# Kerodon

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Corollary 7.2.3.4. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category and let $\overline{f}: A^{\triangleleft } \rightarrow \operatorname{\mathcal{C}}$ be a diagram, where $A$ is a weakly contractible simplicial set. The following conditions are equivalent:

$(1)$

The diagram $\overline{f}$ carries each edge of $A^{\triangleleft }$ to an isomorphism in $\operatorname{\mathcal{C}}$.

$(2)$

The restriction $f = \overline{f}|_{A}$ carries each edge of $A$ to an isomorphism in $\operatorname{\mathcal{C}}$, and $\overline{f}$ is a limit diagram.

Proof. Without loss of generality, we may assume that $f$ carries each edge of $A$ to an isomorphism in $\operatorname{\mathcal{C}}$. Under this assumption, we can restate $(1)$ and $(2)$ as follows:

$(1')$

For every vertex $a \in A$, the edge

$\Delta ^1 \simeq \{ a\} ^{\triangleleft } \hookrightarrow A^{\triangleleft } \xrightarrow { \overline{f} } \operatorname{\mathcal{C}}$

is an isomorphism in the $\infty$-category $\operatorname{\mathcal{C}}$.

$(2')$

The morphism $\overline{f}$ is a colimit diaram.

Using Corollary 3.1.7.2, we can choose an anodyne morphism $i: A \hookrightarrow B$, where $B$ is a Kan complex. Note that $f$ can be regarded as a morphism from $A$ to the core $\operatorname{\mathcal{C}}^{\simeq }$, which is also a Kan complex (Corollary 4.4.3.11). We can therefore extend $f$ to a morphism of Kan complexes $g: B \rightarrow \operatorname{\mathcal{C}}^{\simeq }$. Moreover, the morphism $i$ is right cofinal (Proposition 7.2.1.5) and therefore right anodyne (Proposition 7.2.1.3). It follows that the induced map $B \coprod _{A} A^{\triangleright } \hookrightarrow B^{\triangleright }$ is inner anodyne (Example 4.3.6.5), so that we can choose a functor $\overline{g}: B^{\triangleright } \rightarrow \operatorname{\mathcal{C}}$ satisfying $\overline{g}|_{B} = g$ and $\overline{g}|_{A^{\triangleright }} = \overline{f}$.

It follows from Corollary 7.2.2.3 $\overline{f}$ is a colimit diagram if and only if $\overline{g}$ is a colimit diagram. Since $A$ is weakly contractible, the Kan complex $B$ is contractible. In particular, every vertex $a \in A$ can be regarded as a final object of $B$. The equivalence of $(1')$ and $(2')$ now follows from Corollary 7.2.2.6. $\square$