# Bound spectral radius after multiplied by a zero-row-sum matrix

I'm trying to bound the spectral radius of a matrix product. The first matrix is $$H = (I-M)^{-1}\Gamma,$$ where $$\sum_jM_{ij}<1$$, $$M_{ij}\geq0$$, and $$\Gamma$$ is a diagonal matrix with non-negative entries (when $$\Gamma$$ is a zero matrix then the question is trivial, so let’s focus on the case when $$\Gamma$$ has at least one non-zero entry).

Then the target matrix is $$KH$$. Matrix $$K=P-Q$$ is the difference of two right stochastic matrices (a right stochastic matrix has each row sums to 1 and all elements are non-negative), naturally |$$K_{ij}| \leq 1$$ and $$\sum_{j}K_{ij} = 0$$. In addition, $$Q=\vec{1}\vec{q}'$$ is a rank-1 matrix. That implies $$|K_{ij}-K_{kj}|\leq1$$.

The hypothesis is

$$\rho(KH) = \rho(HK) \leq \rho(H).$$

Alternatively, a bound provided by the norm of H would also be good. The corresponding conjecture is $$\rho(KH)(=\rho(HK))\leq||H||.$$

Does anybody have an idea of how this could be proved or whether this is right? Thanks in advance.

So far I've proved

$$\rho(HK) \leq 2 \|H\|_\infty$$

by the following reasoning: ($$\|\cdot\|_\infty$$ provides an upper bound of the spectral radius of a matrix, according to Gershgorin Disk Theorem.) \begin{align} \rho(HK) & \leq \|HK\|_\infty\\ &=\max_j\sum_i|HK_{ji}|\\ &=\max_j\sum_i|\Sigma_kH_{jk}K_{ki}| \\ &\leq \max_j\sum_i\Sigma_k|K_{ki}||H_{jk}|\\ &\leq \max_j2\sum_k|H_{jk}|\\ &=2\|H\|_\infty \end{align}

Actually any bound tighter than this would be great to see!

• Welcome to MSE. Nice first question! Jun 27 '20 at 8:12

Do you think the result depends on the properties of the matrix $$M$$? I ask because the result is related to $$H$$ and not to $$M$$. In fact, the result you have already proven is in terms of $$H$$, not $$M$$. Or perhaps you think that it is only when $$H$$ satisfies these constraints (i.e., that the matrix $$M$$ associated with $$H$$ satisfies $$\sum_j M_{ij}<1$$ and $$𝑀_{𝑖𝑗}\geq0$$) that you can lower the $$2$$ to a $$1$$.
Now, if it is the case that one needs to use the structure of the underlying $$M$$ matrix, then perhaps using the expansion $$(I-M)^{-1} = I + M + M^2 ...$$ could help.
• I'm framing the question based on properties of $H$ since the conditions on H seems easier to be deducted and can be converted into conditions on $M$ once we specify them, for instance via the method you pointed to. We're only interested in H such that the associated $M$ satisfies $M_{ij}\geq0$; $\sum_jM_{ij}<1$ guarantees the existence of H (by guaranteeing $I-M$ is invertible). Jun 28 '20 at 7:29
We have $$\rho(KH)\leq ||(KH)^{n}||_{\infty}^{1/n},\forall n$$ and $$\rho(KH)=\lim_{n\rightarrow \infty} ||(KH)^{n}||_{\infty}^{1/n},\forall n$$ by the spectral radius formula. Examining the norms of the powers of KH, or just starting with KHK, seems like it might be helpful.