# Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\newcommand\empty{}$

### 5.3.6 Application: Direct Image Fibrations

We now apply Theorem 5.3.5.7 to study the direct image construction of §4.5.9. Our starting point is the following:

Proposition 5.3.6.1. Let $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ be a functor of $\infty$-categories which is either a cartesian fibration or a cocartesian fibration. Then $U$ is exponentiable (in the sense of Definition 4.5.9.10). That is, if we are given any diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}'' \ar [r]^-{F} \ar [d] & \operatorname{\mathcal{E}}' \ar [r] \ar [d] & \operatorname{\mathcal{E}}\ar [d]^{U} \\ \operatorname{\mathcal{C}}'' \ar [r]^-{\overline{F}} & \operatorname{\mathcal{C}}' \ar [r] & \operatorname{\mathcal{C}}}$

where both squares are pullbacks and $\overline{F}$ is a categorical equivalence, then $F$ is also a categorical equivalence.

Remark 5.3.6.2. In the statement of Proposition 5.3.6.1, the hypothesis that $\operatorname{\mathcal{C}}$ is an $\infty$-category is not necessary: see Corollary 5.7.7.6.

Our proof of Proposition 5.3.6.1 will require some preliminaries.

Lemma 5.3.6.3. Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{Set_{\Delta }}$ be a diagram of simplicial sets and suppose we are given morphisms of simplicial sets $A \xrightarrow {f} B \xrightarrow {g} \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$, where $f$ is inner anodyne. Then the induced map

$\theta _{g}: A \times _{ \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) } \underset { \longrightarrow }{\mathrm{holim}}(\mathscr {F} ) \rightarrow B \times _{ \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) } \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {F} )$

is inner anodyne.

Proof. Let $S$ be the collection of all morphisms of simplicial sets $f: A \rightarrow B$ having the property that, for every morphism $g: B \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$, the map $\theta _{g}$ is inner anodyne. It follows immediately from the definitions that $S$ is weakly saturated (in the sense of Definition 1.4.4.15). Consequently, to show that every inner anodyne morphism belongs to $S$, it will suffice to prove that $S$ contained every inner horn inclusion $f: \Lambda ^{n}_{i} \hookrightarrow \Delta ^ n$, $0 < i < n$. Using Remark 5.3.2.3, we can reduce to the case where $\operatorname{\mathcal{C}}= [n]$ and $g: \Delta ^ n \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ is the identity map. In this case, Remark 5.3.2.14 shows that $\theta _{g}$ is a pushout of the inclusion map $\Lambda ^{n}_{i} \times \mathscr {F}(0) \hookrightarrow \Delta ^ n \times \mathscr {F}(0)$, which is inner anodyne by virtue of Lemma 1.4.7.5. $\square$

Lemma 5.3.6.4. Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{Set_{\Delta }}$ be a diagram of simplicial sets, let $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ be a cocartesian fibration of $\infty$-categories, and let $\lambda : \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {F} ) \rightarrow \operatorname{\mathcal{E}}$ be a scaffold. Then, for every morphism of simplicial sets $S \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$, the induced map

$\lambda _{S}: S \times _{ \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) } \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {F} ) \rightarrow S \times _{ \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) } \operatorname{\mathcal{E}}$

is a categorical equivalence of simplicial sets.

Proof. By virtue of Corollary 4.5.7.3, we may assume without loss of generality that $S = \Delta ^ n$ is a standard simplex. Replacing $\operatorname{\mathcal{C}}$ by the category $[n] = \{ 0 < 1 < \cdots < n \}$, we are reduced to proving that $\lambda$ is a categorical equivalence, which follows from Theorem 5.3.5.7. $\square$

Lemma 5.3.6.5. Suppose we are given a pullback diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}' \ar [d] \ar [r]^-{F} & \operatorname{\mathcal{E}}\ar [d]^-{U} \\ \operatorname{\mathcal{C}}' \ar [r]^-{\overline{F}} & \operatorname{\mathcal{C}}, }$

where $\overline{F}$ is inner anodyne. If $U$ is either a cartesian fibration or a cocartesian fibration, then $F$ is a categorical equivalence of simplicial sets.

Proof. We will give the proof under the assumption that $U$ is a cocartesian fibration; the proof when $U$ is a cartesian fibration is similar. Let $S$ be the collection of all monomorphisms of simplicial sets $f: A \hookrightarrow B$ with the following property: for every morphism of simplicial sets $B \rightarrow \operatorname{\mathcal{C}}$, the induced map $A \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}\hookrightarrow B \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}$ is a categorical equivalence. To complete the proof, it will suffice to show that the morphism $\overline{F}: \operatorname{\mathcal{C}}' \hookrightarrow \operatorname{\mathcal{C}}$ belongs to $S$. In fact, we claim that every inner anodyne morphism of simplicial sets belongs to $S$. Using Remark 4.5.3.6, Remark 4.5.3.5, Corollary 4.5.7.2, and Remark 4.5.4.13, we see that $S$ is weakly saturated (see Definition 1.4.4.15). It will therefore suffice to show that $S$ contains every inner horn inclusion $\Lambda ^{n}_{i} \hookrightarrow \Delta ^ n$, $0 < i < n$. In particular, we are reduced to proving Lemma 5.3.6.5 in the special case where $\operatorname{\mathcal{C}}= \operatorname{N}_{\bullet }( \operatorname{\mathcal{C}}_0 )$ is the nerve of a category $\operatorname{\mathcal{C}}_0$. Applying Corollary 5.3.4.9, we deduce that there exists a diagram of $\infty$-categories $\mathscr {G}: \operatorname{\mathcal{C}}_0 \rightarrow \operatorname{QCat}$ and a scaffold $\lambda : \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {G} ) \rightarrow \operatorname{\mathcal{E}}$. We then have a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}' \times _{\operatorname{\mathcal{C}}} \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {G} ) \ar [r]^{ \widetilde{F} } \ar [d] & \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {G} ) \ar [d]^{\lambda } \\ \operatorname{\mathcal{E}}' \ar [r]^-{F} & \operatorname{\mathcal{E}}, }$

where the vertical maps are categorical equivalences (Lemma 5.3.6.4). Consequently, to show that $F$ is a categorical equivalence, it will suffice to show that $\widetilde{F}$ is a categorical equivalence, which follows from Lemma 5.3.6.3. $\square$

Proof of Proposition 5.3.6.1. Without loss of generality we may assume that $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ is a cocartesian fibration of $\infty$-categories. Suppose we are given a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}'' \ar [r]^-{F} \ar [d] & \operatorname{\mathcal{E}}' \ar [r]^-{G} \ar [d] & \operatorname{\mathcal{E}}\ar [d]^{U} \\ \operatorname{\mathcal{C}}'' \ar [r]^-{\overline{F}} & \operatorname{\mathcal{C}}' \ar [r]^-{\overline{G}} & \operatorname{\mathcal{C}}}$

where both squares are pullbacks and $\overline{F}$ is a categorical equivalence. We wish to show that $F$ is also a categorical equivalence. By virtue of Proposition 4.1.3.2, the morphism $\overline{G}$ factors as a composition $\operatorname{\mathcal{C}}' \xrightarrow {\overline{G}'} \operatorname{\mathcal{B}}\xrightarrow { \overline{G}'' } \operatorname{\mathcal{C}}$, where $\overline{G}'$ is inner anodyne and $\overline{G}''$ is an inner fibration. Note that the projection map $V: \operatorname{\mathcal{B}}\times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{B}}$ is a cocartesian fibration of $\infty$-categories. We may therefore replace $\operatorname{\mathcal{C}}$ by $\operatorname{\mathcal{B}}$ and thereby reduce to the special case where $\overline{G}$ is inner anodyne. In this case, the morphism $G: \operatorname{\mathcal{E}}' \rightarrow \operatorname{\mathcal{E}}$ is a categorical equivalence of simplicial sets (Lemma 5.3.6.5). Consequently, to show that $F$ is a categorical equivalence of simplicial sets, it will suffice to show that the composite map $(G \circ F): \operatorname{\mathcal{E}}'' \rightarrow \operatorname{\mathcal{E}}$ is a categorical equivalence of simplicial sets (Remark 4.5.3.5).

Since $\overline{F}$ is a categorical equivalence and $\overline{G}$ is inner anodyne, it follows that the composite map $\overline{G} \circ \overline{F}: \operatorname{\mathcal{C}}'' \rightarrow \operatorname{\mathcal{C}}$ is also a categorical equivalence. Applying Proposition 4.1.3.2, we can factor $\overline{G} \circ \overline{F}$ as a composition $\operatorname{\mathcal{C}}'' \xrightarrow {\overline{F}_0} \operatorname{\mathcal{C}}'_0 \xrightarrow {\overline{G}_0} \operatorname{\mathcal{C}}$, where $\overline{F}_0$ is inner anodyne and $\overline{G}_0$ is an inner fibration. Since $\operatorname{\mathcal{C}}$ is an $\infty$-category, it follows that $\operatorname{\mathcal{C}}'_0$ is also an $\infty$-category (Remark 4.1.1.9). Set $\overline{\operatorname{\mathcal{E}}}'_0 = \operatorname{\mathcal{C}}'_{0} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}$, so that we have a commutative diagram

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}'' \ar [r]^-{F_0} \ar [d] & \operatorname{\mathcal{E}}'_0 \ar [r]^-{G_0} \ar [d] & \operatorname{\mathcal{E}}\ar [d]^{U} \\ \operatorname{\mathcal{C}}'' \ar [r]^-{\overline{F}_0} & \operatorname{\mathcal{C}}'_0 \ar [r]^-{\overline{G}_0} & \operatorname{\mathcal{C}}}$

satisfying $G \circ F = G_0 \circ F_0$. Since $U$ is an isofibration (Proposition 5.1.4.8) and $\overline{G}_0$ is an equivalence of $\infty$-categories, it follows that $G_0$ is an equivalence $\infty$-categories (Corollary 4.5.2.23). Applying Lemma 5.3.6.5 to the square on the left, we see that $F_0$ is a categorical equivalence of simplicial sets. Invoking Remark 4.5.3.5, we deduce that $G \circ F = G_0 \circ F_0$ is also a categorical equivalence, as desired. $\square$

We now formulate the main result of this section. In what follows, we assume that the reader is familiar with the direct image construction introduced in §4.5.9.

Proposition 5.3.6.6. Let $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of simplicial sets and let $V: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$ be an isofibration which satisfies the following condition:

$(\star )$

For $U$-cocartesian edge $e: D' \rightarrow D$ and every vertex $\overline{D} \in \operatorname{\mathcal{E}}$ satisfying $V( \overline{D} ) = D$, there is a $V$-cartesian edge $\overline{e}: \overline{D}' \rightarrow \overline{D}$ of $\operatorname{\mathcal{E}}$ satisfying $V( \overline{e} ) = e$.

Let $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ denote the direct image of $\operatorname{\mathcal{E}}$ along $U$ (see Construction 4.5.9.1). Then the projection map $\pi : \operatorname{Res}_{ \operatorname{\mathcal{D}}/ \operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}) \rightarrow \operatorname{\mathcal{C}}$ is a cartesian fibration of simplicial sets. Moreover, an edge $e$ of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ is $\pi$-cartesian if and only if it satisfies the following condition:

• Form a commutative diagram

$\xymatrix@R =50pt@C=50pt{ \Delta ^{1} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}\ar [r]^-{ V_{e} } \ar [d] & \Delta ^1 \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{D}}\ar [r]^-{ U_{e} } \ar [d] & \Delta ^1 \ar [d]^{ \pi (e) } \\ \operatorname{\mathcal{E}}\ar [r]^-{V} & \operatorname{\mathcal{D}}\ar [r]^-{U} & \operatorname{\mathcal{C}}, }$

so that the edge $e$ can be identified with a morphism of simplicial sets $F_{e}: \Delta ^1 \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{D}}\rightarrow \Delta ^1 \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}$ such that $V_{e} \circ F_{e}$ is the identity. Then the morphism $F_{e}$ carries $U_{e}$-cocartesian morphisms of $\Delta ^{1} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{D}}$ to $V_{e}$-cartesian morphisms of $\Delta ^{1} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}$.

Remark 5.3.6.7. In the situation of Proposition 5.3.6.6, condition $(\star )$ is automatically satisfied if $V$ is a cartesian fibration of simplicial sets.

We will carry out the proof of Proposition 5.3.6.6 in several steps.

Lemma 5.3.6.8. Let $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of simplicial sets, let $V: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$ be an isofibration of simplicial sets, and let $e$ be an edge of the simplicial set $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ which satisfies condition $(\ast )$ of Proposition 5.3.6.6. Then $e$ is $\pi$-cartesian, where $\pi : \operatorname{Res}_{ \operatorname{\mathcal{D}}/ \operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}) \rightarrow \operatorname{\mathcal{C}}$ denotes the projection map.

Proof. Let $n \geq 2$ and suppose we are given a lifting problem

5.30
$$\begin{gathered}\label{equation:cartesian-in-direct-image} \xymatrix@R =50pt@C=50pt{ \Lambda ^{n}_{n} \ar [r]^-{ \sigma _0 } \ar [d] & \operatorname{Res}_{ \operatorname{\mathcal{D}}/ \operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}) \ar [d]^{\pi } \\ \Delta ^{n} \ar [r]^-{ \overline{\sigma } } \ar@ {-->}[ur]^{\sigma } & \operatorname{\mathcal{C}}, } \end{gathered}$$

where $\sigma _0$ carries the final edge $\operatorname{N}_{\bullet }( \{ n-1 < n \} ) \subseteq \Lambda ^{n}_{n}$ to $e$; we wish to show that this lifting problem admits a solution. Replacing $U$ and $V$ with the projection maps $\Delta ^{n} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{D}}\rightarrow \Delta ^ n$ and $\Delta ^{n} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}\rightarrow \Delta ^{n} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{D}}$, we can assume without loss of generality that $\operatorname{\mathcal{C}}= \Delta ^ n$ is a standard simplex and that $\overline{\sigma }: \Delta ^ n \rightarrow \operatorname{\mathcal{C}}$ is the identity map. Set $\operatorname{\mathcal{D}}_0 = \Lambda ^{n}_{n} \times _{\Delta ^ n} \operatorname{\mathcal{D}}$ and $\operatorname{\mathcal{E}}_0 = \Lambda ^{n}_{n} \times _{ \Delta ^{n} } \operatorname{\mathcal{E}}$. Invoking the universal property of the simplicial set $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}})$ (Proposition 4.5.9.2), we can rewrite (5.30) as a lifting problem

5.31
$$\begin{gathered}\label{equation:cartesian-in-direct-image-3} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{D}}_0 \ar [d] \ar [r]^-{F_0} & \operatorname{\mathcal{E}}\ar [d]^{V} \\ \operatorname{\mathcal{D}}\ar [r]^-{ \operatorname{id}_{\operatorname{\mathcal{D}}} } \ar@ {-->}[ur]^{F} & \operatorname{\mathcal{D}}. } \end{gathered}$$

Note that since the edge $e$ satisfies condition $(\ast )$, the morphism $F_0$ satisfies the following condition:

• If $u$ is a $U$-cocartesian edge of $\operatorname{\mathcal{D}}$ lying over the final edge $\operatorname{N}_{\bullet }( \{ n-1 < n \} ) \subseteq \operatorname{\mathcal{C}}$, then $F_0(u)$ is a $V$-cartesian edge of $\operatorname{\mathcal{E}}$.

Using Corollary 5.3.4.9, we can choose a diagram of $\infty$-categories $\mathscr {F}: [n] \rightarrow \operatorname{QCat}$ and a scaffold $\lambda : \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {F} ) \rightarrow \operatorname{\mathcal{D}}$. Set $\operatorname{\mathcal{D}}' = \underset { \longrightarrow }{\mathrm{holim}}( \mathscr {F} )$ and $\operatorname{\mathcal{D}}'_0 = \Lambda ^{n}_{n} \times _{ \Delta ^ n } \operatorname{\mathcal{D}}'$, so that $\lambda$ restricts to a map $\lambda _0: \operatorname{\mathcal{D}}'_0 \rightarrow \operatorname{\mathcal{D}}_0$. We then have a commutative diagram of $\infty$-categories

5.32
$$\begin{gathered}\label{equation:cartesian-in-direct-image-2} \xymatrix@R =50pt@C=50pt{ \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \ar [r]^-{\circ \lambda } \ar [d] & \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}', \operatorname{\mathcal{E}}) \ar [d] \\ \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}_0, \operatorname{\mathcal{E}}) \ar [r]^-{\circ \lambda } & \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}'_0, \operatorname{\mathcal{E}}). } \end{gathered}$$

Since $V$ is an isofibration (Proposition 5.1.4.8), the vertical maps in this diagram are isofibrations (Proposition 4.5.5.14). Since $\lambda$ and $\lambda _0$ are categorical equivalences of simplicial sets (Lemma 5.3.6.4), the horizontal maps are equivalences of $\infty$-categories. Applying Corollary 4.5.2.26, we deduce that the upper horizontal map in the diagram (5.32) restricts to an equivalence from each fiber of the left vertical map to the corresponding fiber of the right vertical map. Consequently, we can replace (5.31) with the lifting problem

5.33
$$\begin{gathered}\label{equation:cartesian-in-direct-image-4} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{D}}'_0 \ar [d] \ar [r]^-{F_0 \circ G_0} & \operatorname{\mathcal{E}}\ar [d]^{V} \\ \operatorname{\mathcal{D}}' \ar [r]^-{G} \ar@ {-->}[ur] & \operatorname{\mathcal{D}}. } \end{gathered}$$

Using Remark 5.3.2.12, we obtain a pushout square

$\xymatrix@R =50pt@C=50pt{ \Lambda ^{n}_{n} \times \mathscr {F}(0) \ar [r]^-{H_0} \ar [d] & \operatorname{\mathcal{D}}'_0 \ar [d] \\ \Delta ^ n \times \mathscr {F}(0) \ar [r]^-{H} & \operatorname{\mathcal{D}}'. }$

Let us identify $F_0 \circ G_0 \circ H_0$ with a morphism of simplicial sets $\tau _0: \Lambda ^{n}_{n} \rightarrow \operatorname{Fun}( \mathscr {F}(0), \operatorname{\mathcal{E}})$, and $G \circ H$ with an $n$-simplex $\overline{\tau }$ of $\operatorname{Fun}( \mathscr {F}(0), \operatorname{\mathcal{D}})$, so that we can rewrite (5.33) again as a lifting problem

$\xymatrix@R =50pt@C=50pt{ \Lambda ^{n}_{n} \ar [d] \ar [r]^-{ \tau _0 } & \operatorname{Fun}( \mathscr {F}(0), \operatorname{\mathcal{E}}) \ar [d]^{V'} \\ \Delta ^{n} \ar [r]^-{\overline{\tau }} \ar@ {-->}[ur]^{\tau } & \operatorname{Fun}(\mathscr {F}(0), \operatorname{\mathcal{D}}). }$

To show that this lifting problem admits a solution, it will suffice to show that $\tau _0$ carries the final edge $\operatorname{N}_{\bullet }( \{ n-1 < n \} )$ of $\Lambda ^{n}_{n}$ to a $V'$-cocartesian edge of $\operatorname{Fun}( \mathscr {F}(0), \operatorname{\mathcal{E}})$. Since $\lambda$ is a scaffold, this follows by combining $(\ast ')$ with the criterion of Theorem 5.2.1.1. $\square$

Lemma 5.3.6.9. Let $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of simplicial sets, let $V: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$ be an isofibration which satisfies condition $(\star )$ of Proposition 5.3.6.6, and let $\pi : \operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}}) \rightarrow \operatorname{\mathcal{C}}$ denote the projection map. Suppose we are given a vertex $Y$ of the simplicial set $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}})$ having image $\overline{Y} = \pi (Y)$, and an edge $\overline{e}: \overline{X} \rightarrow \overline{Y}$ of the simplicial set $\operatorname{\mathcal{C}}$. Then we can write $\overline{e} = \pi (e)$ for some edge $e: X \rightarrow Y$ of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ which satisfies condition $(\ast )$ of Proposition 5.3.6.6.

Proof. As in the proof of Lemma 5.3.6.8, we may assume without loss of generality that $\operatorname{\mathcal{C}}= \Delta ^1$ and that $\overline{e}$ is the nondegenerate edge of $\operatorname{\mathcal{C}}$. Let $\operatorname{\mathcal{D}}(0)$ and $\operatorname{\mathcal{D}}(1)$ denote the fibers of $\operatorname{\mathcal{D}}$ over the vertices $\overline{X} = 0$ and $\overline{Y} = 1$, respectively, and let us identify $Y$ with a morphism of simplicial sets $\operatorname{\mathcal{D}}(1) \rightarrow \operatorname{\mathcal{E}}$. Applying Proposition 5.2.2.8, we can choose a functor $F: \operatorname{\mathcal{D}}(0) \rightarrow \operatorname{\mathcal{D}}(1)$ and a diagram

$\xymatrix@R =50pt@C=50pt{ \Delta ^1 \times \operatorname{\mathcal{D}}(0) \ar [r]^-{H} \ar [d] & \operatorname{\mathcal{D}}\ar [d]^{U} \\ \Delta ^1 \ar@ {=}[r] & \operatorname{\mathcal{C}}}$

which exhibits $F= H|_{ \{ 1\} \times \operatorname{\mathcal{D}}(0) }$ as given by covariant transport along $\overline{e}$. Combining assumption $(\star )$ with Proposition 5.2.1.3, we deduce that the lifting problem

$\xymatrix@R =50pt@C=50pt{ \{ 1\} \times \operatorname{\mathcal{D}}(0) \ar [r]^-{Y \circ F} \ar [d] & \operatorname{\mathcal{E}}\ar [d]^{V} \\ \Delta ^1 \times \operatorname{\mathcal{D}}(0) \ar [r]^-{ H } \ar@ {-->}[ur]^{ \widetilde{H} } & \operatorname{\mathcal{D}}}$

admits a solution with the property that, for every object $D$ of the $\infty$-category $\operatorname{\mathcal{D}}(0)$, the restriction $\widetilde{H}|_{ \Delta ^1 \times \{ D\} }$ is a $V$-cartesian morphism of $\operatorname{\mathcal{E}}$.

Let $\operatorname{\mathcal{D}}' = (\Delta ^1 \times \operatorname{\mathcal{D}}(0)) \coprod _{ (\{ 1\} \times \operatorname{\mathcal{D}}(0)) } \operatorname{\mathcal{D}}(1)$ denote the mapping cylinder of the functor $F$. Amalgamating $H$ with the inclusion map $\operatorname{\mathcal{D}}(1) \hookrightarrow \operatorname{\mathcal{D}}$, we obtain a morphism of simplicial sets $H': \operatorname{\mathcal{D}}' \rightarrow \operatorname{\mathcal{D}}$, which is a categorical equivalence by virtue of Corollary 5.2.4.2. Amalgamating $\widetilde{H}$ with $Y$, we obtain a diagram $\widetilde{H}': \operatorname{\mathcal{D}}' \rightarrow \operatorname{\mathcal{E}}$ satisfying $V \circ \widetilde{H}' = H'$. We have a commutative diagram of $\infty$-categories

$\xymatrix@R =50pt@C=50pt{ \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \ar [r]^-{\circ G} \ar [d] & \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}', \operatorname{\mathcal{E}}) \ar [d] \\ \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}(1), \operatorname{\mathcal{E}}) \ar@ {=}[r] & \operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}(1), \operatorname{\mathcal{E}}), }$

where the horizontal maps are equivalences of $\infty$-categories. Since $V$ is an isofibration, the vertical maps in this diagram are isofibrations (Proposition 4.5.5.14). Applying Corollary 4.5.2.26, we deduce that the upper horizontal map in the diagram (5.32) restricts to an equivalence of the fibers of the vertical maps over the object $Y \in \operatorname{Fun}_{ /\operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}(1), \operatorname{\mathcal{E}})$. It follows that there there exists a functor $E: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ such that $V \circ E = \operatorname{id}_{\operatorname{\mathcal{D}}}$, $E|_{\operatorname{\mathcal{D}}(1)} = Y$, and $E \circ G$ is isomorphic to $\widetilde{H}'$ as an object of the $\infty$-category $\operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}', \operatorname{\mathcal{E}})$. By construction, we can identify $E$ with an edge $e: X \rightarrow Y$ of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ satisfying $\pi (e) = \overline{e}$. To complete the proof, it will suffice to show that $e$ satisfies condition $(\ast )$ of Proposition 5.3.6.6. Let $u: D \rightarrow D'$ be a $U$-cocartesian edge of $\operatorname{\mathcal{D}}$ satisfying $\pi (u) = \overline{e}$; we wish to show that $E(u)$ is a $V$-cartesian edge of $\operatorname{\mathcal{E}}$. By virtue of Remark 5.1.3.8, we can assume without loss of generality that $u: D \rightarrow F(D)$ is the $U$-cocartesian morphism given by the restriction $\widetilde{F}|_{ \Delta ^1 \times \{ D\} }$. In this case, $E(u)$ is isomorphic (as an object of the $\infty$-category $\operatorname{Fun}(\Delta ^1, \operatorname{\mathcal{E}})$) to the $V$-cartesian morphism $\widetilde{H}|_{ \Delta ^1 \times \{ D\} }$, and is therefore also $V$-cartesian (Corollary 5.1.2.5). $\square$

Proof of Proposition 5.3.6.6. Let $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of simplicial sets and let $V: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$ be an isofibration of simplicial sets which satisfies condition $(\star )$. We first claim that the projection map $\pi : \operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}) \rightarrow \operatorname{\mathcal{C}}$ is an inner fibration of $\infty$-categories. To prove this, we may assume without loss of generality that $\operatorname{\mathcal{C}}= \Delta ^ n$ is a standard simplex; in particular, we can assume that $\operatorname{\mathcal{C}}$ is an $\infty$-category. In this case, $\operatorname{\mathcal{D}}$ and $\operatorname{\mathcal{E}}$ are also $\infty$-categories (Remark 4.1.1.9) and $U$ is exponentiable (Proposition 5.3.6.1). Applying Corollary 4.5.9.18, we deduce that $\pi$ is an isofibration of simplicial sets, and therefore an inner fibration (Remark 4.5.5.7).

Let us say that an edge $e$ of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ is special if it satisfies condition $(\ast )$ of Proposition 5.3.6.6. Lemma 5.3.6.8 guarantees that every special edge of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ is $\pi$-cartesian. Moreover, if $Y$ is a vertex of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ and $\overline{e}: \overline{X} \rightarrow \pi (Y)$ is an edge of $\operatorname{\mathcal{C}}$, then Lemma 5.3.6.9 guarantees that there exists a special edge $e: X \rightarrow Y$ of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ satisfying $\pi (e) = \overline{e}$. It follows that $\pi$ is a cartesian fibration of simplicial sets.

To complete the proof of Proposition 5.3.6.6, we must show that every $\pi$-cartesian edge $e: X \rightarrow Y$ of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ is special. Without loss of generality we may assume that $\operatorname{\mathcal{C}}= \Delta ^1$ and that $\pi (e)$ is the nondegenerate edge of $\operatorname{\mathcal{C}}$, so that we can identify $e$ with a functor $E: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ satisfying $V \circ E = \operatorname{id}_{\operatorname{\mathcal{D}}}$. Using Lemma 5.3.6.9, we can choose a special edge $e': X' \rightarrow Y$ of $\operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}})$ satisfying $\pi (e') = \pi (e)$, corresponding to another functor $E': \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$. Since $e'$ is also $\pi$-cartesian, it is isomorphic to $e$ as an object of the $\infty$-category $\operatorname{Fun}_{ / \Delta ^1 }( \Delta ^1, \operatorname{Res}_{\operatorname{\mathcal{D}}/\operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}}) )$, so that $E'$ is isomorphic to $E$ as an object of the $\infty$-category $\operatorname{Fun}_{ /\operatorname{\mathcal{D}}}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}})$. If $u$ is a $U$-cocartesian edge of $\operatorname{\mathcal{D}}$, then $E(u)$ is isomorphic to the $V$-cartesian morphism $E'(u)$ (as an object of the $\infty$-category $\operatorname{Fun}(\Delta ^1, \operatorname{\mathcal{E}})$), and is therefore also $V$-cartesian (Corollary 5.1.2.5). $\square$