Eigenvalues of the sum of a matrix such that all its principal submatrices are stable, and a diagonal matrix with non-positive entries I have a real square matrix $A$ (not necessarily symmetric) with all its principal submatrices (including $A$) having eigenvalues with a negative real part.
On the other hand, I have a matrix $D$ which is diagonal and has only non-positive elements in its diagonal. I think that this implies that $A+D$ must also have eigenvalues with a negative real part. Does anyone know how to prove this, or if this is even true?
I have read some things about this in this link:
Principal minors of sum of a matrix and a diagonal matrix
That implies that the odd (resp. even) minors of $A+D$ will be negative (resp. positive). However, this doesn't prove my assertion.
Until now, I have only found the fact that $A+D$ cannot have a real positive eigenvalue, but could it have a complex-conjugate pair of eigenvalues with a positive real part?
EDIT: It is worth noticing that the condition on all principal submatrices to be stable is a requirement for the claim to be (possibly) true. In fact, if we only consider that $A$ is stable but we allow that some of its submatrices have eigenvalues with a positive real part, then the statement clearly does not hold. For instance, take
$$A=\begin{pmatrix}
1 & -3
\\
1 & -2
\end{pmatrix}.$$
This matrix is clearly stable. Nevertheless, if we consider the diagonal matrix
$$D=\begin{pmatrix}
0 & 0
\\
0 & -3
\end{pmatrix},$$
then
$$A+D=\begin{pmatrix}
1 & -3
\\
1 & -5
\end{pmatrix},$$
which is not a stable matrix. Notice that, in this case, $A$ has one principal submatrix with a positive real eigenvalue.
 A: By going over Nonnegative Matrices in the Mathematical Sciences
by Abraham Berman & Robert J. Plemmons, if we add that $A \in \{ B = (b_{ij}) \in \mathbb{R}^{n \times n}: b_{ij}>=0 \text{ for } i \neq j  \}$, then this is an equivalence. It might hold for a larger class of matrices, but I could not find any proof that holds for the broader case.
Your description that "I have a real square matrix $A$ (not necessarily symmetric) with all its principal submatrices (including $A$) having eigenvalues with a negative real part." Leads to $-A$ having all principal minor being positive (A1 in the book).
If you go over [Berman & Plemmons, Ch.6 M-matrices] you will find that the description above means that $-A$ is a non-singular M-matrix. Which is also equivalent to existing a positive diagonal matrix $P$ such that
$ -AP-PA^\top \succ 0 $ (H24 in the book.)
From there, given any $D$ which is diagonal and has only non-positive elements, since both $-D$ and $P$ matrices are non-negative diagonal matrices then $-PD \succeq0$, we have that
$$ (A+D)P+P(A+D)^\top = AP+PA^\top + 2DP \prec 2DP \preceq 0.$$
Therefore $A+D$ is Hurwitz stable.
