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6.3.5 Fiberwise Localization

Suppose we are given a commutative diagram of $\infty $-categories

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

where $U$ and $U'$ are cocartesian fibrations and the functor $F$ carries $U$-cocartesian morphisms of $\operatorname{\mathcal{E}}$ to $U'$-cocartesian morphisms of $\operatorname{\mathcal{E}}'$. For each object $C \in \operatorname{\mathcal{C}}$, write $F_{C}: \operatorname{\mathcal{E}}_{C} \rightarrow \operatorname{\mathcal{E}}'_{ G(C) }$ for the induced map of fibers. It follows from Theorem 5.1.5.1 that if the functors $\{ F_ C \} _{C \in \operatorname{\mathcal{C}}}$ and $G$ are equivalences of $\infty $-categories, then $F$ is also an equivalence of $\infty $-categories. Our goal in this section is to prove a generalization of this result, which gives a sufficient condition for $F$ to exhibit $\operatorname{\mathcal{E}}'$ as a localization of $\operatorname{\mathcal{E}}$.

Theorem 6.3.5.1. Suppose we are given a commutative diagram of simplicial sets

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

which satisfies the following conditions:

$(1)$

The morphisms $U$ and $U'$ are cocartesian fibrations.

$(2)$

The morphism $F$ carries $U$-cocartesian edges of $\operatorname{\mathcal{E}}$ to $U'$-cocartesian edges of $\operatorname{\mathcal{E}}'$.

$(3)$

For every vertex $C \in \operatorname{\mathcal{C}}$, the induced functor of $\infty $-categories $F_{C}: \operatorname{\mathcal{E}}_{C} \rightarrow \operatorname{\mathcal{E}}'_{G(C)}$ exhibits $\operatorname{\mathcal{E}}'_{C}$ as the localization of $\operatorname{\mathcal{E}}_{C}$ with respect to some collection of morphisms $W_{C}$ of $\operatorname{\mathcal{E}}_{C}$.

$(4)$

The morphism $G$ exhibits $\operatorname{\mathcal{C}}'$ as a localization of $\operatorname{\mathcal{C}}$ with respect to some collection of morphisms $\overline{W}$ of $\operatorname{\mathcal{C}}$.

Set $W_{-} = \bigcup _{C \in \operatorname{\mathcal{C}}} W_{C}$ and let $W_{+}$ be the collection of all $U$-cocartesian edges $e$ such that $U(e)$ belongs to $\overline{W}$. Then $F$ exhibits $\operatorname{\mathcal{E}}'$ as a localization of $\operatorname{\mathcal{E}}$ with respect to $W_{-} \cup W_{+}$.

We begin by proving a special case of Theorem 6.3.5.1, where $\overline{F}$ is assumed to be an isomorphism.

Proposition 6.3.5.2. Suppose we are given a commutative diagram of simplicial sets

6.3
\begin{equation} \begin{gathered}\label{equation:cocartesian-fiberwise-localization} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [rr]^{F} \ar [dr]^{U} & & \operatorname{\mathcal{E}}' \ar [dl]_{U'} \\ & \operatorname{\mathcal{C}}& } \end{gathered} \end{equation}

with the following properties:

$(1)$

The morphisms $U$ and $U'$ are cocartesian fibrations.

$(2)$

The morphism $F$ carries $U$-cocartesian edges of $\operatorname{\mathcal{E}}$ to $U'$-cocartesian edges of $\operatorname{\mathcal{E}}'$.

$(3)$

For every vertex $C \in \operatorname{\mathcal{C}}$, the induced functor of $\infty $-categories $F_{C}: \operatorname{\mathcal{E}}_{C} \rightarrow \operatorname{\mathcal{E}}'_{C}$ exhibits $\operatorname{\mathcal{E}}'_{C}$ as the localization of $\operatorname{\mathcal{E}}_{C}$ with respect to some collection of morphisms $W_{C}$.

Set $W = \bigcup _{C \in \operatorname{\mathcal{C}}} W_{C}$, which we regard as a collection of edges of the simplicial set $\operatorname{\mathcal{E}}$. Then $F$ exhibits $\operatorname{\mathcal{E}}'$ as a localization of $\operatorname{\mathcal{E}}$ with respect to $W$.

Proof of Proposition 6.3.5.2. Let $\operatorname{\mathcal{D}}$ be an $\infty $-category, so that precomposition with $F$ induces a functor $F^{\ast }: \operatorname{Fun}( \operatorname{\mathcal{E}}', \operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}(\operatorname{\mathcal{E}}, \operatorname{\mathcal{D}})$. We wish to show that the functor $F^{\ast }$ is fully faithful, and that its essential image is the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{E}}[W^{-1}], \operatorname{\mathcal{D}}) \subseteq \operatorname{Fun}(\operatorname{\mathcal{E}}, \operatorname{\mathcal{D}})$. To prove this, let $\operatorname{\mathcal{B}}$ denote the direct image $\operatorname{Res}_{ \operatorname{\mathcal{E}}/ \operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}\times \operatorname{\mathcal{D}})$ and let $\pi : \operatorname{\mathcal{B}}\rightarrow \operatorname{\mathcal{C}}$ be the projection map, and define $\operatorname{\mathcal{B}}' = \operatorname{Res}_{\operatorname{\mathcal{E}}'/\operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}' \times \operatorname{\mathcal{D}})$ and $\pi ': \operatorname{\mathcal{B}}' \rightarrow \operatorname{\mathcal{C}}$ similarly. Combining assumption $(1)$ with Proposition 5.3.6.6, we see that $\pi $ and $\pi '$ are cartesian fibrations.

For each vertex $C \in \operatorname{\mathcal{C}}$, let us identify the fibers $\operatorname{\mathcal{B}}_{C} = \{ C\} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{B}}$ and $\operatorname{\mathcal{B}}'_{C} = \{ C\} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{B}}'$ with the $\infty $-categories $\operatorname{Fun}( \operatorname{\mathcal{E}}_{C}, \operatorname{\mathcal{D}})$ and $\operatorname{Fun}( \operatorname{\mathcal{E}}'_{C}, \operatorname{\mathcal{D}})$, respectively. Precomposition with $F$ induces a morphism of simplicial sets $G: \operatorname{\mathcal{B}}' \rightarrow \operatorname{\mathcal{B}}$ satisfying $\pi \circ G = \pi '$, given on each fiber by the functor

\[ G_{C}: \operatorname{\mathcal{B}}'_{C} = \operatorname{Fun}( \operatorname{\mathcal{E}}'_{C},\operatorname{\mathcal{D}}) \xrightarrow { \circ F_ C } \operatorname{Fun}( \operatorname{\mathcal{E}}_ C, \operatorname{\mathcal{D}}) = \operatorname{\mathcal{B}}. \]

Combining assumption $(2)$ with Proposition 5.3.6.6, we see that $G$ carries $\pi '$-cartesian edges of $\operatorname{\mathcal{B}}'$ to $\pi $-cartesian edges of $\operatorname{\mathcal{B}}$. In particular, for every edge $e: X \rightarrow Y$ of $\operatorname{\mathcal{C}}$, the diagram of $\infty $-categories

6.4
\begin{equation} \begin{gathered}\label{equation:proposition:cocartesian-fiberwise-localization} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{B}}'_{Y} \ar [r]^-{e^{\ast }} \ar [d]^{ G_{Y} } & \operatorname{\mathcal{B}}'_{X} \ar [d]^{ G_{X}} \\ \operatorname{\mathcal{B}}_{Y} \ar [r]^-{ e^{\ast } } & \operatorname{\mathcal{B}}_{X} } \end{gathered} \end{equation}

commutes up to isomorphism, where the horizontal functors are given by contravariant transport along $e$ (see Remark 5.2.8.5).

Let us identify the vertices of $\operatorname{\mathcal{B}}$ with pairs $(C, \rho )$, where $C$ is a vertex of $\operatorname{\mathcal{C}}$ and $\rho : \operatorname{\mathcal{E}}_{C} \rightarrow \operatorname{\mathcal{D}}$ is a functor of $\infty $-categories. Let $\operatorname{\mathcal{B}}^{0} \subseteq \operatorname{\mathcal{B}}$ denote the full simplicial subset spanned by those vertices $(C,\rho )$ for which the functor $\rho $ carries each edge of $W_{C}$ to an isomorphism in the $\infty $-category $\operatorname{\mathcal{D}}$. It follows from assumption $(3)$ that for every vertex $C$, the functor $G_{C}: \operatorname{\mathcal{B}}'_{C} \rightarrow \operatorname{\mathcal{B}}_{C}$ is fully faithful, and its essential image can be identified with the full subcategory $\operatorname{\mathcal{B}}^{0}_{C} = \{ C\} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{B}}^{0} \subseteq \operatorname{\mathcal{B}}_{C}$. Combining this observation with the homotopy commutativity of the diagram (6.3.5.2), we see that for every edge $e: X \rightarrow Y$ in $\operatorname{\mathcal{E}}$, the contravariant transport functor $e^{\ast }: \operatorname{\mathcal{B}}_{Y} \rightarrow \operatorname{\mathcal{B}}_{X}$ carries $\operatorname{\mathcal{B}}^{0}_{Y}$ into $\operatorname{\mathcal{B}}^{0}_{X}$. It follows that $\pi $ restricts to a cartesian fibration of simplicial sets $\pi ^{0}: \operatorname{\mathcal{B}}^{0} \rightarrow \operatorname{\mathcal{C}}$, and that an edge of $\operatorname{\mathcal{B}}^{0}$ is $\pi ^{0}$-cartesian if and only if it is $\pi $-cartesian when viewed as an edge of $\operatorname{\mathcal{B}}$ (Proposition 5.1.4.16). In particular, the morphism $G: \operatorname{\mathcal{B}}' \rightarrow \operatorname{\mathcal{B}}^{0} \subseteq \operatorname{\mathcal{B}}$ carries $\pi '$-cartesian edges of $\operatorname{\mathcal{B}}'$ to $\pi ^{0}$-cartesian edges of $\operatorname{\mathcal{B}}^{0}$, and therefore induces an equivalence $\operatorname{\mathcal{B}}' \rightarrow \operatorname{\mathcal{B}}^{0}$ of cartesian fibrations over $\operatorname{\mathcal{C}}$ (Proposition 5.1.6.14). We complete the proof by observing that $F^{\ast }: \operatorname{Fun}( \operatorname{\mathcal{E}}', \operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}(\operatorname{\mathcal{E}}[W^{-1}], \operatorname{\mathcal{D}})$ can be identified with the functor

\[ \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{B}}' ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{B}}^{0} ) \]

given by precomposition with $G$, and is therefore an equivalence of $\infty $-categories. $\square$

Corollary 6.3.5.3. Let $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of simplicial sets. Suppose that, for every vertex $C \in \operatorname{\mathcal{C}}$, the $\infty $-category $\operatorname{\mathcal{E}}_{C} = \{ C\} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{E}}$ is weakly contractible. Let $W$ be the collection of all edges $e$ of $\operatorname{\mathcal{E}}$ having the property that $U(e)$ is a degenerate edge of $\operatorname{\mathcal{C}}$. Then $U$ exhibits $\operatorname{\mathcal{C}}$ as a localization of $\operatorname{\mathcal{E}}$ with respect to $W$.

Proof. For each vertex $C \in \operatorname{\mathcal{C}}$, let $W_{C}$ be the collection of all morphisms in the $\infty $-category $\operatorname{\mathcal{E}}_{C}$. Since $\operatorname{\mathcal{E}}_{C}$ is weakly contractible, the projection map $\operatorname{\mathcal{E}}_{C} \rightarrow \{ C\} $ exhibits $\{ C\} $ as a localization of $\operatorname{\mathcal{E}}_{C}$ with respect to $W_{C}$ (Proposition 6.3.1.20). The desired result now follows by applying Proposition 6.3.5.2 to the commutative diagram

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [dr]^{U} \ar [rr]^{U} & & \operatorname{\mathcal{C}}\ar [dl]_{\operatorname{id}_{\operatorname{\mathcal{C}}} } \\ & \operatorname{\mathcal{C}}. & } \]
$\square$

We now consider another special case of Theorem 6.3.5.1.

Proposition 6.3.5.4. Suppose we are given a pullback diagram of simplicial sets

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

where $U$ and $U'$ are cocartesian fibrations. Suppose that $\overline{F}$ exhibits $\operatorname{\mathcal{C}}'$ as a localization of $\operatorname{\mathcal{C}}$ with respect to some collection of edges $\overline{W}$, and let $W$ denote the collection of $U$-cocartesian edges $e$ of $\operatorname{\mathcal{E}}$ which satisfy $U(e) \in \overline{W}$. Then $F$ exhibits $\operatorname{\mathcal{E}}'$ as a localization of $\operatorname{\mathcal{E}}$ with respect to $W$.

Proof. Using Corollary 4.1.3.3, we can choose an inner anodyne map $\operatorname{\mathcal{C}}' \hookrightarrow \operatorname{\mathcal{C}}''$, where $\operatorname{\mathcal{C}}''$ is an $\infty $-category. By virtue of Proposition 5.7.7.2, we can assume that $U'$ is the pullback of a cocartesian fibration of simplicial sets $U'': \operatorname{\mathcal{E}}'' \rightarrow \operatorname{\mathcal{C}}''$. Applying Proposition 5.3.6.1, we deduce that the inclusion map $\operatorname{\mathcal{E}}' \hookrightarrow \operatorname{\mathcal{E}}''$ is a categorical equivalence of simplicial sets. We may therefore replace $U'$ by $U''$, and thereby reduce to proving Proposition 6.3.5.4 in the special case where $\operatorname{\mathcal{C}}'$ is an $\infty $-category.

Fix an $\infty $-category $\operatorname{\mathcal{D}}$. We wish to show that the functor $F^{\ast }: \operatorname{Fun}( \operatorname{\mathcal{E}}', \operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}( \operatorname{\mathcal{E}}, \operatorname{\mathcal{D}})$ is fully faithful and that its essential image is the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{E}}[W^{-1} ], \operatorname{\mathcal{D}}) \subseteq \operatorname{Fun}(\operatorname{\mathcal{E}}, \operatorname{\mathcal{D}})$. Let $\operatorname{\mathcal{B}}' = \operatorname{Res}_{\operatorname{\mathcal{E}}'/\operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}' \times \operatorname{\mathcal{D}})$ and $\pi ': \operatorname{\mathcal{B}}' \rightarrow \operatorname{\mathcal{C}}$ be as in the proof of Proposition 6.3.5.2, so that we have canonical isomorphisms

\[ \operatorname{Fun}(\operatorname{\mathcal{E}}, \operatorname{\mathcal{D}}) \simeq \operatorname{Fun}_{ / \operatorname{\mathcal{C}}'}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{B}}') \quad \quad \operatorname{Fun}(\operatorname{\mathcal{E}}, \operatorname{\mathcal{D}}) \simeq \operatorname{Fun}_{ / \operatorname{\mathcal{C}}'}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{B}}') \]

Note that a morphism $G: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$ carries each edge of $W$ to an isomorphism in $\operatorname{\mathcal{D}}$ if and only if the corresponding object $g \in \operatorname{Fun}_{ / \operatorname{\mathcal{C}}'}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{B}}')$ carries each element $\overline{e} \in \overline{W}$ to a $\pi '$-cartesian edge of $\operatorname{\mathcal{B}}'$ (see Proposition 5.3.6.6). Since $\overline{F}$ carries each edge $\overline{e} \in \overline{W}$ to an isomorphism in $\operatorname{\mathcal{C}}'$, this is equivalent to the requirement that $g( \overline{e} )$ is an isomorphism in $\operatorname{\mathcal{B}}'$ (Proposition 5.1.1.8). We are therefore reduced to showing that composition with $\overline{F}$ induces a fully faithful functor $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}' }( \operatorname{\mathcal{C}}', \operatorname{\mathcal{B}}' ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}' }( \operatorname{\mathcal{C}}, \operatorname{\mathcal{B}}' )$, whose essential image is spanned by those functors $g \in \operatorname{Fun}_{ / \operatorname{\mathcal{C}}' }( \operatorname{\mathcal{C}}, \operatorname{\mathcal{B}}' )$ which carry each edge of $\overline{W}$ to an isomorphism in $\operatorname{\mathcal{B}}'$. This is a special case of Remark 6.3.1.15. $\square$

Corollary 6.3.5.5. Suppose we are given a pullback diagram of simplicial sets

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

where $U$ and $U'$ are left fibrations. Suppose that $\overline{F}$ exhibits $\operatorname{\mathcal{C}}'$ as a localization of $\operatorname{\mathcal{C}}$ with respect to some collection of edges $\overline{W}$. Then $F$ exhibits $\operatorname{\mathcal{E}}'$ as a localization of $\operatorname{\mathcal{E}}$ with respect to $W = U^{-1}( \overline{W} )$.

Proof of Theorem 6.3.5.1. Suppose we are given a commutative diagram of simplicial sets

6.5
\begin{equation} \begin{gathered}\label{equation:fiberwise-localization} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [d]^{U} \ar [r]^{F} & \operatorname{\mathcal{E}}' \ar [d]^{U'} \\ \operatorname{\mathcal{C}}\ar [r]^{\overline{F}} & \operatorname{\mathcal{C}}' } \end{gathered} \end{equation}

which satisfies the hypotheses of Theorem 6.3.5.1. Fix an $\infty $-category $\operatorname{\mathcal{D}}$. We wish to show that precomposition with $F$ induces a fully faithful functor $F^{\ast }: \operatorname{Fun}( \operatorname{\mathcal{E}}', \operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}(\operatorname{\mathcal{E}}, \operatorname{\mathcal{D}})$, whose essential image consists of those morphisms $G: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$ which carry each edge of $W_{-} \cup W_{+}$ to an isomorphism in $\operatorname{\mathcal{D}}$. Let $\pi : \operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}' \rightarrow \operatorname{\mathcal{E}}'$ be given by projection onto the second factor. Note that the pair $(U, F)$ determines a morphism of simplicial sets $\widetilde{F}: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}'$ satisfying $\pi \circ \widetilde{F} = F$, so that $F^{\ast }$ factors as a composition

\[ \operatorname{Fun}( \operatorname{\mathcal{E}}', \operatorname{\mathcal{D}}) \xrightarrow {\pi ^{\ast }} \operatorname{Fun}( \operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}', \operatorname{\mathcal{D}}) \xrightarrow {\widetilde{F}^{\ast }} \operatorname{Fun}( \operatorname{\mathcal{E}}, \operatorname{\mathcal{D}}). \]

Let $W'$ be the collection of all edges of $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}'$ of the form $( \overline{e}, f)$, where $\overline{e}$ belongs to $\overline{W}$ and $f$ is a $U'$-cocartesian edge of $\operatorname{\mathcal{E}}'$. It follows from Proposition 6.3.5.4 that the functor $\pi ^{\ast }$ is fully faithful, and that its essential image consists of those morphisms $G': \operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}' \rightarrow \operatorname{\mathcal{D}}$ which carry each edge of $W'$ to an isomorphism in $\operatorname{\mathcal{D}}$. Applying Proposition 6.3.5.2, we see that the functor $\widetilde{F}^{\ast }$ is also fully faithful, and that its essential image consists of those morphisms $G: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$ which carry each edge of $W_{-}$ to an isomorphism in $\operatorname{\mathcal{D}}$. To complete the proof, it will suffice to show the following:

$(\ast )$

A morphism of simplicial sets $G': \operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}' \rightarrow \operatorname{\mathcal{D}}$ carries each edge of $W'$ to an isomorphism in $\operatorname{\mathcal{D}}$ if and only if $G' \circ \widetilde{F}$ carries each edge of $W_{+}$ to an isomorphism in $\operatorname{\mathcal{D}}$.

The “only if” assertion is immediate (since $\widetilde{F}(W_{+} )$ is contained in $W'$). The converse follows from the observation that every edge $(\overline{e}, f)$ is isomorphic, when viewed as an object of the $\infty $-category $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}( \Delta ^1, \operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}')$, to $\widetilde{F}(e)$, where $e: X \rightarrow Y$ is any $U$-cocartesian edge of $\operatorname{\mathcal{E}}$ for which $U(e) = \overline{e}$ and $F(X)$ is isomorphic to the domain of $f$ as an object of the $\infty $-category $\{ X\} \times _{\operatorname{\mathcal{C}}'} \operatorname{\mathcal{E}}'$. $\square$

We close this section with a variant of the preceding results.

Proposition 6.3.5.6. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets which exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to a collection of edges $W$. Let $K$ be any simplicial set, and let $W_{K}$ denote the collection of edges $e = (e', e'')$ of the product $K \times \operatorname{\mathcal{C}}$ for which $e'$ is a degenerate edge of $K$ and $e''$ belongs to $W$. Then the induced map $F_{K}: K \times \operatorname{\mathcal{C}}\rightarrow K \times \operatorname{\mathcal{D}}$ exhibits $K \times \operatorname{\mathcal{D}}$ as the localization of $K \times \operatorname{\mathcal{C}}$ with respect to $W_{K}$.

Proof. Let $\operatorname{\mathcal{E}}$ be an $\infty $-category, and let

\[ \theta : \operatorname{Fun}( K \times \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \rightarrow \operatorname{Fun}(K \times \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \]

be the functor given by precomposition with $F_ K$. We wish to show that $F_{K}$ is fully faithful, and that its essential image is the full subcategory $\operatorname{Fun}( (K \times \operatorname{\mathcal{C}})[W_{K}^{-1}], \operatorname{\mathcal{E}})$ of Notation 6.3.1.1. Unwinding the definitions, we can identify $\theta $ with the functor

\[ \theta ': \operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{Fun}(K,\operatorname{\mathcal{E}}) ) \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{Fun}(K, \operatorname{\mathcal{E}})) \]

given by precomposition with $F$. Under this identification $\operatorname{Fun}( (K \times \operatorname{\mathcal{C}})[W_{K}^{-1}], \operatorname{\mathcal{E}})$ corresponds to the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{Fun}(K,\operatorname{\mathcal{E}}) ) \subseteq \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{Fun}(K,\operatorname{\mathcal{E}}) )$ (see Theorem 4.4.4.4), so that the desired result follows from our assumption on the functor $F$. $\square$