Notation 6.3.1.1. Let $\operatorname{\mathcal{C}}$ be a simplicial set, let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$, and let $\operatorname{\mathcal{E}}$ be an $\infty $-category. We let $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})$ denote the full subcategory of $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ spanned by those morphisms $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ that carry each edge of $W$ to an isomorphism in $\operatorname{\mathcal{E}}$.
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
6.3.1 Localizations of $\infty $-Categories
We begin by introducing some terminology.
Remark 6.3.1.2. In the context of Notation 6.3.1.1, we will usually be interested in the situation where the simplicial set $\operatorname{\mathcal{C}}$ is an $\infty $-category (as suggested by the notation). However, it will be technically convenient to allow more general simplicial sets as well.
Example 6.3.1.3. Let $\operatorname{\mathcal{C}}$ be a simplicial set and let $W$ be a collection of degenerate edges of $\operatorname{\mathcal{C}}$. Then, for every $\infty $-category $\operatorname{\mathcal{E}}$, we have $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) = \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$.
Example 6.3.1.4. Let $\operatorname{\mathcal{C}}$ be a simplicial set and let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$. If $\operatorname{\mathcal{E}}$ is a Kan complex, then $\operatorname{Fun}(\operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) = \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ (see Proposition 1.4.6.10).
Example 6.3.1.5. Let $W = \{ \operatorname{id}_{\Delta ^1} \} $ consist of the single nondegenerate edge of the standard $1$-simplex $\Delta ^1$. For every $\infty $-category $\operatorname{\mathcal{E}}$, $\operatorname{Fun}(\Delta ^1[W^{-1}], \operatorname{\mathcal{E}})$ is the full subcategory $\operatorname{Isom}(\operatorname{\mathcal{E}}) \subseteq \operatorname{Fun}(\Delta ^1, \operatorname{\mathcal{E}})$ spanned by the isomorphisms in $\operatorname{\mathcal{E}}$ (Example 4.4.1.14).
Example 6.3.1.6. Let $\operatorname{\mathcal{C}}$ be a simplicial set and let $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$ denote its homotopy category (Definition 1.3.6.1). Let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$, let $[W]$ denote the collection of morphisms in $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$ which belong to the image of $W$, and let $F: \mathrm{h} \mathit{\operatorname{\mathcal{C}}} \rightarrow \operatorname{\mathcal{D}}$ be a functor of ordinary categories which exhibits $\operatorname{\mathcal{D}}$ as a strict localization of $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$ with respect to $[W]$ (Definition 6.3.0.1). If $\operatorname{\mathcal{E}}$ is an ordinary category, then we have a canonical isomorphism of simplicial sets
\[ \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{N}_{\bullet }(\operatorname{\mathcal{E}}) ) \simeq \operatorname{N}_{\bullet }( \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) ). \]
Remark 6.3.1.7. Let $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ be simplicial sets and let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$. For every $\infty $-category $\operatorname{\mathcal{E}}$, the canonical isomorphism $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) ) \simeq \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}))$ restricts to an isomorphism of full subcategories This follows immediately from the criterion of Theorem 4.4.4.4.
\[ \operatorname{Fun}(\operatorname{\mathcal{C}}[W^{-1}], \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) ) \simeq \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{Fun}(\operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) ). \]
Remark 6.3.1.8. Let $\operatorname{\mathcal{C}}$ be a simplicial set, let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$, and let $\operatorname{\mathcal{E}}$ be an $\infty $-category. Then the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) \subseteq \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is replete. That is, if $F,F': \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ are isomorphic objects of $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$, then $F$ carries edges of $W$ to isomorphisms in $\operatorname{\mathcal{E}}$ if and only if $F'$ carries edges of $W$ to isomorphisms in $\operatorname{\mathcal{E}}$ (see Example 4.4.1.14).
Definition 6.3.1.9. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets and let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$. We say that $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$ if, for every $\infty $-category $\operatorname{\mathcal{E}}$, the precomposition map $\operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \xrightarrow { \circ F} \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is fully faithful, and its essential image is the full subcategory $\operatorname{Fun}(\operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) \subseteq \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$.
Remark 6.3.1.10. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets. If $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to a collection of edges $W$, then, for every $\infty $-category $\operatorname{\mathcal{E}}$ and every morphism $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$, the composite map $(G \circ F): \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ carries each element of $W$ to an isomorphism in $\operatorname{\mathcal{E}}$. In particular, if $\operatorname{\mathcal{D}}$ itself is an $\infty $-category, then $F$ carries each element of $W$ to an isomorphism in $\operatorname{\mathcal{D}}$.
Exercise 6.3.1.11. Let $\operatorname{\mathcal{C}}$ be a simplicial set, let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$, and let $F,F': \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a pair of diagrams taking values in an $\infty $-category $\operatorname{\mathcal{D}}$. Suppose that $F$ and $F'$ are isomorphic when viewed as objects of the $\infty $-category $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$. Show that $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$ if and only if $F'$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$.
Example 6.3.1.12. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets and let $W$ be a collection of degenerate edges of $\operatorname{\mathcal{C}}$. Then $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$ if and only if it is a categorical equivalence of simplicial sets (see Proposition 4.5.3.8).
Proposition 6.3.1.13. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets and let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$. The following conditions are equivalent:
The morphism $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$ (Definition 6.3.1.9).
For every $\infty $-category $\operatorname{\mathcal{E}}$, the functor $\operatorname{Fun}( \operatorname{\mathcal{D}},\operatorname{\mathcal{E}}) \xrightarrow { \circ F} \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ factors through the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})$ and induces an equivalence of $\infty $-categories $\operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})$.
For every $\infty $-category $\operatorname{\mathcal{E}}$, the functor $\operatorname{Fun}( \operatorname{\mathcal{D}},\operatorname{\mathcal{E}}) \xrightarrow { \circ F} \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ factors through the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})$ and induces a homotopy equivalence of Kan complexes $\operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}})^{\simeq } \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})^{\simeq }$.
For every $\infty $-category $\operatorname{\mathcal{E}}$, the functor $\operatorname{Fun}( \operatorname{\mathcal{D}},\operatorname{\mathcal{E}}) \xrightarrow { \circ F} \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ factors through the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})$ and induces a bijection of sets $\pi _0( \operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}})^{\simeq } ) \rightarrow \pi _0( \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})^{\simeq } )$.
Proof. The equivalence $(1) \Leftrightarrow (2)$ follows from Corollary 4.6.2.23 (and the repleteness of the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) \subseteq \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$). The implication $(2) \Rightarrow (3)$ follows from Remark 4.5.1.19 and the implication $(3) \Rightarrow (4)$ from Remark 3.1.6.5. We will complete the proof by showing that $(4) \Rightarrow (2)$. Assume that $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ satisfies condition $(4)$, and let $\operatorname{\mathcal{E}}$ be an $\infty $-category; we wish to show that the precomposition functor $\operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \xrightarrow { \circ F} \operatorname{Fun}(\operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})$ is an equivalence of $\infty $-categories. For this, it will suffice to show that for every simplicial set $\operatorname{\mathcal{B}}$, the induced map
\[ \theta : \pi _0( \operatorname{Fun}(\operatorname{\mathcal{B}}, \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}))^{\simeq }) \rightarrow \pi _0( \operatorname{Fun}(\operatorname{\mathcal{B}}, \operatorname{Fun}(\operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) )^{\simeq } ) \]
is a bijection. Using Remark 6.3.1.7, we can identify $\theta $ with the map
\[ \pi _0( \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{Fun}(\operatorname{\mathcal{B}}, \operatorname{\mathcal{E}}) )^{\simeq } ) \rightarrow \pi _0( \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{Fun}(\operatorname{\mathcal{B}}, \operatorname{\mathcal{E}}) )^{\simeq } ), \]
which is bijective by virtue of assumption $(4)$. $\square$
Example 6.3.1.14. Let $W = \{ \operatorname{id}_{\Delta ^1} \} $ consist of the single nondegenerate edge of the standard $1$-simplex $\Delta ^1$. Then the projection map $\Delta ^1 \rightarrow \Delta ^0$ exhibits $\Delta ^0$ as a localization of $\Delta ^1$ with respect to $W$. To prove this, it will suffice to show that for every $\infty $-category $\operatorname{\mathcal{E}}$, the construction $X \mapsto \operatorname{id}_{X}$ induces an equivalence of $\infty $-categories $\operatorname{\mathcal{E}}= \operatorname{Fun}( \Delta ^0, \operatorname{\mathcal{E}}) \rightarrow \operatorname{Fun}( \Delta ^1[W^{-1}], \operatorname{\mathcal{E}}) = \operatorname{Isom}(\operatorname{\mathcal{E}})$, which follows from Corollary 4.5.3.13.
Remark 6.3.1.15. 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 $W$, and let $U: \overline{\operatorname{\mathcal{E}}} \rightarrow \operatorname{\mathcal{E}}$ be an isofibration of $\infty $-categories. Then, for every diagram $\operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$, precomposition with $F$ induces a fully faithful functor whose essential image is spanned by those functors $G: \operatorname{\mathcal{C}}\rightarrow \overline{\operatorname{\mathcal{E}}}$ which carry each edge of $W$ to an isomorphism in the $\infty $-category $\overline{\operatorname{\mathcal{E}}}$. This follows by applying Corollary 4.5.2.32 to the diagram
\[ \operatorname{Fun}_{ / \operatorname{\mathcal{E}}}( \operatorname{\mathcal{D}}, \overline{\operatorname{\mathcal{E}}} ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{E}}}( \operatorname{\mathcal{C}}, \overline{\operatorname{\mathcal{E}}} ), \]
\[ \xymatrix@R =50pt@C=50pt{ \operatorname{Fun}( \operatorname{\mathcal{D}}, \overline{\operatorname{\mathcal{E}}} ) \ar [r]^{\circ F} \ar [d]^{ U \circ } & \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \overline{\operatorname{\mathcal{E}}} ) \ar [d] \\ \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \ar [r] & \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}). } \]
Remark 6.3.1.16. 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 $W$. Then, for every Kan complex $\operatorname{\mathcal{E}}$, precomposition with $F$ induces a homotopy equivalence of Kan complexes (see Example 6.3.1.4). It follows that $F$ is a weak homotopy equivalence of simplicial sets.
\[ \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \xrightarrow { \circ F} \operatorname{Fun}(\operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}}) = \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \]
Remark 6.3.1.17. 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 $[W]$ denote the collection of morphisms in the homotopy category $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$ which belong to the image of $W$. Then the induced functor $\mathrm{h} \mathit{F}: \mathrm{h} \mathit{\operatorname{\mathcal{C}}} \rightarrow \mathrm{h} \mathit{\operatorname{\mathcal{D}}}$ exhibits the homotopy category $\mathrm{h} \mathit{\operatorname{\mathcal{D}}}$ as a $1$-categorical localization of $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$ with respect to $[W]$, in the sense of Definition 6.3.0.6. This follows immediately from Example 6.3.1.6.
Example 6.3.1.18. Let $\operatorname{\mathcal{C}}$ be an ordinary category and let $W$ be a collection of morphisms of $\operatorname{\mathcal{C}}$, which we identify with edges of the simplicial set $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$. Let $F: \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) \rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets which exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ with respect to $W$. Then the induced functor $\operatorname{\mathcal{C}}\simeq \mathrm{h} \mathit{\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})} \xrightarrow { \mathrm{h} \mathit{F} } \mathrm{h} \mathit{\operatorname{\mathcal{D}}}$ exhibits the homotopy category $\mathrm{h} \mathit{\operatorname{\mathcal{D}}}$ as a $1$-categorical localization of $\operatorname{\mathcal{C}}$ with respect to $W$, in the sense of in the sense of Definition 6.3.0.6.
Remark 6.3.1.19. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ and $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ be morphisms of simplicial sets, and let $W$ be a collection of edges of $\operatorname{\mathcal{C}}$. If any two of the following three conditions is satisfied, then so is the third:
The morphism $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$.
The morphism $G \circ F$ exhibits $\operatorname{\mathcal{E}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$.
The morphism $G$ is a categorical equivalence of simplicial sets.
Proposition 6.3.1.20. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets, where $\operatorname{\mathcal{D}}$ is an $\infty $-category, and let $W$ be the collection of all edges of $\operatorname{\mathcal{C}}$. The following conditions are equivalent:
The morphism $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$.
The $\infty $-category $\operatorname{\mathcal{D}}$ is a Kan complex and $F$ is a weak homotopy equivalence of simplicial sets.
Proof. We first prove that $(2)$ implies $(1)$. Assume that $\operatorname{\mathcal{D}}$ is a Kan complex and that $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ is a weak homotopy equivalence; we wish to show that $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$. By virtue of Proposition 6.3.1.13, it will suffice to show that for every $\infty $-category $\operatorname{\mathcal{E}}$, composition with $F$ induces a homotopy equivalence of Kan complexes $\theta : \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}})^{\simeq } \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1}], \operatorname{\mathcal{E}})^{\simeq }$. Since $\operatorname{\mathcal{D}}$ is a Kan complex, Proposition 4.4.3.22 allows us to identify $\theta $ with the canonical map
\[ \operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}^{\simeq }) \xrightarrow { \circ F} \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}^{\simeq } ), \]
which is a homotopy equivalence by virtue of our assumption that $F$ is a weak homotopy equivalence.
We now show that $(1)$ implies $(2)$. Assume that $F$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$. Invoking Remark 6.3.1.16, we deduce that $F$ is a weak homotopy equivalence. We wish to show that $\operatorname{\mathcal{D}}$ is a Kan complex. Choose a weak homotopy equivalence $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$, where $\operatorname{\mathcal{E}}$ is a Kan complex (Corollary 3.1.7.2). Then the composite map $(G \circ F): \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ is also a weak homotopy equivalence (Remark 3.1.6.16). Invoking the implication $(2) \Rightarrow (1)$, we conclude that $G \circ F$ exhibits $\operatorname{\mathcal{E}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$. It follows from Remark 6.3.1.19 that $G$ is an equivalence of $\infty $-categories. Since $\operatorname{\mathcal{E}}$ is a Kan complex, it follows that the $\infty $-category $\operatorname{\mathcal{D}}$ is also a Kan complex (Remark 4.5.1.21). $\square$
Proposition 6.3.1.21 (Transitivity). Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{C}}'$ and $F': \operatorname{\mathcal{C}}' \rightarrow \operatorname{\mathcal{C}}''$ be morphisms of simplicial sets. Let $W$ and $W'$ be collections of edges of $\operatorname{\mathcal{C}}$ satisfying the following conditions:
The morphism $F$ exhibits $\operatorname{\mathcal{C}}'$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W$.
The morphism $F'$ exhibits $\operatorname{\mathcal{C}}''$ as a localization of $\operatorname{\mathcal{C}}'$ with respect to $F(W')$.
Then the composite morphism $(F' \circ F): \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{C}}''$ exhibits $\operatorname{\mathcal{C}}''$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W \cup W'$.
Proof. Let $\operatorname{\mathcal{E}}$ be an $\infty $-category; we wish to prove that precomposition with $F' \circ F$ induces an equivalence from $\operatorname{Fun}(\operatorname{\mathcal{C}}'', \operatorname{\mathcal{E}})$ to the full subcategory $\operatorname{Fun}( \operatorname{\mathcal{C}}[ (W \cup W')^{-1} ], \operatorname{\mathcal{E}}) \subseteq \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$. We have a commutative diagram
\[ \xymatrix@R =50pt@C=50pt{ \operatorname{Fun}( \operatorname{\mathcal{C}}'', \operatorname{\mathcal{E}}) \ar [r]^-{\circ F'} & \operatorname{Fun}( \operatorname{\mathcal{C}}'[ F(W')^{-1} ], \operatorname{\mathcal{E}}) \ar [r]^-{\circ F} \ar [d] & \operatorname{Fun}( \operatorname{\mathcal{C}}[ (W \cup W')^{-1} ], \operatorname{\mathcal{E}}) \ar [d] \\ & \operatorname{Fun}( \operatorname{\mathcal{C}}', \operatorname{\mathcal{E}}) \ar [r]^-{\circ F} & \operatorname{Fun}( \operatorname{\mathcal{C}}[W^{-1} ], } \]
where the horizontal functors on the left and lower right are equivalences of $\infty $-categories. Since the square is a pullback and the vertical maps are isofibrations (Remark 6.3.1.8), it follows that the horizontal map on the upper right is also an equivalence of $\infty $-categories (Corollary 4.5.2.29). $\square$
Corollary 6.3.1.22. Let $\operatorname{\mathcal{C}}$ be a simplicial set, let $W$ and $W'$ be collections of edges of $\operatorname{\mathcal{C}}$, and 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 $W$. Suppose that, for every edge $w \in W'$, the image $F(w)$ is a degenerate edge of $\operatorname{\mathcal{D}}$. Then $F$ also exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{C}}$ with respect to $W \cup W'$.
Proof. Combine Proposition 6.3.1.21 with Example 6.3.1.12. $\square$