Some hybrid matrix triangle inequalities
A recent result due to Teng Zhang compares the sum of $m$ matrices and the sum of their quadratic symmetric moduli: $$ \left\| \sum_{k=1}^m A_k\right\| \le \sqrt{2} \left\| \sum_{k=1}^m |A_k|_{\qsym}\right\| $$ for every unitarily invariant norm. Here $|A|_{\qsym}$ is the quadratic mean of $|A|$ and $|A^*|$. We derive operator and eigenvalue refinements of Zhang's inequality from a new polar decomposition for the quadratic symmetric modulus. For instance, $$ \left| \sum_{k=1}^m A_k\right| \le \frac{\sqrt{2}}{2} \left\{ \sum_{k=1}^m \left(|A_k|_{\qsym}+V|A_k|_{\qsym}V^*\right)\right\} $$ for some unitary matrix $V$. We also establish the polar decomposition for the maximal modulus associated with Olson's order, and derive, as in the quadratic case, a series of estimates.