# Kerodon

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Proposition 7.2.7.7. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. The following conditions are equivalent:

$(1)$

There exists a filtered diagram of simplicial sets $\{ \operatorname{\mathcal{C}}_{\alpha } \}$, where each $\operatorname{\mathcal{C}}_{\alpha }$ is an $\infty$-category with a final object, and an equivalence of $\infty$-categories $F: \operatorname{\mathcal{C}}\rightarrow \varinjlim _{\alpha } \operatorname{\mathcal{C}}_{\alpha }$.

$(2)$

There exists a filtered diagram of simplicial sets $\{ \operatorname{\mathcal{C}}_{\alpha } \}$, where each $\operatorname{\mathcal{C}}_{\alpha }$ is a filtered $\infty$-category, and an equivalence of $\infty$-categories $F: \operatorname{\mathcal{C}}\rightarrow \varinjlim _{\alpha } \operatorname{\mathcal{C}}_{\alpha }$.

$(3)$

There exists an equivalence of $\infty$-categories $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{C}}'$, where $\operatorname{\mathcal{C}}'$ is filtered.

$(4)$

The $\infty$-category $\operatorname{\mathcal{C}}$ is filtered.

Proof. The implication $(1) \Rightarrow (2)$ follows from Example 7.2.4.5, the implication $(2) \Rightarrow (3)$ from Remark 7.2.4.6, and the implication $(3) \Rightarrow (4)$ from Corollary 7.2.4.11. We will complete the proof by showing that every filtered $\infty$-category $\operatorname{\mathcal{C}}$ satisfies condition $(1)$. Without loss of generality, we may assume that $\operatorname{\mathcal{C}}$ satisfies condition $(\ast )$ of Lemma 7.2.7.3. Let $A$ be the directed partially ordered set defined in the proof of Theorem 7.2.7.2. For each $\alpha \in A$, let $L_{\alpha } \subseteq \operatorname{\mathcal{C}}$ denote the corresponding subset of $\operatorname{\mathcal{C}}$. By virtue of Corollary 4.1.3.3, we can choose an $\infty$-category $\operatorname{\mathcal{C}}_{\alpha }$ and an inner anodyne morphism $F_{\alpha }: L_{\alpha } \hookrightarrow \operatorname{\mathcal{C}}_{\alpha }$, which depend functorially on $\alpha$. Applying Corollary 4.5.7.2, we see that the morphisms $F_{\alpha }$ induce an equivalence of $\infty$-categories

$\operatorname{\mathcal{C}}\simeq \varinjlim _{\alpha \in A} L_{\alpha } \xrightarrow { \{ F_{\alpha } \} _{\alpha \in A} } \varinjlim _{\alpha \in A} \operatorname{\mathcal{C}}_{\alpha }.$

To complete the proof, it will suffice to show that each of the $\infty$-categories $\operatorname{\mathcal{C}}_{\alpha }$ contains a final object. By construction, there exists an isomorphism of simplicial sets $u: L_{\alpha } \simeq K^{\triangleright }$, for some finite simplicial set $K$. Using Corollary 4.1.3.3, we can choose a categorical equivalence $v: K \rightarrow \operatorname{\mathcal{D}}$, where $\operatorname{\mathcal{D}}$ is an $\infty$-category. Applying Corollary 4.5.8.9, we deduce that the map $v^{\triangleright }: K^{\triangleright } \rightarrow \operatorname{\mathcal{D}}^{\triangleright }$ is also a categorical equivalence of simplicial sets. Since $F_{\alpha }$ is inner anodyne, there exists a functor $G: \operatorname{\mathcal{C}}_{\alpha } \rightarrow \operatorname{\mathcal{D}}^{\triangleright }$ satisfying $G \circ F_{\alpha } = v^{\triangleright } \circ u$. Applying the two-out-of-three property (Remark 4.5.3.5), we see that $G$ is an equivalence of $\infty$-categories. Since the $\infty$-category $\operatorname{\mathcal{D}}^{\triangleright }$ has a final object (given by the cone point; see Example 4.6.6.5), it follows that $\operatorname{\mathcal{C}}_{\alpha }$ also has a final object (Corollary 4.6.6.22). $\square$