Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\newcommand\empty{}$
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$

Definition 5.4.1.1. Let $X$ be a simplicial set and let $\sigma : \Delta ^2 \rightarrow X$ be a $2$-simplex of $X$. We will say that $\sigma $ is left-degenerate if it factors through the map $\sigma ^{0}: \Delta ^2 \rightarrow \Delta ^1$ given on vertices by $\sigma ^{0}(0) = 0 = \sigma ^{0}(1)$ and $\sigma ^{0}(2) = 1$ (Notation 1.1.1.9). We say that $\sigma $ is right-degenerate if it factors through the map $\sigma ^{1}: \Delta ^2 \rightarrow \Delta ^1$ given on vertices $\sigma ^{1}(0) = 0$ and $\sigma ^{1}(1) = 1 = \sigma ^{1}(2)$.