\sigma = \left( \begin{array}{l}
 \begin{array}{*{20}c}
 {a_1 } & {a_2 } & {a_3 } \\
\end{array} \\ 
 \begin{array}{*{20}c}
 {a_2 } & {a_3 } & {a_1 } \\
\end{array} \\ 
 \end{array} \right)