Counterexamples to a conjectured Schatten norm inequality

Abstract

For $p\ge 1$ and $m\ge 2$, let $c_p(m)$ be the least constant such that \[ \left\|\sum_{k=1}^m A_k\right\|_p \leq c_p(m)\left\|\sum_{k=1}^m |A_k|\right\|_p \] for all square complex matrices. Tang and Zhang recently conjectured a closed formula for $c_p(m)$, obtained from a common-left equiangular rank-one family. We disprove that formula for every $m\ge 2$ and every $1<p<2$.

5 pages