$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.56
\begin{equation} \label{equation:pinched-morphism-in-dg-nerve} \partial f_{I} = \begin{cases} 0 & \textnormal{ if } k=0 \\ \sum _{a=0}^{k} (-1)^{a+1} f_{I \setminus \{ i_ a\} } & \textnormal{ if } k > 0. \end{cases} \end{equation}