# Analogue of simplicial sets

This question is prompted by this one (and some of the comments that it drew).

Simplicial complex is to ordered simplicial complex as $X$ is to simplicial set. The question is about $X$.

Let $\text{Unord}$ be the full subcategory of $\text{Set}$ whose objects are the sets $[n]=\lbrace 0,\dots ,n\rbrace$ for $n\ge 0$. An object of $\text{Set}^{\text{Unord}^{\text{op}}}$ is basically a simplicial set $X_\bullet$ with a suitable $\Sigma_{n+1}$-action on $X_n$ for all $n$. The obvious functor $\Delta:\text{Unord} \to \text{Top}$ (which creates a simplex with given vertex set) determines a functor $$\text{Set}^{\text{Unord}^{\text{op}}} \to \text{Top}$$

This has a left adjoint analogous to the realization of simplicial sets.

Is the counit of this adjunction a weak homotopy equivalence for all spaces?

If so, is this adjunction a Quillen equivalence for some model structure on $\text{Set}^{\text{Unord}^{\text{op}}}$?

If not, is there something else along these lines that works?

(This must be known. It seems like an obvious question, and from some comments at the other question I gather that at least on the homology side this is something people thought about a long time ago.)

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X sounds like the "symmetric simplicial sets" studied in the paper "Left-determined model categories and universal homotopy theories" by Rosický and Tholen. –  Mike Shulman Mar 4 '11 at 19:22
@Tom: for some reason, the text was rendered incorrectly on my screen, so I made some changes. –  John Klein Mar 4 '11 at 19:35
@Tom: one thing I didn't change: "This has a left adjoint analogous to the realization of simplicial sets." Don't you mean right adjoint here? –  John Klein Mar 4 '11 at 19:40
@Mike: I looked at the paper and I think that's correct. It is stated there that "symmetric simplicial sets" and simplicial sets are Quillen equivalent. –  John Klein Mar 4 '11 at 20:24
@John: Realization is left adjoint; singular complex is right adjoint. Yes? –  Tom Goodwillie Mar 4 '11 at 23:23

## 1 Answer

I have never thought of a counter example, but I would not bet on a positive answer to the first question. However, the answer to the second question is yes: this version of the singular functor is a right Quillen equivalence for a suitable model category on $\mathrm{Set}^{\mathrm{Unord}^{op}}$.

As mentioned above by Mike Shulman, this homotopy theory of symmetric simplicial sets is studied in the paper

J. Rosický and W. Tholen, Left-determined model categories and universal homotopy theories, Trans. Amer. Math. Soc. 355 (2003), 3611-3623.

In their paper, they construct the model structure on $\mathrm{Set}^{\mathrm{Unord}^{op}}$, but this has to be completed by

J. Rosický and W. Tholen, Erratum to "Left-determined model categories and universal homotopy theories", Trans. Amer. Math. Soc. 360 (2008), 6179-6179.

in which the authors explain that, after all, the class of cofibrations they considered is not the class of all monomorphisms (this is why I have some doubts about the fact that the topological realisation would be well behaved if we do not consider a cofibrant resolution in the picture).

You may find another construction of this model category (with an explicit description of what the cofibrations are) as well as its precise link with Kan's subdivision functor in section 8.3 of

D.-C. Cisinski, Les préfaisceaux comme modèles des types d'homotopie, Astérisque 308 (2006),

as an example of a test category in the sense of Grothendieck; see also this MO question.

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