4
$\begingroup$

Let $\mathcal{C}$ be a cofibrantly generated model category, which is generated by $I$ and $J$. According to the small object argument (Hovey Theorem 2.1.14) of cofibrantly generated model categories, each map $X\to Y$ in $\mathcal{C}$ could be factored as $$X\xrightarrow{i} Z \xrightarrow{r} Y,$$ where $i$ is a relative $I$-cell complex and $r$ is a weak equivalence. But now, I need the cofibrantly generated model category $\mathcal{C}$ to satisfy the extra following property:

For any finite $I$-cell complex $X$ in $\mathcal{C}$, the fold map $(\mathsf{id}_X,\mathsf{id}_X)\colon X\amalg X \to X$ could always be factored as $$X\amalg X \xrightarrow{(i_0,i_1)} X' \xrightarrow{r} X$$ where $(i_0,i_1)$ is a relative finite $I$-cell complex and $r$ is a weak equivalence.

(To avoid misunderstanding, a relative finite $ I $‑cell complex means that both the factorization and the coproduct of morphisms in $I$ are finite.)

I think this property is not established for every cofibrantly generated model categories, but I also think it is not a very strong restriction. So I was wondering if there are some "deeper meaning" of this property (in a more general view)? or some large classes of cofibrantly generated model categories (maybe with extra structure such as enrich structure) has this property?

Additionally, the category $\mathbf{Top}$ (or $\mathbf{SSet}$) is too special for an example of this property, since the "middle object" $X'=X\times [0,1]$ of such factorization is generated by the catesian product, and the finiteness of $X\times [0,1]$ also rely on the catesian product. And I also read Williamson's thesis Cylindrical model structures , it looks like it satisfies this property, but as I said I think this property may not depend on the "cylindrical structure".

$\endgroup$

1 Answer 1

6
$\begingroup$

I've encountered that condition a few time. Here is what I know about it:

If that property is satisfied for a model category $M$ then the full subcategory $C$ of finite $I$-cell complex is itself a Brown category of cofibrant object.

So they both have $\infty$-category attached to them: $h_\infty(M)$ which is locally presentable and $h_\infty(C)$ which his finitely co-complete. If we add the assumption that all maps in $I$ are between finitely presentable objects of $C$, then $h_\infty(M)$ is $\omega$-presentable with

$$h_\infty(M) \simeq Ind(h_\infty(C)) $$

(the ind completion)

Conversely, given any small Brown category of cofibrant objects $C$ you can build canonically a $\omega$-combinatorial left semi-model category $M$ such that $C$ identitfies with the finite $I$-cell complex in $M$. (and hence $M$ is a model for the infinity category cal ind completion of $C$).

Another remark is that all this can be generalized from "finite" to "$\lambda$-small" for any $\lambda$ and if $M$ is a combinatorial model category then there is $\lambda$ such that it satisfies these conditions (i.e. $\lambda$-small $I$-complex have cylinder that are relative $\lambda$-small complex ).

Some of these claims appears in this paper of mine, the rest can be justified from there (except the clain about $\infty$-categories, that will be in an upcoming work).

$\endgroup$
7
  • $\begingroup$ Thank you so much for your answer! It really looks like what I want to know! I can see most of your points, except the $h_{\infty}$ part (I will read your paper seriously later). However, I have a stupid question. I am not sure that a cofibration between finite $I$-cell complexes is always a relative $I$-cell complex (if it is relative, the finiteness is automatic). $\endgroup$
    – Frank
    Commented Nov 19 at 14:26
  • $\begingroup$ So when we try to construct a semi-model category with that property as you described, we should consider whether the factorization axioms of the Brown category satisfying finiteness (the first cofibration in the factorization is a relative (finite) $I$-cell complex) at first? $\endgroup$
    – Frank
    Commented Nov 19 at 14:27
  • $\begingroup$ I'm not sure I understood the question correctly but: Theorem 2.4 in the linked paper implies that if you start with a finitely presentable category and $I$ generating cofibrations between finitely presentable objects. Then if the category of finite $I$-cell complex is a Brown category where the cofibration are the relative $I$-cell complexes (which is closely related to having a factorization as in the question) then this extend into a left semi-structure on the whole category which is cofibrantly generated by $I$ and by the set of "trivial cofibrations" of the Brown category. $\endgroup$ Commented Nov 19 at 15:51
  • $\begingroup$ ... Note that condition (iii') of the theorem is obtained from a relative cylinder by taking B=B' and C to be the relative cylinder $B \coprod_A B \hookrightarrow C \overset{\sim}{\to} B$ $\endgroup$ Commented Nov 19 at 16:24
  • $\begingroup$ Sorry for my stupidity. May I ask you what will the class $J$ in Theorem 2.4 be in this case? Thank you so much! $\endgroup$
    – Frank
    Commented Nov 21 at 1:59

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .