身份矩阵

例子：

$\left[\begin{array}{cc}1& 0.\\ 0.& 1\end{array}\right]\cdot \left[\begin{array}{rr}\hfill 4.& \hfill 7.\\ \hfill -1& \hfill 3.\end{array}\right]=\left[\begin{array}{cc}1（4.）+0.（-1）& 1（7.）+0.（3.）\\ 0.（4.）+1（-1）& 0.（7.）+1（3.）\end{array}\right]$

$=\left[\begin{array}{rr}\hfill 4.& \hfill 7.\\ \hfill -1& \hfill 3.\end{array}\right]$