# Kerodon

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Proposition 7.1.2.13. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category, let $K$ be a simplicial set, and let $\overline{\rho }: \overline{F} \rightarrow \overline{G}$ be a natural transformation between diagrams $\overline{F}, \overline{G}: K^{\triangleleft } \rightarrow \operatorname{\mathcal{C}}$. Assume that, for every vertex $x \in K$, the morphism $\overline{\rho }_{x}: \overline{F}(x) \rightarrow \overline{F}(x)$ is an isomorphism in $\operatorname{\mathcal{C}}$. Then any two of the following conditions imply the third:

$(1)$

The morphism of simplicial sets $\overline{F}$ is a limit diagram in $\operatorname{\mathcal{C}}$.

$(2)$

The morphism of simplicial sets $\overline{G}$ is a limit diagram in $\operatorname{\mathcal{C}}$.

$(3)$

The natural transformation $\overline{\rho }$ carries the cone point ${\bf 0} \in K^{\triangleleft }$ to an isomorphism $\overline{\rho }_{\bf 0}: \overline{F}( {\bf 0} ) \rightarrow \overline{F}( {\bf 0} )$.

Proof. Set $F = \overline{F}|_{K}$ and $G = \overline{G}|_{K}$, so that $\overline{\rho }$ restricts to an isomorphism $\rho : F \rightarrow G$ in the $\infty$-category $\operatorname{Fun}(K, \operatorname{\mathcal{C}})$ (Theorem 4.4.4.4). Set $X = \overline{F}( {\bf 0} )$ and $Y = \overline{F}( {\bf 0} )$, and let $\underline{X}, \underline{Y}: K \rightarrow \operatorname{\mathcal{C}}$ be the constant maps taking the values $X$ and $Y$, respectively. Let $c$ denote the composition $\Delta ^1 \times K \simeq K \star _{K} K \rightarrow \Delta ^{0} \star _{ \Delta ^{0} } K = K^{\triangleleft }$. Then the composition

$\Delta ^1 \times \Delta ^1 \times K \xrightarrow { \operatorname{id}\times c} \Delta ^1 \times K^{\triangleleft } \xrightarrow { \overline{\rho } }$

can be identified with a commutative diagram

$\xymatrix { \underline{X} \ar [d]^{ \alpha } \ar [dr]^{\gamma } \ar [r]^{ \underline{f} } & \underline{Y} \ar [d]^{\beta } \\ F \ar [r]^{ \rho }_{\sim } & G }$

in the $\infty$-category $\operatorname{Fun}(K, \operatorname{\mathcal{C}})$. Using Remark 7.1.2.6, we can reformulate conditions $(1)$ and $(2)$ as follows:

$(1')$

The natural transformation $\alpha$ exhibits $X$ as a limit of $F$.

$(2')$

The natural transformation $\beta$ exhibits $Y$ as a limit of $G$.

Since $\rho$ is an isomorphism, we can use Remark 7.1.1.8 restate $(1')$ as follows:

$(1'')$

The natural transformation $\gamma$ exhibits $X$ as a limit of $G$.

It will therefore suffice to show that any two of the conditions $(1'')$, $(2')$, and $(3)$ imply the third, which is a special case of Remark 7.1.1.9. $\square$