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I'm unsure if my question is advanced enough for this site, but let's see.

Let $\mathcal{C}$ be a locally cartesian closed category, so that it always has dependent products $\Pi_f$, i.e., right adjoints to base change functors $f^{\ast}$, along with dependent sums $\Sigma_f$, i.e., left adjoints to base change functors. In the article Simplicial Model of Univ. Foundations, Definition 1.4.1 is an expression $$U^{\sqcap}\equiv \underbrace{\Sigma_{U \rightarrow 1}}_{F(U)} \underbrace{\Pi_{\tilde{U} \rightarrow U}}_{G(U)}\left(\underbrace{\pi_{2}: U \times \tilde{U} \rightarrow \tilde{U}}_{H(U)}\right) $$ where $\tilde{U}\to U$ refers to a particular morphism $p$ with which $U$ comes equipped. (This morphism gives the structure of a so-called universe in $\mathcal{C}$, but this shouldn't be essential to my question.)

Now, the authors refer to the "evident functoriality" of $U^{\sqcap}$ in $U$. But I just don't see this.

$U^{\sqcap}$ has the form $F(U)(G(U)(H(U)))$ where $F$ and $G$ refer to functors defined in terms of $U$ and $H$ is a functor on pairs of objects in $\mathcal{C}$. If we are given a morphism $(f,g)$ in the arrow category of $\mathcal{C}$ from $\tilde{U}'\to U'$ to $\tilde{U}\to U$, we get an induced map $H(f,g):\pi_2' \to \pi_2$. I also believe that we can view $\Pi_{-}$ and $\Sigma_{-}$ as functors from the arrow category of $\mathcal{C}$ to the arrow category of $\text{CAT}$. But I'm getting nowhere with trying put this all together to find an induced map $(f,h)^{\sqcap}:\left(U'\right)^{\sqcap} \to U^{\sqcap}$.

I'm likely missing something obvious, but any help would be much appreciated.

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You are right that $(-)^{\mathsf \Pi}$ is not functorial on the arrow category of $\mathcal{C}$. However, it is functorial on the category whose objects are arrows in $\mathcal{C}$ and whose morphisms are pullback squares, and I believe that's what the authors were referring to, since in their case $\tilde{U_0}$ is defined as the pullback of $\tilde{U}$ to $U_0$.

Given a pullback square $\require{AMScd}$ \begin{CD} \tilde{U}' @>f>> \tilde{U} \\ @V p' VV @VV p V \\ U' @>>g> U \end{CD} we have a map $\tilde{U}' \times U' \to \tilde{U} \times U$ over $f$ and hence a map $\tilde{U}' \times U' \to f^*(\tilde{U} \times U)$ over $\tilde{U}'$, thus by functoriality a map $(U')^{\mathsf{\Pi}} = \Pi_{p'}(\tilde{U}'\times U') \to \Pi_{p'}(f^*(\tilde{U}\times U))$. But by the Beck-Chevalley condition for the pullback square we have $\Pi_{p'}(f^*(\tilde{U}\times U)) \cong g^*(\Pi_p(\tilde{U}\times U)) = g^*(U^{\mathsf{\Pi}})$, so we get a map $(U')^{\mathsf{\Pi}} \to g^*(U^{\mathsf{\Pi}})$ over $U'$, hence a map $(U')^{\mathsf{\Pi}} \to U^{\mathsf{\Pi}}$ over $g$.

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  • $\begingroup$ I still can't see how it would act on morphisms. Would you mind saying a bit more about what the induced map would look like? $\endgroup$ – CuriousKid7 Nov 18 at 4:12
  • $\begingroup$ @CuriousKid7 I added some details. $\endgroup$ – Mike Shulman Nov 18 at 14:18
  • $\begingroup$ Sorry, another clarification: How do you know that the Beck-Chevalley condition is satisfied? It seems from nLab that the condition holds so long as the square in our case is a pullback (which it is here). Is this fact true due to some general theorem about the Beck-Chevalley condition that I'm unaware of, or is this just a matter of direct verification? $\endgroup$ – CuriousKid7 Nov 20 at 2:27
  • $\begingroup$ @CuriousKid7 The fact that the Beck-Chevalley condition holds for any pullback square is indeed a general theorem. Probably the easiest way to prove it is to pass to left adjoints of everything and use the pullback pasting lemma. $\endgroup$ – Mike Shulman Nov 20 at 6:28
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    $\begingroup$ @CuriousKid7 Joyal's proof is of a different and much subtler Beck-Chevalley condition, involving derived Kan extensions of diagrams on quasicategories. This Beck-Chevalley condition is much simpler and can be proven in a few lines; my last comment explains how (very briefly, to be sure). I'm not sure offhand where to find this written out in more detail, but it's a nice exercise to work out yourself. $\endgroup$ – Mike Shulman Nov 20 at 17:27

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