Further improvements of generalized numerical radius inequalities for Hilbert space operators

TL;DR

This paper presents new inequalities and refinements for the $A$-numerical radius and semi-norm of operators on semi-Hilbert spaces, improving existing bounds and relations.

Contribution

It introduces novel inequalities for the $A$-numerical radius and semi-norm, including refinements of the triangle inequality, advancing the theoretical understanding of operator bounds in semi-Hilbert spaces.

Findings

Established new bounds for the $A$-numerical radius involving operator norms.

Proved inequalities relating $A$-adjoint operators and their numerical radii.

Refined the triangle inequality for the $A$-semi-norm.

Abstract

Several new improvements of the $A$-numerical radius inequalities for operators acting on a semi-Hilbert space, i.e., a space generated by a positive operator $A$, are proved. In particular, among other inequalities, we show that \begin{align*} \frac{1}{4}\|T^{\sharp_A} T+TT^{\sharp_A}\|_A \leq\frac{1}{4}\Big(2\omega_A^2(T)+\gamma(T)\Big) \leq \omega_A^2(T), \end{align*} where $$\gamma(T)=\sqrt{\left(\|\Re_A(T)\|_A^2-\|\Im_A(T)\|_A^2\right)^2+4\|\Re_A(T)\Im_A(T)\|_A^2}.$$ Here $\omega_A(X)$ and $\|X\|_A$ denote respectively the $A$-numerical radius and the $A$-seminorm of an operator $X$. Also, $\Re_A(T):=\frac{T+T^{\sharp_A}}{2}$ and $\Im_A(T):=\frac{T-T^{\sharp_A}}{2i}$, where $T^{\sharp_A}$ is a distinguished $A$-adjoint operator of $T$. Further, some new refinements of the triangle inequality related to $\|\cdot\|_A$ are established.