$A$-Davis-Wielandt Radius Bounds of Semi-Hilbertian Space Operators

TL;DR

This paper introduces new bounds for the $A$-Davis-Wielandt radius of operators on semi-Hilbertian spaces, refining existing estimates and extending results to certain block matrices.

Contribution

It provides novel bounds for the $A$-Davis-Wielandt radius, improving upon previous bounds and applying to off-diagonal block matrices in semi-Hilbertian spaces.

Findings

New bounds for $d\omega_A(S)$ are established.

Bounds for $2\times 2$ off-diagonal block matrices are derived.

Results refine and extend existing radius bounds.

Abstract

Consider $\mathcal{H}$ is a complex Hilbert space and $A$ is a positive operator on $\mathcal{H}.$ The mapping $\langle\cdot,\cdot\rangle_A: \mathcal{H}\times \mathcal{H} \to \mathbb {C}$, defined as $\left\langle y,z\right\rangle_{A}=\left\langle Ay,z\right\rangle $ for all $y,z$ $\in $ ${\mathcal{H}}$, induces a seminorm $ \left\Vert \cdot\right\Vert_{A}$. The $A$-Davis-Wielandt radius of an operator $S$ on $\mathcal{H}$ is defined as $d\omega_{A}\left( S\right) =\sup \left\{ \sqrt{\left\vert \left\langle Sz,z\right\rangle_{A}\right\vert ^{2}+\left\Vert Sz\right\Vert_{A}^{4}} :\left\Vert z\right\Vert_{A}=1\right\} \text{.} $ We investigate some new bounds for $d\omega_{A}\left( S\right)$ which refine the existing bounds. We also give some bounds for the $2\times 2$ off-diagonal block matrices.