Lemma 14.3.2. Let $\mathcal{C}$ be a category.

Given a simplicial object $U$ in $\mathcal{C}$ we obtain a sequence of objects $U_ n = U([n])$ endowed with the morphisms $d^ n_ j = U(\delta ^ n_ j) : U_ n \to U_{n-1}$ and $s^ n_ j = U(\sigma ^ n_ j) : U_ n \to U_{n + 1}$. These morphisms satisfy the opposites of the relations displayed in Lemma 14.2.3.

Conversely, given a sequence of objects $U_ n$ and morphisms $d^ n_ j$, $s^ n_ j$ satisfying these relations there exists a unique simplicial object $U$ in $\mathcal{C}$ such that $U_ n = U([n])$, $d^ n_ j = U(\delta ^ n_ j)$, and $s^ n_ j = U(\sigma ^ n_ j)$.

A morphism between simplicial objects $U$ and $U'$ is given by a family of morphisms $U_ n \to U'_ n$ commuting with the morphisms $d^ n_ j$ and $s^ n_ j$.

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