An analog of H\"older's inequality for the spectral radius of Hadamard products

Abstract

We prove new inequalities related to the spectral radius $\rho$ of Hadamard products (denoted by $\circ$) of complex matrices. Let $p,q\in [1,\infty]$ satisfy $\frac{1}{p}+\frac{1}{q}=1$, we show an analog of H\"older's inequality on the space of $n\times n$ complex matrices $$\rho(A\circ B) \le \rho(|A|^{\circ p})^{\frac{1}{p}} \rho(|B|^{\circ q})^{\frac{1}{q}} \quad \text{for all $A,B\in \mathbb{C}^{n\times n}$,} $$ where $|\cdot|$ denotes entry-wise absolute values, and $(\cdot)^{\circ p}$ represents the entry-wise Hadamard power. We derive a sharper inequality for the special case $p=q=2$. Given $A,B\in \mathbb{C}^{n\times n}$, for some $\beta \in (0,1]$ depending on $A$ and $B$, $$\rho(A\circ B) \le \beta \rho(|A\circ A|)^{\frac{1}{2}} \rho(|B\circ B|)^{\frac{1}{2}} .$$ Analysis for another special case $p=1$ and $q=\infty$ is also included.