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Proposition Let $\operatorname{Set_{\Delta }}$ denote the category of simplicial sets and let $\operatorname{Set}_{\Delta }^{\leq 1} \subseteq \operatorname{Set_{\Delta }}$ denote the full subcategory spanned by the simplicial sets of dimension $\leq 1$. Then the construction $X_{\bullet } \mapsto \mathrm{Gr}( X_{\bullet } )$ induces an equivalence of categories $\operatorname{Set}_{\Delta }^{\leq 1} \rightarrow \operatorname{Graph}$.

Proof. It follows from Proposition that the functor $X_{\bullet } \mapsto \mathrm{Gr}(X_{\bullet } )$ is fully faithful when restricted to simplicial sets of dimension $\leq 1$. It will therefore suffice to show that it is essentially surjective. Let $G$ be any directed graph, and form a pushout diagram of simplicial sets

\[ \xymatrix { \underset { e \in \operatorname{Edge}( G )}{\coprod } \operatorname{\partial \Delta }^{1} \ar [r] \ar [d]^{(s,t)} & \underset { e \in \operatorname{Edge}( G)}{\coprod } \Delta ^{1} \ar [d] \\ \underset { v \in \operatorname{Vert}( G )}{\coprod } \Delta ^{0} \ar [r] & X_{\bullet }. } \]

Then $X_{\bullet }$ is a simplicial set of dimension $\leq 1$, and the directed graph $\mathrm{Gr}( X_{\bullet } )$ is isomorphic to $G$. $\square$