± : pt G β‘ x) β p (Ο β Ξ±) β‘ f # Ο β p Ξ± Our family sends a point $x : \mathbf{B} G$ to a point $y : \mathbf{B} H$ with a function $p$ that sends based paths ending at $x$ to based paths ending at $y$, with the additional constraint that $p$ must βextendβ $f$, in the sense that a loop on the left can be factored out using $f$. For the centre of contraction, we simply pick $p$ to be $f$, sending loops on $\bullet_{G}$ to loops on $\bullet_{H}$. C-contr : is-contr (C (pt G)) C-contr .centre .C.y = pt H C-contr .centre .C.p = f .hom C-contr .centre .C.f-p = f .preserves .pres-β As it turns out, such a structure is entirely determined by the pair $(y, p(\mathrm{refl}) : \bullet_{H} \equiv y)$, which means it is contractible. C-contr .paths (mk y p f-p) i = mk (ptβ‘y i) (funextP fβ‘p i) (β‘β‘β‘ i) where ptβ‘y : pt H β‘ y ptβ‘y = p refl fβ‘p : β Ο β Square refl (f # Ο) (p Ο) (p refl) fβ‘p Ο = β-filler (f # Ο) (p refl) β· (sym (f-p Ο refl) β ap p (β-idr Ο)) β‘β‘β‘ : PathP (Ξ» i β β Ο Ξ± β fβ‘p (Ο β Ξ±) i β‘ f # Ο β fβ‘p Ξ± i) (f .preserves .pres-β) f-p β‘β‘β‘ = prop! We can now apply the elimination principle and unpack our data: c : β x β C x c = B-elim-contr G C-contr g : B G ββ B H p : {x : β G β} β pt G β‘ x β pt H β‘ g .fst x g .fst x = c x .C.y g .snd = sym (p refl) p {x} = c x .C.p f-p : (Ο : pt G β‘ pt G) (Ξ± : pt G β‘ pt G) β p (Ο β Ξ±) β‘ f # Ο β p Ξ± f-p = c (pt G) .C.f-p In order to show that this is a delooping of $f$ (i.e.Β that $\Pi_1(g) \equiv f$), we need one more thing: that $p$ extends $g$ on the right. We get this essentially for free, by path induction, because $p(Ξ±)$ ends at $g(x)$ by definition. p-g : (Ξ± : pt G β‘ pt G) {x' : β G β} (l : pt G β‘ x') β p (Ξ± β l) β‘ p Ξ± β ap (g .fst) l p-g Ξ± = J (Ξ» _ l β p (Ξ± β l) β‘ p Ξ± β ap (g .fst) l) (ap p (β-idr _) β sym (β-idr _)) Since $g$ is pointed by $p(\mathrm{refl})$, this lets us conclude that we have found a right inverse to $\Pi_1$: fβ‘apg : (Ο : pt G β‘ pt G) β Square (p refl) (f # Ο) (ap (g .fst) Ο) (p refl) fβ‘apg Ο = commutesβsquare$
p refl β ap (g .fst) Ο β‘Λ
p (refl β Ο)           β‘Λ
p (Ο β refl)           β‘
f # Ο β p refl         β

rinv : Ξ β .Fβ g β‘ f
rinv = Homomorphism-path Ξ» Ο β sym (Β·Β·-unique' (symP (fβ‘apg Ο)))

We are most of the way there. In order to get a proper equivalence, we must check that delooping $\Pi_1(f)$ gives us back the same pointed map $f$.

We use the same trick, building upon what weβve done before: start by defining a family that asserts that $p_x$ agrees with $f$ all the way, not just on loops:

module Deloop-Hom-Ξ β {G H : ConcreteGroup β} (f : B G ββ B H) where
open Deloop-Hom {G = G} {H} (Ξ β .Fβ f) public
open ConcreteGroup H using (H-Level-B)

Cβ² : β x β Type _
Cβ² x = Ξ£ (f .fst x β‘ g .fst x) Ξ» eq
β (Ξ± : pt G β‘ x) β Square (f .snd) (ap (f .fst) Ξ±) (p Ξ±) eq

This is a property, and $\bullet_{G}$ has it:

Cβ²-contr : is-contr (Cβ² (pt G))
Cβ²-contr .centre .fst = f .snd β sym (g .snd)
Cβ²-contr .centre .snd Ξ± = transport (sym Squareβ‘double-composite-path) $Β·Β·-β-assoc β sym (f-p Ξ± refl) β ap p (β-idr _) Cβ²-contr .paths (eq , eq-paths) = Ξ£-prop-path!$
sym (β-unique _ (transpose (eq-paths refl)))

Using the elimination principle again, we get enough information about g to conclude that it is equal to f, so that we have a left inverse.

cβ² : β x β Cβ² x
cβ² = B-elim-contr G Cβ²-contr

gβ‘f : β x β g .fst x β‘ f .fst x
gβ‘f x = sym (cβ² x .fst)

The homotopy gβ‘f is pointed by definition, but we need to bend the path into a Square:

Ξ² : gβ‘f (pt G) β‘ sym (f .snd β sym (g .snd))
Ξ² = ap (sym β fst) (sym (Cβ²-contr .paths (cβ² (pt G))))

ptgβ‘ptf : Square (gβ‘f (pt G)) (g .snd) (f .snd) refl
ptgβ‘ptf i j = hcomp (β i β¨ β j) Ξ» where
k (k = i0) β β-filler (f .snd) (sym (g .snd)) (~ j) (~ i)
k (i = i0) β g .snd j
k (i = i1) β f .snd (j β§ k)
k (j = i0) β Ξ² (~ k) i
k (j = i1) β f .snd (~ i β¨ k)

linv : g β‘ f
linv = funextβ gβ‘f ptgβ‘ptf

Phew. At last, Ξ β is fully faithful.

Ξ β-is-ff : is-fully-faithful (Ξ β {β})
Ξ β-is-ff {β} {G} {H} = is-isoβis-equiv \$
iso Deloop-Hom.g Deloop-Hom.rinv (Deloop-Hom-Ξ β.linv {G = G} {H})

Weβve shown that Ξ β is fully faithful and essentially surjective; this lets us conclude with the desired equivalence.

ConcreteβAbstract : is-equivalence (Ξ β {β})
ConcreteβAbstract = ff+split-esoβis-equivalence Ξ β-is-ff Ξ β-is-split-eso