Notation 2.5.5.5. Let $A_{\bullet }$ be a simplicial abelian group. For each $n \geq 0$, let $\mathrm{D}_{n}(A)$ denote the subgroup of $\mathrm{C}_{n}(A) = A_ n$ generated by the images of the degeneracy operators $\{ s^{n-1}_{i}: A_{n-1} \rightarrow A_ n \} _{0 \leq i \leq n-1}$. By convention, we set $\mathrm{D}_{n}(A) = 0$ for $n < 0$.
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$