
> 
> **Important Edit**: I e-mailed Jacob Lurie, and he said that the statement of condition (\*) is incorrect as printed.
> 
> 
> Here is the correct statement of (\*):
> 
> 
> For any cofibration $f:A\to B$ and any *trivial* fibration $g:X\to Y$ in $C$, the induced morphism:
> 
> 
> $$\operatorname{Map}(B,X)\to \operatorname{Map}(B,Y)\times\_{\operatorname{Map}(A,Y)} \operatorname{Map}(A,X)$$
> 
> 
> is a *trivial* Kan fibration. (Where $\operatorname{Map}$ is the (sSet)-enriched $\operatorname{Hom}$).
> 
> 
> 


Let $C$ be a simplicially-enriched category with a model structure, not necessarily compatible with the simplicial enrichment.


Suppose that all objects of $C$ are cofibrant, that $C$ is tensored and cotensored over $SSet$, and that the class of weak equivalences of $C$ is closed under filtered colimits.


(\*)Suppose further that for any cofibration $f:A\to B$ and any fibration $g:X\to Y$ in $C$, the induced morphism:


$$\operatorname{Map}(B,X)\to \operatorname{Map}(B,Y)\times\_{\operatorname{Map}(A,Y)} \operatorname{Map}(A,X)$$


is a Kan fibration. (Where $\operatorname{Map}$ is the (sSet)-enriched $\operatorname{Hom}$).


Lastly, assume that $A\otimes \Delta^n\tilde{\to}A\otimes \Delta^0=A$ is a weak equivalence for any object $A$ in $C$ and any $n\in \mathbf{N}$. (Here, the tensor $A\otimes K$, where $A$ is in $C$ and $K$ is in $sSet$, is the object of $C$ representing the functor $Map(A,-)^K$).


Let $L\subseteq K$ be an inclusion of simplicial sets. Suppose $\sigma:\Delta^n\hookrightarrow K$ is a nondegenerate simplex of $K$ with all of its faces living in $L$. That is, we can factor the map $\partial\sigma:=\sigma|\_{\partial\Delta^n}$ through the inclusion $L\subseteq K$ (in fact, we will assume that the target of this map actually is $L$).


Then for any object $D$ in $C$, the pushout $$D\otimes \Delta^n\coprod\_{D\otimes\partial\Delta^n} D\otimes L\cong D\otimes (\Delta^n\coprod\_{\partial\Delta^n}L)$$ is a homotopy pushout. Now, the question here is, why is this the case?


The proof I'm reading says that it follows from the line marked (\*) above (and the fact that $C$ is left-proper (which follows from the fact that all objects of $C$ are cofibrant)), but it's not clear to me how to apply that hypothesis.


That is, how does the line marked (\*) imply anything relevant?


If you'd like to look up the original source, it is Higher Topos Theory Proposition A.3.1.7 (in the appendix).


Edit: It's probable that a few of the hypotheses are unrelated to the actual question. I included everything because I'm not sure what's important.



Verifying a technical lemma regarding homotopy pushouts in the theory of simplicial model categories 
Assuming I understand the question, isn't the following a counterexample? Let $\cal C$ be sSet with the usual model structure, but with the trivial simplicial enrichment in which the simplicial set $Map(A,X)$ is the discrete (constant) set of sSet morphisms $A\to X$. So $A\otimes K$ is coproduct of $\pi\_0(K)$ copies of $A$. Then the inclusion $\partial\Delta^1\to \Delta^1$ induces a map $A\otimes \partial\Delta^1\to A\otimes \Delta^1$ that is not in general a cofibration, and the pushout $A\otimes \* \leftarrow A\otimes \partial\Delta^1\to A\otimes \Delta^1$ is not a homotopy pushout.


Or am I making some mistake about the meaning of "enriched, tensored, and cotensored", or about the meaning of "homotopy pushout" in the present context?


