# Kerodon

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Corollary 11.8.0.1. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category and let $W$ be a collection of morphisms in $\operatorname{\mathcal{C}}$. Let $K$ be a simplicial set, and let $\widetilde{W}$ be the collection of morphisms in $\operatorname{Fun}(K, \operatorname{\mathcal{C}})$ consisting of those natural transformations $w: F \rightarrow F'$ with the property that, for each vertex $x \in K$, the image $w_{x}: F(x) \rightarrow F'(x)$ belongs to $W$. If $G: K \rightarrow \operatorname{\mathcal{C}}$ is a diagram which carries each vertex of $K$ to a $W$-local object of $\operatorname{\mathcal{C}}$, then $G$ is $\widetilde{W}$-local when viewed as an object of $\operatorname{Fun}(K, \operatorname{\mathcal{C}})$ (see Definition 9.1.1.1)

Proof. Using Corollary 4.1.3.3, we can choose an inner anodyne morphism $K \hookrightarrow \operatorname{\mathcal{K}}$, where $\operatorname{\mathcal{K}}$ is an $\infty$-category. Extend $G$ to a functor $\overline{G}: \operatorname{\mathcal{K}}\rightarrow \operatorname{\mathcal{C}}$. Since the restriction map $\operatorname{Fun}( \operatorname{\mathcal{K}}, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}(K, \operatorname{\mathcal{C}})$ is a trivial Kan fibration (Proposition 1.5.7.6), we can replace $K$ by $\operatorname{\mathcal{K}}$ (and $G$ by $\overline{G}$) and thereby reduce to proving Corollary 11.8.0.1 in the special case where $K = \operatorname{\mathcal{K}}$ is an $\infty$-category.

Let $w: F \rightarrow F'$ be a morphism in $\operatorname{Fun}(\operatorname{\mathcal{K}}, \operatorname{\mathcal{C}})$ which belongs to $\widetilde{W}$; we wish to show that precomposition with $w$ induces a homotopy equivalence of Kan complexes $\theta : \operatorname{Hom}_{ \operatorname{Fun}(\operatorname{\mathcal{K}}, \operatorname{\mathcal{C}}) }(F',G) \xrightarrow { \circ [w] } \operatorname{Hom}_{ \operatorname{Fun}(\operatorname{\mathcal{K}}, \operatorname{\mathcal{C}}) }( F, G )$. Let $\mathscr {H}: \operatorname{\mathcal{C}}^{\operatorname{op}} \times \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{S}}$ be a $\operatorname{Hom}$-functor for $\operatorname{\mathcal{C}}$, let $U: \operatorname{Tw}(\operatorname{\mathcal{K}}) \rightarrow \operatorname{\mathcal{S}}$ be the functor given by the composition

$\operatorname{Tw}(\operatorname{\mathcal{K}}) \rightarrow \operatorname{\mathcal{K}}^{\operatorname{op}} \times \operatorname{\mathcal{K}}\xrightarrow {F^{\operatorname{op}} \times G} \operatorname{\mathcal{C}}^{\operatorname{op}} \times \operatorname{\mathcal{C}}\xrightarrow { \mathscr {H}} \operatorname{\mathcal{S}}$

and define $U': \operatorname{Tw}(\operatorname{\mathcal{K}}) \rightarrow \operatorname{\mathcal{S}}$ similarly. The morphism $w$ induces a natural transformation $\overline{w}: U' \rightarrow U$, which carries each object $(f: X \rightarrow Y)$ of $\operatorname{Tw}(\operatorname{\mathcal{K}})$ to the morphism of Kan complexes

$\overline{w}_{f}: U'(f) \simeq \operatorname{Hom}_{\operatorname{\mathcal{C}}}( F'(X), G(Y) ) \rightarrow \operatorname{Hom}_{\operatorname{\mathcal{C}}}( F(X), G(Y) ) \simeq U(f)$

given by precomposition with the morphism $w_{X}: F'(X) \rightarrow F(X)$ of $\operatorname{\mathcal{C}}$. Since $w_{X}$ belongs to $W$ and the object $G(Y) \in \operatorname{\mathcal{C}}$ is $W$-local, it follows that $\overline{w}_{f}$ is a homotopy equivalence. Applying Theorem 4.4.4.4, we see that $\overline{w}$ is an isomorphism (in the functor $\infty$-category $\operatorname{Fun}( \operatorname{Tw}(\operatorname{\mathcal{K}}), \operatorname{\mathcal{S}})$). We conclude by observing that Example 8.3.4.22 supplies an identification of $\theta$ with the image of $\overline{w}$ under the limit functor $\varprojlim : \operatorname{Fun}( \operatorname{Tw}(\operatorname{\mathcal{K}}), \operatorname{\mathcal{S}}) \rightarrow \operatorname{\mathcal{S}}$. $\square$