Eigenvalues of block matrix of oreder $2n$ related What are the eigenvalues of following block matrix?
$\begin{bmatrix}
A_0 & A_1 \\
A_1 & A_0 \\
\end{bmatrix}_{2n \times 2n}$
Where $A_0$ and $A_1$ are $n \times n$ matrices.

The eigenvalues of matrix
$\begin{bmatrix}
A_0 & A_1 \\
A_1 & A_0 \\
\end{bmatrix}_{2n \times 2n}$ are union of matrices  $A_0+A_1$ and $A_0-A_1$.
