Exercise 1.1.2.8. Let $n \geq 0$ be an integer. For $0 \leq j \leq n$, the map $\delta ^{j}: [n-1] \rightarrow [n]$ of Notation 1.1.1.8 determines a map of simplicial sets $\Delta ^{n-1} \rightarrow \Delta ^{n}$ which factors through the simplicial subset $\operatorname{\partial \Delta }^ n \subseteq \Delta ^ n$. We therefore obtain a map of simplicial sets $\Delta ^{n-1} \rightarrow \operatorname{\partial \Delta }^{n}$, which we will also denote by $\delta ^{j}$. Show that, for any simplicial set $S_{\bullet }$, the construction
determines an injective map
whose image is the collection of sequences of $(n-1)$-simplices $(\sigma _0, \sigma _1, \ldots , \sigma _ n)$ satisfying the identities $d_ j(\sigma _ k) = d_{k-1}( \sigma _{j})$ for $0 \leq j < k \leq n$.