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$\DeclareMathOperator\Cat{Cat}$Suppose we have a span in $\Cat$ $$ \require{AMScd} \begin{CD} A @> G>> X \\ @VVFV \\ B \end{CD} $$ We can view this as a span in $\Cat_\infty$. What useful conditions can we impose to ensure the pushout is still a 1-category?

As a specific example, is either of the following conditions sufficient?

  • $F$ is injective on objects and arrows
  • $F$ is injective on objects and arrows, and every isomorphism of the form $F(X) \cong F(Y)$ is in the image of $F : A(X,Y) \to B(F(X), F(Y))$
  • Both $F$ and $G$ satisfy the property above

Remark: This second proposition is the property $F$ is a monomorphism in $Cat_\infty$ together with the proposition that $F$ is an isocofibration in $Cat$ so that if the pushout in $Cat_\infty$ is a 1-category, it's given by taking the the pushout in $Cat$. For the question as asked we can drop the isocofibration condition.

Being injective on objects is not sufficient, since we have a pushout square in $\Cat_\infty$ $$ \require{AMScd} \begin{CD} S^1 @>>> 1 \\ @VVV @VVV \\ 1 @>>> S^2 \end{CD} $$ and $S^1 \to 1$ can be given by a functor between 1-object categories.

As @AchimKrause points out in the comments, injective on objects and arrows is not sufficient either.


An example of a sufficient condition that does work (but is too restrictive for me) is if $A$, $B$, $X$ are all free categories and $F$ is obtained from an inclusion of the generating graphs.

In this case, we can compute this in the Bergner model structure on simplicially enriched categories. The map $A \to B$, when viewed in simplicial categories, is a cofibration between cofibrant objects (it is $\mathfrak{C}[-]$ applied to the inclusion of the generating graphs viewed as simplicial sets), and $X$ is cofibrant as well, and thus the pushout (which is obviously a 1-category) is a homotopy pushout, and thus computes the pushout in $\Cat_\infty$.

Another case that works, as described in the comments, is when $A$ and $B$ are groupoids and $F$ is a monomorphism in $Cat_\infty$; in this case, $B \cong A \amalg A'$, and thus the pushout in $Cat_\infty$ is $X \amalg A'$.

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  • $\begingroup$ If F is an equivalence then I imagine the pushout is still a 1-category, but this is too restrictive. $\endgroup$ Oct 25, 2020 at 2:07
  • $\begingroup$ Note that being injective on objects or on morphisms is not a condition stable under equivalence, and so it is unlikely to be of help. Maybe asking for one leg to be a replete inclusion will work, but I'm honestly skeptical there's a sensible condition for this. $\endgroup$ Oct 25, 2020 at 6:39
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    $\begingroup$ For groupoids, something akin to your first condition should indeed work, namely that the functors are injective on $\pi_0$ and faithful. This should reduce to the corresponding statement for classifying spaces of groups. For general categories, condition 1 is definitely not enough. You can consider a pushout where the upper right corner is an arbitrary category, the upper left corner is a disjoint union of multiple $\Delta^1$, and the l.l. corner is the localisation of the u.l. corner. Then the pushout is a localisation of the category you started with, and not generally a $1$-category. $\endgroup$ Oct 25, 2020 at 7:37
  • $\begingroup$ @DenisNardin I was thinking about computing the pushout in the canonical model structure on Cat which is why I was focusing on the "injective on objects" condition. IIRC, the second of the two conditions I list the proposition "$F$ is a monomorphism in $Cat_\infty$" restricted to the case $F$ is injective on objects. So the equivalence-respecting condition would be to consider $F$ being a monomorphism in $Cat_\infty$. $\endgroup$ Oct 25, 2020 at 9:04
  • $\begingroup$ @PushoutOfCategories "F is a monomorphism in $Cat_∞$" is precisely the condition of being (equivalent to) the inclusion of a replete subcategory. Unfortunately Achim's example shows that this is of course not nearly enough (every localization can be realized as a pushout along a replete subcategory!). $\endgroup$ Oct 25, 2020 at 9:09

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Martina Rovelli and I have indeed thought about the case of Dwyer morphisms before. Originally, we were also trying to employ Barwick-Kan, but I think there is the following subtle point there.

You have to specify how to look at a category as a particular relative category, and the natural way is to assign to a category $\mathcal{A}$ the pair $(\mathcal{A}, \mathrm{iso}(\mathcal{A}))$. However, I think this functor does not take Dwyer morphisms in categories to Dwyer morphisms in relative categories. Indeed, I think that already the inclusion of the object $a$ into the category ${a<b}$ is not a Dwyer morphism of relative categories. It seems that checking Barwick-Kan §§3.2-3.5 shows that you would need your homotopy to be a relative functor $$ (a<b, \mathrm{id}) \times (0<1, \mathrm{max}) \to (a<b, \mathrm{id}) $$ which maps $b0$ to $a$ and $b1$ to $b$, so that the weak equivalence $b0\to b1$ would map to a map which is not a weak equivalence.

Edit May 2022: Instead, we believe to have found an explicit proof using anodyne extensions now The question turned out to be far more subtle. As we were trying to use it in a joint work with Philip Hackney and Emily Riehl (https://arxiv.org/abs/2106.03660), the referee was pointing out that our proof did not work in a full generality. We have been thinking about these pushouts for a while since then, and we still believe that the statement holds true, although the proof is much more involved now (https://arxiv.org/abs/2205.02353).

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