A note on the generalized maximal numerical range of operators

TL;DR

This paper explores new properties of the $A$-maximal numerical range of operators, characterizes $A$-normaloid operators, and introduces improved $A$-numerical radius inequalities, extending recent theoretical results.

Contribution

It provides new characterizations of $A$-normaloid operators and generalizes $A$-numerical radius inequalities, extending prior work by Spitkovsky.

Findings

Characterization of $A$-normaloid operators via $A$-maximal numerical range intersection.

Extension of Spitkovsky's recent results on $A$-normaloid operators.

New inequalities for $A$-numerical radius that improve previous bounds.

Abstract

The paper considers some new properties of the so-called $A$-maximal numerical range of operators, denoted by $W_{\max}^A(\cdot)$, where $A$ is a positive bounded linear operator acting on a complex Hilbert space $\mathcal{H}$. Some characterizations of $A$-normaloid operators are also given. In particular, we extend a recent recent by Spitkovsky in [Oper. Matrices, 13, 3(2019)]. Namely, it is shown that an $A$-bounded linear operator $T$ acting on $\mathcal{H}$ is $A$-normaloid if and only if $W_{\max}^A(T)\cap \partial W_A(T)\neq\varnothing$. Here $\partial W_A(T)$ stands for the boundary of $A$-numerical range of $T$. Some new $A$-numerical radius inequalities generalizing and improving earlier well-known results are also given.