# Kerodon

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Definition 7.3.1.2 (Right Kan Extensions). Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor of $\infty$-categories. Suppose we are given a simplicial set $K$ together with diagrams $\delta : K \rightarrow \operatorname{\mathcal{C}}$ and $F_0: K \rightarrow \operatorname{\mathcal{D}}$ and a natural transformation $\alpha : F \circ \delta \rightarrow F_0$, as indicated in the diagram

$\xymatrix@R =50pt@C=50pt{ & \operatorname{\mathcal{C}}\ar [dr]^{F} \ar@ {=>}[]+<0pt,-25pt>;+<0pt,-50pt>^-{\alpha } & \\ K \ar [ur]^{\delta } \ar [rr]_{F_0} & & \operatorname{\mathcal{D}}. }$

We will say that $\alpha$ exhibits $F$ as a right Kan extension of $F_0$ along $\delta$ if, for every object $C \in \operatorname{\mathcal{C}}$, the following condition is satisfied:

$(\ast _ C)$

Let $\alpha _{C}$ denote a composition of the natural transformations

$\underline{ F(C) } \xrightarrow { F( \gamma ' ) } (F \circ \delta )|_{ K_{C/} } \xrightarrow { \alpha } F_0|_{ K_{C/} }$

(formed in the $\infty$-category $\operatorname{Fun}( K_{C/}, \operatorname{\mathcal{D}})$), where $\gamma ': \underline{C} \rightarrow \delta |_{ K_{C/} }$ is defined in Notation 7.3.1.1. Then $\alpha _{C}$ exhibits $F(C)$ as a limit of the diagram

$K_{C/} = \operatorname{\mathcal{C}}_{C/} \times _{\operatorname{\mathcal{C}}} K \rightarrow K \xrightarrow {F_0} \operatorname{\mathcal{D}},$

in the sense of Definition 7.1.1.1.