$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.64
\begin{equation} \begin{gathered}\label{equation:sparse-horn} \xymatrix@R =50pt@C=50pt{ K \ar [r] \ar [d] & \Delta ^{2n} \ar [d]^{ \sigma _ n } \\ \mathcal{R}(n-1) \ar [r] & \mathcal{R}(n). } \end{gathered} \end{equation}