Notation 1.1.1.13 (Degeneracy Operators). Let $\operatorname{{\bf \Delta }}_{\operatorname{surj}}$ denote the category whose objects are the linearly ordered sets $[n] = \{ 0 < 1 < \cdots < n \} $ for $n \geq 0$, and whose morphisms are nondecreasing surjective functions $[m] \twoheadrightarrow [n]$. For every pair of integers $0 \leq i \leq n$ we let $\sigma ^{i}_{n}: [n+1] \twoheadrightarrow [n]$ denote the morphism of $\operatorname{{\bf \Delta }}_{\operatorname{surj}}$ given by the formula
If $C_{\bullet }$ is a simplicial object of a category $\operatorname{\mathcal{C}}$, then we can evaluate $C_{\bullet }$ on the morphism $\sigma ^{i}_{n}$ to obtain a morphism from $C_{n}$ to $C_{n+1}$. We will denote this map by $s^{n}_{i}: C_{n} \rightarrow C_{n+1}$ and refer to it as the $i$th degeneracy operator.