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If $\mathcal{C}$ is a category, then surely the category of simplicial objects $s\mathcal{C}$ is not automatically a model category. What conditions must $\mathcal{C}$ satisfy in order for $s\mathcal{C}$ to have a reasonable model structure?

flag	asked Jan 12 2011 at 18:46 Jeff Strom 3,514●1●6●19

Harry Gindi · Accepted Answer · 2011-01-12 19:04:38Z UTC

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It always has a model structure using Kan's theory of Reedy categories. For a proof, see Hirschhorn Model Categories and their Localizations 15.3.

This is because $\Delta$ and $\Delta^{op}$ are both Reedy categories.

I will address the more general question as well:

If $C$ is any small category, the condition for $M^C$ to be a model category is that $M$ be cofibrantly generated. However, it is not necessarily true that $M^C$ is itself cofibrantly generated. In general, the condition we need for that is called combinatoriality. However, the existence of such a cofibrantly generated model structure that is not combinatorial would disprove Vopenka's principle (a large cardinal axiom), so I cannot give an example of one.

edited Jan 12 2011 at 19:04

answered Jan 12 2011 at 18:51

Harry Gindi
8,868●2●33●96

I stand corrected and informed, thanks! – Jeff Strom Jan 12 2011 at 18:53

No problem! – Harry Gindi Jan 12 2011 at 19:00

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I thought the questions was: when does the category of simplicial objects in $\mathcal{C}$ have a model structure, without assuming $\mathcal{C}$ is a model category. – Oscar Randal-Williams Jan 12 2011 at 19:14

I'm tempted to let you think that I asked the smartest possible question, but I didn't. I originally intended $\mathcal{C}$ to be any category. But I think we can see that if $s\mathcal{C}$ has a model structure, then $\mathcal{C}$ must be complete-cocomplete, and then we can use the trivial structure on $\mathcal{C}$ and follow Harry's method. So the question in which we assume a given model category on $\mathcal{C}$ is harder and more interesting, and what was answered. – Jeff Strom Jan 12 2011 at 19:31

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The real question is: what is a "reasonable" model category structure on $s\mathcal{C}$? What constraints should it satisfy? – Charles Rezk Jan 12 2011 at 21:11

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Charles Rezk · Answer 2 · 2011-01-12 19:28:33Z UTC

Quillen gives a couple of sets of sufficient conditions for $s\mathcal{C}$ to be a model category, in his Homotopical Algebra book. Notably, this includes the case when $\mathcal{C}$ is a complete and cocomplete category with a small set of projective generators. This gives model structures for categories of simplicial groups, simplicial rings, simplicial lie algebras, etc.

Model categories of simplicial objects

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