Theorem 5.2.4.1. Let $U: \operatorname{\mathcal{M}}\rightarrow \Delta ^1$ be a functor of $\infty $-categories, and suppose we are given a commutative diagram $\sigma :$
\[ \xymatrix@R =50pt@C=50pt{ \{ 1\} \times \operatorname{\mathcal{C}}\ar [r] \ar [d] & \{ 1\} \times \operatorname{\mathcal{D}}\ar [d]^-{g} \\ \Delta ^1 \times \operatorname{\mathcal{C}}\ar [r]^-{h} & \operatorname{\mathcal{M}}} \]
in the category $(\operatorname{Set_{\Delta }})_{/\Delta ^1}$. Then $\sigma $ is a categorical pushout diagram of simplicial sets (Definition 4.5.4.1) if and only if the following conditions are satisfied:
- $(1)$
The restriction $h|_{ \{ 0\} \times \operatorname{\mathcal{C}}}: \operatorname{\mathcal{C}}\rightarrow \{ 0\} \times _{\Delta ^1} \operatorname{\mathcal{M}}$ is a categorical equivalence of simplicial sets.
- $(2)$
The morphism $g: \operatorname{\mathcal{D}}\rightarrow \{ 1\} \times _{\Delta ^1} \operatorname{\mathcal{M}}$ is a categorical equivalence of simplicial sets.
- $(3)$
For every vertex $C \in \operatorname{\mathcal{C}}$, the restriction $h|_{ \Delta ^1 \times \{ C\} }$ is a $U$-cocartesian morphism of $\operatorname{\mathcal{M}}$.
Moreover, if these conditions are satisfied, then $U$ is a cocartesian fibration.
Proof of Theorem 5.2.4.1.
Let $U: \operatorname{\mathcal{M}}\rightarrow \Delta ^1$ be a functor of $\infty $-categories and suppose we are given a commutative diagram $\sigma :$
\[ \xymatrix@R =50pt@C=50pt{ \{ 1\} \times \operatorname{\mathcal{C}}\ar [r]^-{F} \ar [d] & \{ 1\} \times \operatorname{\mathcal{D}}\ar [d]^-{g} \\ \Delta ^1 \times \operatorname{\mathcal{C}}\ar [r]^-{h} & \operatorname{\mathcal{M}}} \]
in the category $(\operatorname{Set_{\Delta }})_{/\Delta ^1}$. We wish to show that $\sigma $ is a categorical pushout square if and only if conditions $(1)$ through $(3)$ of Theorem 5.2.4.1 are satisfied.
We first reduce to the case where $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ are $\infty $-categories. Choose inner anodyne morphisms $\operatorname{\mathcal{C}}\hookrightarrow \operatorname{\mathcal{C}}'$ and $\operatorname{\mathcal{D}}\hookrightarrow \operatorname{\mathcal{D}}'$, where $\operatorname{\mathcal{C}}'$ and $\operatorname{\mathcal{D}}'$ are $\infty $-categories (Corollary 4.1.3.3). Since the fiber $\{ 1\} \times _{\Delta ^1} \operatorname{\mathcal{M}}$ is an $\infty $-category, we can extend $g$ to a functor $g': \operatorname{\mathcal{D}}' \rightarrow \{ 1\} \times _{\Delta ^1} \operatorname{\mathcal{M}}$. Similarly, the composition $\operatorname{\mathcal{C}}\xrightarrow {F} \operatorname{\mathcal{D}}\hookrightarrow \operatorname{\mathcal{D}}'$ extends to a functor of $\infty $-categories $F': \operatorname{\mathcal{C}}' \rightarrow \operatorname{\mathcal{D}}'$. Using Exercise 3.1.7.11, we can factor $F'$ as a composition $\operatorname{\mathcal{C}}' \xrightarrow {F''} \operatorname{\mathcal{D}}'' \xrightarrow {v} \operatorname{\mathcal{D}}'$, where $F''$ is a monomorphism and $v$ is a trivial Kan fibration. It follows from Lemma 1.5.7.5 that the inclusion map
\[ ( \Delta ^1 \times \operatorname{\mathcal{C}}) {\coprod }_{ ( \{ 1\} \times \operatorname{\mathcal{C}}) } (\{ 1\} \times \operatorname{\mathcal{C}}' ) \hookrightarrow \Delta ^1 \times \operatorname{\mathcal{C}}' \]
is inner anodyne, so that we can extend $h$ to a functor $h': \Delta ^1 \times \operatorname{\mathcal{C}}' \rightarrow \operatorname{\mathcal{M}}$ satisfying $h'|_{ \{ 1\} \times \operatorname{\mathcal{C}}'} = g' \circ F'$. By virtue of Proposition 4.5.4.9, $\sigma $ is a categorical pushout square if and only if the diagram $\sigma :$
\[ \xymatrix@R =50pt@C=50pt{ \{ 1\} \times \operatorname{\mathcal{C}}' \ar [r]^-{F''} \ar [d] & \{ 1\} \times \operatorname{\mathcal{D}}'' \ar [d]^-{g' \circ v} \\ \Delta ^1 \times \operatorname{\mathcal{C}}' \ar [r]^-{h'} & \operatorname{\mathcal{M}}} \]
is a categorical pushout square. We may therefore replace $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ by $\operatorname{\mathcal{C}}'$ and $\operatorname{\mathcal{D}}''$, and thereby reduce to the case where $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ are $\infty $-categories and $F$ is a monomorphism.
The assumption that $F$ is a monomorphism guarantees that the natural map
\[ \iota : (\Delta ^1 \times \operatorname{\mathcal{C}}) {\coprod }_{ ( \{ 1\} \times \operatorname{\mathcal{C}})} \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}\star _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}} \]
is also a monomorphism, and Proposition 5.2.4.5 guarantees that $\iota $ is a categorical equivalence of simplicial sets. Since $\operatorname{\mathcal{M}}$ is an $\infty $-category, Lemma 4.5.5.2 guarantees the existence of a functor $G: \operatorname{\mathcal{C}}\star _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{M}}$ satisfying $G|_{ \Delta ^1 \times \operatorname{\mathcal{C}}} = h$ and $G|_{\operatorname{\mathcal{D}}} = g$. By virtue of Proposition 4.5.4.9, the diagram $\sigma $ is a categorical pushout square if and only if the functor $G$ is an equivalence of $\infty $-categories.
Note that the functor $G$ fits into a commutative diagram
\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\star _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}\ar [rr]^-{G} \ar [dr]_{U'} & & \operatorname{\mathcal{M}}\ar [dl]^{U} \\ & \Delta ^1, & } \]
where $U'$ is the cocartesian fibration of Proposition 5.2.3.15, and the functor $U$ is an isofibration (Example 4.4.1.6). The desired result now follows by applying the criterion of Theorem 5.1.6.1 (and invoking Remark 5.1.6.8).
$\square$