Kerodon

Proof. Let us say that an edge $e$ of $\operatorname{\mathcal{C}}_0 \operatorname{\vec{\times }}_{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}_1$ is special if it satisfies condition $(\ast )$. We first show that if $e$ is a special edge of $\operatorname{\mathcal{C}}_0 \operatorname{\vec{\times }}_{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}_1$, then $e$ is $V$-cocartesian. Let $e_0$ and $e_1$ denote the images of $e$ in $\operatorname{\mathcal{C}}_0$ and $\operatorname{\mathcal{C}}_1$, respectively. Note that $V$ factors as a composition

Here $V'$ is a pullback of the projection map $\overline{V}': \operatorname{Fun}( \Delta ^1, \operatorname{\mathcal{C}}) = \operatorname{\mathcal{C}}\operatorname{\vec{\times }}_{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{C}}\operatorname{\vec{\times }}_{\operatorname{\mathcal{D}}} \operatorname{\mathcal{C}}$. Since $F( e_0 )$ is $U$-cocartesian, Lemma 5.3.7.1 implies that $e$ is $V'$-cocartesian. Moreover, $V''$ is a pullback of the product map $(U_0 \times U_1): \operatorname{\mathcal{C}}_0 \times \operatorname{\mathcal{C}}_1 \rightarrow \operatorname{\mathcal{D}}_0 \times \operatorname{\mathcal{D}}_1$. By assumption, $e_0$ is $U_0$-cocartesian and $e_1$ is $U_1$-cocartesian. It follows that $V'(e)$ is $V''$-cocartesian, so that $e$ is $V$-cocartesian by virtue of Remark 5.1.1.6.

Since $U_0$, $U_1$, and $U$ are inner fibrations, the morphisms $\overline{V}'$ and $(U_0 \times U_1)$ are also inner fibrations (see Proposition 4.1.4.1). It follows that $V'$ and $V''$ are inner fibrations (Remark 4.1.1.5), so that $V$ is an inner fibration (Remark 4.1.1.8). To show that $V$ is a cocartesian fibration, it will suffice to show that if $C$ is an object of $\operatorname{\mathcal{C}}_0 \operatorname{\vec{\times }}_{ \operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}_1$ and $\overline{e}: V(C) \rightarrow \overline{C}'$ is an edge of $\operatorname{\mathcal{D}}_0 \operatorname{\vec{\times }}_{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_1$, then there exists a special edge $e: C \rightarrow C'$ satisfying $V(e) = \overline{e}$. Let us identify $C$ with a triple $(C_0, C_1, u)$ where $C_0$ is a vertex of $\operatorname{\mathcal{C}}_0$, $C_1$ is a vertex of $\operatorname{\mathcal{C}}_1$, and $u: F_0(C_0) \rightarrow F_1(C_1)$ is an edge of $\operatorname{\mathcal{C}}$. Similarly, we can identify $\overline{C}'$ with a triple $(\overline{C}'_0, \overline{C}'_1, \overline{u}' )$ where $\overline{C}_0$ is a vertex of $\operatorname{\mathcal{D}}_0$, $\overline{C}'_1$ is a vertex of $\operatorname{\mathcal{D}}_1$, and $\overline{u}': G_0( \overline{C}'_0 ) \rightarrow G_1( \overline{C}'_1 )$ is an edge of $\operatorname{\mathcal{D}}$. The edge $\overline{e}$ has images $\overline{e}_{0}: U_0(C_0) \rightarrow \overline{C}'_0$ an $\overline{e}_{1}: U_1(C_1) \rightarrow \overline{C}'_{1}$ in $\operatorname{\mathcal{D}}_0$ and $\operatorname{\mathcal{D}}_1$-respectively. Since $U_0$ is a cocartesian fibration, we can lift $\overline{e}_0$ to a $U_0$-cocartesian edge $e_0: C_0 \rightarrow C'_0$ of $\operatorname{\mathcal{C}}_0$. Similarly, we can lift $\overline{e}_1$ to a $U_1$-cocartesian edge $e_1: C_1 \rightarrow C'_1$ of $\operatorname{\mathcal{C}}_1$. The edge $\overline{e}$ also determines a map $\Delta ^1 \times \Delta ^1 \rightarrow \operatorname{\mathcal{D}}$, which we depict informally in the diagram

Using our assumption that $U$ is an inner fibration, we can lift the upper right triangle to a $2$-simplex $\sigma $:

of the simplicial set $\operatorname{\mathcal{C}}$. Using the fact that $F_0( e_0 )$ is $U$-cocartesian, we can lift the lower triangle to a $2$-simplex $\tau $

of $\operatorname{\mathcal{C}}$. Setting $C' = (C'_0, C'_1, w) \in \operatorname{\mathcal{C}}_0 \operatorname{\vec{\times }}_{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}_1$, we observe that the tuple $( e_0, e_1, \sigma , \tau )$ determines a special edge $e: C \rightarrow C'$ satisfying $V(e) = \overline{e}$.

We now complete the proof by showing that every $V$-cocartesian edge $f: C \rightarrow C''$ in $\operatorname{\mathcal{C}}_0 \operatorname{\vec{\times }}_{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}_1$ is special. Using the preceding argument, we can choose a special edge $e: C \rightarrow C'$ satisfying $V(e) = V(f)$. Set $\overline{C}' = V( C' ) = V(C'' )$. Applying Remark 5.1.3.8, we deduce that there is a $2$-simplex $\rho :$

of the simplicial set $\operatorname{\mathcal{C}}_0 \operatorname{\vec{\times }}_{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}_1$, where $s$ is an isomorphism in the $\infty $-category $V^{-1}( \{ \overline{C}' \} )$. Applying Example 5.1.3.6, we deduce that the images of $s$ in $\operatorname{\mathcal{C}}_0$ is $U_0$-cocartesian, and the image of $s$ in $\operatorname{\mathcal{C}}_1$ is $U_1$-cocartesian. Since the collections of $U_0$-cocartesian and $U_1$-cocartesian edges are closed under composition (Corollary 5.1.2.4), we conclude that $f$ is also special. $\square$