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Consider the pushout of a diagram $A\leftarrow B\rightarrow C$ of categories and assume that at least one of the arrows is an embedding, i.e. injective on objects and arrows. When applying the nerve functor to the pushout diagram, do we get a homotopy pushout in simplicial sets?

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  • $\begingroup$ "injective on objects" is an evil condition. Don't you want something like $F(X)\cong F(Y)\implies X\cong Y$ or (stronger) "F reflects isomorphisms" ? $\endgroup$ Commented Mar 22, 2014 at 18:56
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    $\begingroup$ @JohannesHahn: "injective on objects" is evil, but so is "pushout"; if the question involves evil concepts, then the answer may do as well. $\endgroup$ Commented Mar 22, 2014 at 19:08
  • $\begingroup$ @NeilStrickland Why is a pushout evil? It is a limit... $\endgroup$ Commented Mar 22, 2014 at 19:17
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    $\begingroup$ @JohannesHahn: limits and colimits of categories are usually evil. That is why people consider 2-(co)limits or homotopy (co)limits instead. $\endgroup$ Commented Mar 22, 2014 at 19:51
  • $\begingroup$ Do all these "evil" connotations apply to groupoids, and higher groupoids? I like colimits of groupoids and higher groupoids, and have used them for specific homotopy invariant calculations. The higher case led to the notion of nonabelian tensor product of groups, in work with Loday. Can someone please give me an example of an algebraic calculation using homotopy 2-colimits? $\endgroup$ Commented Mar 23, 2014 at 18:40

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This is wrong. Let $B=\mathbb{N}$, $A$ the cone over $B$ and $C$ the cocone over $B$. The pushout category is contractible, since it has an initial and a terminal object. But the homotopy pushout on the nerve level is the homotopy type of a $2$-sphere.

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  • $\begingroup$ I guess you are taking $B$ to be a one-object category rather than a poset-category? $\endgroup$
    – Zhen Lin
    Commented Mar 23, 2014 at 12:21
  • $\begingroup$ Yes, one object category. You can take any monoid which is not contractible. $\endgroup$ Commented Mar 23, 2014 at 13:41

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