# Kerodon

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Proposition 8.1.2.9. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. For every object $X \in \operatorname{\mathcal{C}}$, the coslice inclusion

$\iota _{X}: \operatorname{\mathcal{C}}_{X/} \hookrightarrow \{ X\} \times _{ \operatorname{\mathcal{C}}^{\operatorname{op}} } \operatorname{Tw}(\operatorname{\mathcal{C}})$

is an equivalence of $\infty$-categories.

Proof. By construction, we have a commutative diagram

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}_{X/} \ar [rr]^-{ \iota _{X} } \ar [dr] & & \{ X\} \times _{ \operatorname{\mathcal{C}}^{\operatorname{op}} } \operatorname{Tw}(\operatorname{\mathcal{C}}) \ar [dl]^-{ \lambda _{+} } \\ & \operatorname{\mathcal{C}}, & }$

where the vertical maps are left fibrations of $\infty$-categories (Propositions 4.3.6.1 and 8.1.1.11). Moreover, the $\infty$-category $\operatorname{\mathcal{C}}_{X/}$ has an initial object $\widetilde{X}$, given by the identity morphism $\operatorname{id}_{X}: X \rightarrow X$ (Proposition 4.6.7.22). Proposition 8.1.2.1 guarantees that $\iota _{X}( \widetilde{X} )$ is an initial object of the $\infty$-category $\{ X\} \times _{ \operatorname{\mathcal{C}}^{\operatorname{op}} } \operatorname{Tw}(\operatorname{\mathcal{C}})$, so that $\iota _{X}$ is an equivalence of $\infty$-categories by virtue of Corollary 5.6.6.20. $\square$