定义四个2阶单位矩阵如下：E11 = &#92；(&#92；begin{bmatrix} 1 & 0 &#92；&#92；0 & 1 &#92；end{bmatrix}&#92；)。E12 = &#92；(&#92；begin{bmatrix} 0 & 1 &#92；&#92；0 & 0 &#92；end{bmatrix}&#92；)。E21 = &#92；(&#92；begin{bmatrix} 0 & 0 &#92；&#92；1 & 0 &#92；end{bmatrix}&#92；)。E22 = &#92；(&#92；begin{bmatrix} 0 & 0 &#92；&#92；0 & 1 &#92；end{bmatrix}&#92；)。