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Proposition 7.3.2.6. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor of $\infty $-categories, let $F_0$ denote the restriction of $F$ to a full subcategory $\operatorname{\mathcal{C}}^{0} \subseteq \operatorname{\mathcal{C}}$, and let $\iota : \operatorname{\mathcal{C}}^{0} \hookrightarrow \operatorname{\mathcal{C}}$ denote the inclusion functor. Then:

  • The functor $F$ is left Kan extended from $\operatorname{\mathcal{C}}^{0}$ (in the sense of Definition 7.3.2.1) if and only if the identity transformation $\operatorname{id}: F_0 \rightarrow F \circ \iota $ exhibits $F$ as a left Kan extension of $F_0$ along $\iota $ (in the sense of Variant 7.3.1.5).

  • The functor $F$ is right Kan extended from $\operatorname{\mathcal{C}}^{0}$ (in the sense of Definition 7.3.2.1) if and only if the identity transformation $\operatorname{id}_{F_0}: F \circ \iota \rightarrow F_0$ exhibits $F$ as a right Kan extension of $F_0$ along $\iota $ (in the sense of Definition 7.3.1.2).

Proof. Fix an object $C \in \operatorname{\mathcal{C}}$. It follows from Remark 7.1.3.6 that the composition

\[ (\operatorname{\mathcal{C}}^{0}_{/C})^{\triangleright } \hookrightarrow (\operatorname{\mathcal{C}}_{/C})^{\triangleright } \rightarrow \operatorname{\mathcal{C}}\xrightarrow {F} \operatorname{\mathcal{D}} \]

is a colimit diagram in $\operatorname{\mathcal{D}}$ if and only if the natural transformation $\operatorname{id}_{ F_0 }$ satisfies condition $(\ast _ C)$ of Variant 7.3.1.5. The first assertion follows by allowing the object $C$ to vary, and the second follows by a similar argument. $\square$