Solution
= Solution
The <characteristic polynomial> is
$$
\det\begin{pmatrix}-\lambda&a\\a&-\lambda\end{pmatrix}=\lambda^2-a^2.
$$
Its roots give the <eigenvalues>
$$
\boxed{\lambda=a,\quad\lambda=-a}.
$$
= Solution
The <characteristic polynomial> is
$$
\det\begin{pmatrix}-\lambda&a\\a&-\lambda\end{pmatrix}=\lambda^2-a^2.
$$
Its roots give the <eigenvalues>
$$
\boxed{\lambda=a,\quad\lambda=-a}.
$$