Contents

model category

for ∞-groupoids

# Contents

## Idea

The model structure on reduced simplicial sets is a presentation of the full sub-(∞,1)-category

∞Grpd${}^{*/}_{\geq 1} \hookrightarrow$ ∞Grpd${}^{*/}$ $\simeq$ Top${}^{*/}$

of pointed ∞-groupoids on those that are connected.

By the looping and delooping-equivalence, this is equivalent to the (∞,1)-category of ∞-groups and this equivalence is presented by a Quillen equivalence to the model structure on simplicial groups.

## Definition

###### Definition

A reduced simplicial set is a simplicial set $S$ with a single vertex:

$S_0 = * \,.$

Write $sSet_0 \subset$ sSet for the full subcategory of the category of simplicial sets on those that are reduced.

###### Proposition

There is a model category structure on $sSet_0$ (def. ) whose

• weak equivalences

• and cofibrations

are those in the classical model structure on simplicial sets (i.e. the weak homotopy equivalences and the monomorphisms, respectively.)

This appears as (Goerss-Jardine, ch V, prop. 6.2).

## Properties

### Relation to ordinary simplicial sets

###### Proposition

Under the forgetful functor $U \colon sSet_0 \hookrightarrow sSet$

In particular

• every fibrant object maps to a fibrant object.
###### Proof

The first statment appears as (Goerss-Jardine, ch. V, lemma 6.6.). The second is an immediate consequence. It appears as (Goerss-Jardine, ch. V, corollary 6.8).

### Relation to pointed simplicial sets

###### Proposition
$sSet^{\ast/} \underoverset {\underset{cn}{\longrightarrow}} {\overset{i}{\longleftarrow}} {\bot} sSet_{0}$

into pointed simplicial sets (where $i$ the obvious inclusion, and $cn$ forms the 1st Eilenberg subcomplex over the basepoint) is a Quillen adjunction (with respect to the reduced model structure from prop. and the coslice model structure under the point of the classical model structure on simplicial sets).

###### Proof

By prop. the left adjoint preserves cofibrations and acyclic cofibrations (in fact all weak equivalences).

###### Proposition

The operation of reduced suspension $\Sigma_\ast$ (smash product with the simplicial circle $S^1 \coloneqq \Delta[1]/\partial \Delta[1]$) and forming loop space $\Omega_\ast$ (pointed mapping space out of the circle) constitute a Quillen adjunction

$sSet_0 \underoverset {\underset{\Omega_\bullet}{\longrightarrow}} {\overset{\Sigma_\ast}{\longleftarrow}} {\bot} sSet^{\ast/}$
###### Proof

By the internal hom construction we have the adjunction

$sSet^{\ast/} \underoverset {\underset{\Omega_\bullet}{\longrightarrow}} {\overset{\Sigma_\ast}{\longleftarrow}} {\bot} sSet^{\ast/}$

But by the formula at smash product the reduced suspension clearly lands in reduced simplicial sets, so that we do have the restricted adjunction as claimed. Now by the fact that the classical model structure on simplicial sets is a simplicial model category, the above is a Quillen adjunction and $\Sigma_\ast$ preserves cofibrations and acyclic cofibrations. Hence, by prop. , so does its factorization through the model structure on reduced simplicial sets.

###### Corollary

The operations of reduced suspension and loop space (co-)restrict to a Quillen adjunction on the reduced model structure (prop. ) itself, by composition of the Quillen adjunctions from prop. and prop. .

$sSet_0 \underoverset {\underset{\Omega_\bullet}{\longrightarrow}} {\overset{\Sigma_\ast}{\longleftarrow}} {\bot} sSet^{\ast/} \underoverset {\underset{cn}{\longrightarrow}} {\overset{i}{\longleftarrow}} {\bot} sSet_{0}$

### Relation to simplicial groups

###### Proposition

The simplicial loop space functor $G$ and the delooping functor $\overline{W}(-)$ (discussed at simplicial group) constitute a Quillen equivalence

$(G \dashv \bar W) \colon sGr \underoverset {\underset{\bar W}{\longrightarrow}} {\overset{G}{\longleftarrow}} {\bot} sSet_0$

between the model structure on reduced simplicial sets from prop. and model structure on simplicial groups.

This appears as (Goerss-Jardine, ch. V prop. 6.3).

## References

A standard textbook reference is chapter V of

Last revised on March 6, 2017 at 11:48:53. See the history of this page for a list of all contributions to it.