# A construction with homotopy colimits and homotopy pullbacks for descent.

I have some troubles in trying to give a meaningful interpretation to the following property which is stated in this preprint by professor Rezk (see Definition 6.5) as part of the requirement for a model category $\mathbf{M}$ to have what the author calls descent. Here is the problematic condition:

(P1) Let $X\colon I\longrightarrow \mathbf{M}$ be a functor from a small category $I$ to a model category $\mathbf{M}$. Set $$\bar{X}:=\text{hocolim}X$$ and consider any map $f\colon\bar{Y}\rightarrow\bar{X}$ in $\mathbf{M}$. Form a functor $Y\colon I\longrightarrow\mathbf{M}$ by setting, for $i\in I$, $$Y(i):=X(i)\times^{h}_{\bar{X}}\bar{Y}.$$ Then the evident map $$\text{hocolim Y}\rightarrow \bar{Y}$$ is a weak equivalence.

(Here $(-)\times^{h}_{\bar{X}}(-)$ denotes a homotopy pullback over $\bar{X}$, I guess).

My doubts arise as I do not see a clear way to understand the construction made. Here are some points that are obscure to me:

1. To have a chance to construct something as $Y(i)$ or the "evident" map $\text{hocolim}Y\rightarrow\bar{Y}$, I should be able to get maps $X(i)\rightarrow \bar{X}$ and $Y(i)\rightarrow\bar{Y}$. The latter family of maps should then induce the alleged arrow $\text{hocolim}Y\rightarrow\bar{Y}$, or at least this is what I believe. The point now is: are there some models for the homotopy colimits and the homotopy pullbacks (for which they form functors from the appropriate diagram categories) that allow me to get maps $X(i)\rightarrow \text{hocolim}X$ and $Y(i)\rightarrow\bar{Y}$ (as for ordinary colimits and pullbacks)? Can these maps be built so as to form a (co)cone (over $\text{hocolim}X$ or over $\bar{Y}$) either in a strict sense or in some homotopically relaxed version (to be defined), so that I can use, for example, the maps $Y(i)\rightarrow\bar{Y}$ to get $\text{hocolim}Y\rightarrow\bar{Y}$?

2. Is the evidence of the map $\text{hocolim}Y\rightarrow\bar{Y}$ really so evident?

3. Are there some canonical properties of the homotopy colimits or of the homotopy pullbacks which allow me to make the construction done by Rezk (and to answer the questions in 1.) in a way which is independent of the chosen model to realize those homotopy colimits and pullbacks?

Note: Rezk considers arbitrary model categories when formulating the property (P1) above, but I would be very happy to get an answer to my questions (which ultimately boil down to the final question: "How should I interpret property (P1)?") in the case where $\mathbf{M}$ is (at most) a combinatorial model category (but not simplicial, if possible).

There is a somehow related question on MO which suggests, in the case where $\mathbf{M}$ is a category of spaces (say, simplicial sets), to feel free to cofibrantly and fibrantly replace everything (objects, arrows, diagrams) as long as this may be needed to give a meaning to constructions like the one in (P1) above. I have then tried to see if I could work out something with this underlying philosophy.

Here are my trials: in what follows, I assume $\mathbf{M}$ to be a cofibrantly generated category and keep the same notations as in (P1) above.

Let $Q$ be the functorial cofibrant replacement in the projective model structure on $\mathbf{M}^{I}$ and let $R$ be the fibrant replacement functor for the Reedy model structure on the category of cospans in $\mathbf{M}$. Given a cospan $$A\rightarrow B\leftarrow C$$ in $\mathbf{M}$, I will adopt a little abuse of notation and denote the image of this cospan under $R$ by $$RA\rightarrow RB\leftarrow RC.$$

Now, a model for the homotopy colimit of $X$ is given by $\bar{X}=\text{colim}\ QX$. In this case, I thus get a cocone $$(QX)(i)\rightarrow \text{hocolim} X$$ and, for each $i\in I$, I can consider the cospan in $\mathbf{M}$ $$(QX)(i)\rightarrow \text{hocolim} X \leftarrow \bar{Y}$$ A model for the homotopy pullback of this cospan is given by doing the ordinary pullback of $$R((QX)(i))\rightarrow R(\bar{X})\leftarrow R(\bar{Y})$$ Thus, my (corrected) functor $Y$ is given by $$Y(i):=R((QX)(i))\times_{R(\bar{X})}R(\bar{Y})$$ and I have an evident natural transformation $$Y\Rightarrow c(R(\bar{Y})),$$ where $c$ is the constant functor from $I$ at $R(\bar{Y})$. Thus, I get a map $$\text{hocolim}(Y)=\text{colim}(QY)\rightarrow \text{hocolim}(c(R(\bar{Y})))=\text{colim}(Q(c(R(\bar{Y})))),$$ hence also a map $$t\colon\text{hocolim}(Y)\rightarrow R(\bar{Y})$$ because I have a natural transformation $Q(c(R(\bar{Y})))\Rightarrow c(R(\bar{Y}))$ and the colimit of the RHS has a canonical map to $R(\bar{Y})$. I may then require this $t$ to be a weak equivalence.

Question: Does this make any kind of sense?

I would really appreciate any kind of partial answer to any of my questions above or even some general suggestions about how to interpret these kinds of constructions with homotopy colimits, pullbacks and so on.

EDIT: I reposted the question on MO as I realized it may suit it as well.

• All the morphisms in question come immediately from the universal property of the homotopy colimit. Are you familiar with that?
– user314
Mar 29, 2015 at 18:22
• @Adeel No, I am not, at least not in the most general setting of (arbitrary) model categories. I would be very grateful if you could provide a reference or an answer about that! Mar 29, 2015 at 19:40
• There are good references at the nLab page.
– user314
Mar 29, 2015 at 20:51
• @Adeel Thanks! I have to say I already knew that page and found the part where the alleged universal property for holims and hocolims a little bit too sketchy for me. I will try to take a look at some references there (for example Shulman's work), but I am afraid everything is carried over under the hypotheses of having simplicial model categories at hand, which is not my case (maybe one can use (co)simplicial resolutions to readapt those results, I don't know yet). Thus, if you would bother to give further explanations in an answer, that would be really appreciated! :) Mar 30, 2015 at 12:57
• The universal property of homotopy colimits is formulated in almost the same way as for ordinary colimits, but with additional weak equivalences. Specifically, if D: I→C is an I-indexed diagram in a model category C, then its homotopy colimit is a pair (D'→D,τ:D'→X), where D'→D is a weak equivalence of diagrams and τ is a cocone over some object X (the homotopy colimit) that satisfies the universal property: the derived space of maps Map(X,Y) maps via a weak equivalence to the derived end Map(D',const(Y)). Feb 7, 2016 at 13:05