Bounding the Frobenius norm of a q-deformed commutator

Abstract

For two $n\times n$ complex matrices $A$ and $B$, we define the $q$-deformed commutator as $[A,B{]}_q:=AB-qBA$ for a real parameter $q$. In this paper, we investigate a generalization of the Böttcher-Wenzel inequality which gives the sharp upper bound of the (Frobenius) norm of the commutator. In our generalisation, we investigate sharp upper bounds on the $q$-deformed commutator. This generalization can be studied in two different scenarios: firstly bounds for general matrices, and secondly for traceless matrices. For both scenarios, partial answers and conjectures are given for positive and negative $q$. In particular, denoting the Frobenius norm by $||.|{|}_F$, when either $A$ or $B$ is normal, we prove the following inequality to be true and sharp: $||[A,B{]}_q|{|}_F^2\le \left(1+q^2\right)||A|{|}_F^2||B|{|}_F^2$ for positive $q$. Also, we conjecture that the same bound is true for positive $q$ when either $A$ or $B$ is traceless. For negative $q$, we conjecture other sharp upper bounds to be true for the generic scenarios and the scenario when either of $A$ or $B$ is traceless. All conjectures are supported with numerics and proved for $n=2$.

A shortcoming in the earlier proof of Proposition 1 has been remedied. A new and more elementary proof is provided for Proposition 3; the older proof is also retained. 18 pages and 4 figures. All comments are welcome