Matrix Inequalities between $f(A)\sigma f(B)$ and $A\sigma B$

Abstract

Let $A$ and $B$ be $n\times n$ positive definite complex matrices, let $\sigma$ be a matrix mean, and let $f:[0,\infty )\to [0,\infty )$ be a differentiable convex function with $f(0)=0$. We prove that $$f^{\prime }(0)(A\sigma B)\le \frac{f(m)}{m}(A\sigma B)\le f(A)\sigma f(B)\le \frac{f(M)}{M}(A\sigma B)\le f^{\prime }(M)(A\sigma B),$$ where $m$ represents the smallest eigenvalues of $A$ and $B$ and $M$ represents the largest eigenvalues of $A$ and $B$. If $f$ is differentiable and concave, then the reverse inequalities hold. We use our result to improve some known subadditivity inequalities involving unitarily invariant norms under certain mild conditions. In particular, if $f(x)/x$ is increasing, then $$|||f(A)+f(B)|||\le \frac{f(M)}{M}|||A+B|||\le |||f(A+B)|||$$ holds for all $A$ and $B$ with $M\le A+B$. Furthermore, we apply our results to explore some related inequalities. As an application, we present a generalization of Minkowski's determinant inequality.equality.

to appear in Aequationes Mathematicae