# Maximal Cisinski model structure on simplicial sets

This is a very simple question coming from the observation that every (pre)sheaf category has the maximal Cisinski model structure on it. This is the Cisinski model structure with the smallest class of weak equivalences possible.

Now, it is natural to ask: what is the maximal Cisinski model structure on the most canonical category in this setting, namely the category of simplicial sets? Is it larger than the Joyal model structure? Assuming that the answer is "yes", can we give an explicit description of its weak equivalences and fibrant objects?

• So taking the class of weak equivalences to be the isos doesn't work? – David Roberts Jul 29 '17 at 9:41
• @DavidRoberts No since trivial fibrations must be weak equivalences. If $X$ is a nontrivial contractible Kan complex, then $X \to 1$ is a trivial fibration, but not an iso. – Valery Isaev Jul 29 '17 at 9:47
• Ah, I was under the impression that the condition was that the cofibrations were the monos. – David Roberts Jul 29 '17 at 10:06
• @DavidRoberts Yes, this is the condition and it implies that the trivial fibrations are the same as in the Quillen (or Joyal) model structure. – Valery Isaev Jul 29 '17 at 10:08
• And there is even another issue. On his paper Catégories dérivables he defines a variation of the notion of model structure, which has all the right to be called a Cisinski model structure (although it is way less popular then model structures on presheaves categories). – Andrea Gagna Aug 1 '17 at 7:03

I will refer by [Ara] to this article by Ara for terminology and notation, in particular to section 2, since it is synthetic and in English; and I will refer by [Cis] to Cisinski's book.

Let $A$ be a small category.

Theorem 1.4.3 of [Cis] (see also Theorem 2.2 of [Ara]) states that one has a cofibrantly generated model structure on $\widehat{A}$ where cofibrations are monomorphisms precisely when the class $\mathbf W$ of weak equivalences are an accessible $A$-localizer (see §2.1 of [Ara]). Further, this is the model structure that one gets by mean of the homotopical structure (or homotopical datum) $(\mathfrak{L}, S)$, where $\mathfrak L$ is the functorial cylinder associated to the subobject classifier (also known as Lawvere object) and $S$ is a set of monomorphisms of $\widehat A$ (cf. Theorem 1.3.22 of [Cis] and Theorem 2.14 of [Ara]).

In particular, a homotopical structure allows you to define a good notion of anodyne extensions and to determine fibrant objects (and fibrations between fibrant objects), see §2.10 of [Ara] and what follows.

From all this, it results that the model structure you seek has the minimal $\Delta$-localizer as weak equivalences and can be described by the homotopical structure where the cylinder is the Lawvere object of $\widehat \Delta$ and the small set of monomorphisms $S$ is the empty set. Then, following §2.10 in [Ara] or equivalently §1.3.2 in [Cis], you can construct a certain set $\Lambda_{\mathfrak{L}}(\emptyset, \mathcal M)$ (for $\widehat \Delta$ you can choose $\mathcal M$ to be the set of boundary inclusions of representables $\{\partial \Delta_m \to \Delta_m\}$) so that the class of relative anodyne extensions $\mathbf{An}_{\mathfrak L}$ will be the saturated class generated by $\Lambda_{\mathfrak{L}}(\emptyset, \mathcal M)$. The fibrant objects for this model structure will be simplicial sets with the right lifting property with respect to the anodyne extensions and therefore to the set $\Lambda_{\mathfrak{L}}(\emptyset, \mathcal M)$.

Unfortunately, I do not know of any better combinatrial description of $\Lambda_{\mathfrak{L}}(\emptyset, \mathcal M)$. Joyal's model structure is a left Bousfield localisation of this one and it is determined by the $\Delta$-localizer $\mathbf{W}(\mathrm{Sp})$, where $\mathrm{Sp}$ is the set of spine inclusions of representables $\{I_n \to \Delta_n\}$ (cf. Theorem 5.20 of [Ara]); in particular, every quasi-category is a fibrant object for the minimal model structure. We know that the minimal localizer $\mathbf W$ is strictly smaller that $\mathbf{W}(\mathrm{Sp})$, since the model structure given by the minimal localizer is proper (see Remarque 1.5.5 of [Cis]), while Joyal's model structure is not right proper.

• I was referring to these constructions in the first paragraph of the post. The question is whether we can prove that $\mathbf{W}(\mathrm{Sp})$ is strictly smaller than $\mathbf{W}$. The second question is about an explicit description of fibrant objects (and weak equivalence). I would not be surprised if there is no explicit description for them, but maybe we can prove at least some of their properties. A simple example of such a property is that every contractible Kan complex is fibrant. It would be nice to see other such properties. – Valery Isaev Jul 29 '17 at 14:35
• A less trivial example is that every fibrant object must have the right lifting property with respect to the maps ${\Lambda'}^k_n \to \Delta'_n$, where $\Delta'_n$ is the nerve of the trivial groupoid on $n$ objects and ${\Lambda'}^k_n$ is the obvious colimit of $\Delta'_{n-1}$. Now, the question is whether this condition is sufficient. – Valery Isaev Jul 29 '17 at 15:25
• I edited the last paragraph trying to answer better to your questions. Anyway, the minimal model structure is indeed strictly smaller than Joyal's for properness reasons and every quasi-category is fibrant in the minimal structure. – Andrea Gagna Jul 29 '17 at 17:15
• According to Remarque 1.3.15 in [Cis] we have $\mathbf{An}_{\mathfrak{L}} = \operatorname{l}(\operatorname{r}(\Lambda'_L(\mathcal{M})))$, where $\Lambda'_L(\mathcal{M})$ is the set of monomorphisms of the form $L \times \partial \Delta^n \cup \{\epsilon\} \times \Delta^n \to L \times \Delta^n$ with $n \in \mathbb{N}$ and $\epsilon \in \{0,1\}$. – Daniel Gerigk Nov 8 '17 at 17:53

I think I've worked out a relatively nice description of the fibrant objects. I won't be able to include the full proofs here, but I will give the description and explain a bit about why it works. (I'm planning to include the details in a longer note, to which I'll post a link here once I make it available, but that will probably take at least a couple more months.)

Let me first recap some of Andrea's answer and the comments following it: the fibrant objects in the minimal model structure are those with lifts against $$\textbf{An}_{\mathfrak{L}}$$. The class $$\textbf{An}_{\mathfrak{L}}$$ is generated by the pushout-products of the boundary inclusions $$\partial\Delta[n]\hookrightarrow\Delta[n]$$ with the maps $$\{\varepsilon\}\hookrightarrow L$$ for $$\varepsilon=0,1$$, where $$L$$ is the subobject classifier in simplicial sets.

The first thing to notice is that we can actually use $$J$$, the nerve of the groupoid with two objects and an isomorphism between them, instead of $$L$$. One way to see why is to work out what $$L$$ is as a simplicial set, and see that $$J$$ is actually a retract of $$L$$. I talked to Alex Campbell, however, and he told me that if you trace through Cisinski's arguments in his book, you can actually see that everything he says about $$L$$ would apply to a separating cylinder which is trivially fibrant, such as $$J$$.

So, at any rate, now we know the fibrant objects are those with lifts against $$\textbf{An}_{J}$$, which is generated by the pushout-products of the boundary inclusions $$\partial\Delta[n]\hookrightarrow\Delta[n]$$ with the map $$\{0\}\hookrightarrow J$$. With this description already, there is some nice intuition: having lifts against the pushout-product

$$(\partial\Delta[n]\times J) \cup (\Delta[n]\times \{0\})\hookrightarrow \Delta[n]\times J$$

tells you that any time you have an $$n$$-simplex and an "isomorphism of boundaries" which starts at the boundary of that $$n$$-simplex, then it extends to an "isomorphism of $$n$$-simplices" which starts at that $$n$$-simplex.

The description so far, which is not new, was not satisfying to me. I wanted something which felt more like the horn extension definition of Kan complexes and quasi-categories, so I played around a bit and found the following:

Define an "isoplex," denoted $$\mathfrak{D}_i[n]$$, to be the nerve of the category

$$c_0\rightarrow c_1 \rightarrow \ldots c_{i-1} \rightarrow c_i \leftrightarrow c_{i+1} \rightarrow c_{i+2} \rightarrow \ldots \rightarrow c_n$$,

i.e., where the morphism $$c_i\rightarrow c_{i+1}$$ is an isomorphism. Think of this as analogous to $$\Delta[n]$$, which is the nerve of the category $$c_0\rightarrow c_1 \rightarrow \ldots \rightarrow c_n$$. In particular, we can define faces of $$\mathfrak{D}_i[n]$$ in a similar manner, where $$d_j\mathfrak{D}_i[n]$$ is the maximal subcomplex not containing the $$j$$ vertex. Then we can define the iso-horn of $$\mathfrak{D}_i[n]$$, denoted $$\mathbb{V}_i[n]$$, to be the union of all of its faces except the $$i$$th face. Let $$\text{IsoHorn}$$ be the set of all iso-horn inclusions $$\mathbb{V}_i[n]\hookrightarrow \mathfrak{D}_i[n]$$.

It turns out that $$\text{IsoHorn}$$ generates the class $$\textbf{An}_J$$. In particular, the fibrant objects in the minimal model structure are the simplicial sets with lifts against $$\text{IsoHorn}$$.

That $$\overline{\text{IsoHorn}}\subseteq \textbf{An}_J$$ is not so bad to check, because each iso-horn extension is a retract of a generator of $$\textbf{An}_J$$. The other direction follows because each of the generators of $$\textbf{An}_J$$ can be built out of iso-horn extensions (by transfinite composition of pushouts), which I think is pretty intuitive, but which did take a couple pages of combinatorics for me to check carefully.

As for the minimal model structure being different from Joyal's, in addition to Andrea's properness argument, we can also just see directly that the horn $$\Lambda^1[2]$$ is fibrant in the minimal model structure.

But not only is the minimal model structure different from Joyal's, I'm confident there will be multiple interesting Cisinski model structures between them. In particular, my thesis project is to find a Cisinski model structure where the fibrant objects model up-to-homotopy versions of 2-Segal sets (which is why I was thinking about the minimal model structure in the first place).

• This looks great! I will accept your answer after the details become available. – Valery Isaev Aug 2 '19 at 8:42