Is there a procedure to find the eigenvalues of $\textbf{M}$? ‎ $$\begin{eqnarray} ‎\textbf{M}=\left[‎ ‎\begin {array}{ccccc}‎ ‎\textbf{A} & \textbf{B} & & &\\‎ ‎\textbf{B}^T & \textbf{ A} & \textbf{B} & &\\‎ ‎&\ddots &\ddots & \ddots &\\‎ ‎& & & & \textbf{B} \\‎ ‎& & & \textbf{B}^T & \textbf{A} ‎\end {array}‎ ‎\right]‎, ‎\end{eqnarray}‎‎ $$

where $\textbf{B}^T$ is transpose of matrix $\textbf{B}$.