# Kerodon

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Proposition 7.3.4.1. Let $\delta : \operatorname{\mathcal{K}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of $\infty$-categories. Suppose we are given functors of $\infty$-categories $F_0: \operatorname{\mathcal{K}}\rightarrow \operatorname{\mathcal{D}}$ and $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ and a natural transformation $\beta : F_0 \rightarrow F \circ \delta$. The following conditions are equivalent:

$(1)$

The natural transformation $\beta$ exhibits $F$ as a left Kan extension of $F_0$ along $\delta$.

$(2)$

For each object $C \in \operatorname{\mathcal{C}}$, the restriction of $\beta$ to the fiber $\operatorname{\mathcal{K}}_{C} = \{ C\} \times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{K}}$ determines a natural transformation $F_0|_{ \operatorname{\mathcal{K}}_{C} } \rightarrow \underline{ F(C) }$ which exhibits $F(C)$ as a colimit of the diagram $F_0|_{ \operatorname{\mathcal{K}}_{C} }$ in the $\infty$-category $\operatorname{\mathcal{D}}$.

Proof. By virtue of Corollary 7.2.2.7, it will suffice to show that for each object $C \in \operatorname{\mathcal{C}}$, the tautological map

$\operatorname{\mathcal{K}}_{C} = \operatorname{\mathcal{K}}\times _{\operatorname{\mathcal{C}}} \{ C\} \hookrightarrow \operatorname{\mathcal{K}}\times _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}_{/C}$

is right cofinal. Since $\delta$ is a cocartesian fibration, it will suffice to show that the inclusion map $\{ \operatorname{id}_ C \} \hookrightarrow \operatorname{\mathcal{C}}_{/C}$ is right cofinal (Proposition 7.2.3.13). This follows from Corollary 4.6.6.25, since $\operatorname{id}_{C}$ is a final object of the $\infty$-category $\operatorname{\mathcal{C}}_{/C}$ (Proposition 4.6.6.23). $\square$