Formula_2627

\[e_{0}=\left\|\begin{array}{ll}1 & 0 \\0 & 1\end{array}\right\|, \quad e_{1}=i\|\| \begin{array}{ll}0 & 1 \\1 & 0\end{array}\left\|, \quad e_{2}=i\right\| \begin{array}{cc}0 & i \\-i & 0\end{array}\left\|, \quad e_{3}=i\right\| \begin{array}{cc}1 & 0 \\0 & -1\end{array} \| .\]