[行列解析2.4.p6]

A =
\begin{pmatrix}
1 & 0 & 0 \\
0 & 2 & 0 \\
0 & 0 & 3
\end{pmatrix}, \quad
B =
\begin{pmatrix}
-2 & 1 & 2 \\
-1 & -2 & -1 \\
1 & 1 & 1
\end{pmatrix}

\sigma(aA + bB) = \{ a - 2b, \; 2a - 2b, \; 3a + b \}