# Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$

Construction 4.3.5.1 (Slice Simplicial Sets). Let $f: K \rightarrow X$ be a morphism of simplicial sets. We define a simplicial set $X_{/f}$ as follows:

• For each $n \geq 0$, an $n$-simplex of $X_{/f}$ is a map of simplicial sets $\overline{f}: \Delta ^ n \star K \rightarrow X$ satisfying $\overline{f}|_{K} = f$.

• For every nondecreasing function $\alpha : [m] \rightarrow [n]$ in $\operatorname{{\bf \Delta }}$, the associated map

$\alpha ^{\ast }: \{ \textnormal{n-simplices of X_{/f}} \} \rightarrow \{ \textnormal{m-Simplices of X_{/f}} \}$

is carries an $n$-simplex $\overline{f}: \Delta ^ n \star K \rightarrow X$ to the composite map

$\Delta ^{m} \star K \xrightarrow { \operatorname{id}_{K} \star \alpha } \Delta ^{n} \star K \xrightarrow { \overline{f} } X.$

We will refer to $X_{/f}$ as the slice simplicial set of $X$ over $f$.