[行列解析1.4.p16]

A =
\begin{pmatrix}
a & b & & & 0 \\
c & a & b & & \\
& \ddots & \ddots & \ddots & \\
& & \ddots & a & b \\
0 & & & c & a
\end{pmatrix} \in M_n, \quad bc \neq 0

\sigma(A) = \{\, a + 2 \sqrt{bc} \cos\frac{\pi \kappa}{n+1} : \kappa = 1, \dots, n \,\}