Consider the block matrix $M = \begin{bmatrix} A & B \\ C & D \end{bmatrix}$ of order $(m+n) \times (m+n)$. I block matrix
$$M = \begin{bmatrix} A & B \\ C & D \end{bmatrix}$$
I need references where they are talking about the relation between the eigenvalues of $M$ and the eigenvalues of $A$ and $D$. I want to learn under what circumstances such a relation exists. The simplest of such a possibility is $B=0$ or $C=0$. I believe there are other non-trivial cases. Kindly share some references. Thank you.