model category, model $\infty$-category
Definitions
Morphisms
Universal constructions
Refinements
Producing new model structures
Presentation of $(\infty,1)$-categories
Model structures
for $\infty$-groupoids
on chain complexes/model structure on cosimplicial abelian groups
related by the Dold-Kan correspondence
for equivariant $\infty$-groupoids
for rational $\infty$-groupoids
for rational equivariant $\infty$-groupoids
for $n$-groupoids
for $\infty$-groups
for $\infty$-algebras
general $\infty$-algebras
specific $\infty$-algebras
for stable/spectrum objects
for $(\infty,1)$-categories
for stable $(\infty,1)$-categories
for $(\infty,1)$-operads
for $(n,r)$-categories
for $(\infty,1)$-sheaves / $\infty$-stacks
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.
A reduced simplicial set is a simplicial set $S$ with a single vertex, hence with $X_0$ the singleton set
Write $sSet_0 \subset$ sSet for the full subcategory of the category of simplicial sets on those that are reduced.
There is a model category structure on $sSet_0$ (def. ) whose
weak equivalences
and cofibrations
are those whose underlying maps are such in the classical model structure on simplicial sets (i.e. the simplicial weak homotopy equivalences and the monomorphisms, respectively).
This appears as Goerss & Jardine, Ch V, Prop. 6.2.
Under the forgetful functor $U \colon sSet_0 \hookrightarrow sSet$
In particular:
The first statment is proven in Goerss & Jardine, Ch. V, Lemma 6.6.. The second is an immediate consequence, stated there as as Goerss & Jardine, Ch. V, Corollary 6.8.
Let $f \colon X \longrightarrow Y$ be a fibration in the model structure on reduced simplicial sets (Prop. ) such that both $X$ and $Y$ are Kan complexes. Then $f$ is a Kan fibration precisely if it induces a surjection on the first simplicial homotopy group $\pi_1(f) \colon \pi_1(X) \twoheadrightarrow \pi_1(Y)$.
(Goerss & Jardine, Ch. V, Cor. 6.9)
As an example:
Let $\mathcal{G}_1 \xrightarrow{\phi} \mathcal{G}_2$ be a homomorphism of simplicial groups which is a Kan fibration. Then the induced morphism of simplicial classifying spaces $\overline{W}\mathcal{G}_1 \xrightarrow{ \overline{W}(\phi)} \overline{W}\mathcal{G}_2$ is a Kan fibration if and only if $\phi$ is a surjection on connected components: $\pi_0(\phi) \colon \pi_0(\mathcal{G}_1) \twoheadrightarrow{\;} \pi_0(\mathcal{G}_1)$.
Since $\overline{W}(-)$ is a right Quillen functor to the model structure on reduced simplicial sets (Prop. ) it follows that $\overline{W}(\phi)$ is in any case a fibration in that model structure. Hence Cor. implies that $\overline{W}(\phi)$ is a Kan fibration precisely of $\pi_1 \circ \overline{W}(\phi)$ is surjective. But $\pi_1 \circ \overline{W} = \pi_0$, by this Prop.
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).
By prop. the left adjoint preserves cofibrations and acyclic cofibrations (in fact all weak equivalences).
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
By the internal hom construction we have the adjunction
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.
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. .
(Quillen equivalence between simplicial groups and reduced simplicial sets)
The simplicial loop space functor $G$ and the simplicial classifying space-construction $\overline{W}(-)$ constitute a Quillen equivalence
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).
model structure on reduced simplicial sets
groupoid object in an (∞,1)-category, ∞-group, looping and delooping
Textbook account for the Kan model structure on reduced simplicial sets:
The analog (in fact transfer) of the Joyal model structure for reduced simplicial sets, modelling quasi-categories with a single object:
Last revised on May 16, 2023 at 17:37:36. See the history of this page for a list of all contributions to it.