An extension of Birkhoff--James orthogonality relations in semi-Hilbertian space operators

TL;DR

This paper extends Birkhoff--James orthogonality relations in semi-Hilbertian space operators by introducing a family of seminorms that interpolate between the $A$-numerical radius and the $A$-operator seminorm, generalizing classical results.

Contribution

It introduces a new family of seminorms parametrized by rom 0 to 1, providing a unified framework for orthogonality relations in semi-Hilbertian spaces, extending classical operator theory.

Findings

Characterization of Birkhoff--James orthogonality for the new seminorms.

Interpolation of seminorms between $A$-numerical radius and $A$-operator seminorm.

Special case reduction to classical operator norm and numerical radius when $A=I$.

Abstract

Let $\mathbb{B}(\mathcal{H})$ denote the $C^{\ast}$-algebra of all bounded linear operators on a Hilbert space $\big(\mathcal{H}, \langle\cdot, \cdot\rangle\big)$. Given a positive operator $A\in\B(\h)$, and a number $\lambda\in [0,1]$, a seminorm ${\|\cdot\|}_{(A,\lambda)}$ is defined on the set $\B_{A^{1/2}}(\h)$ of all operators in $\B(\h)$ having an $A^{1/2}$-adjoint. The seminorm ${\|\cdot\|}_{(A,\lambda)}$ is a combination of the sesquilinear form ${\langle \cdot, \cdot\rangle}_A$ and its induced seminorm ${\|\cdot\|}_A$. A characterization of Birkhoff--James orthogonality for operators with respect to the discussed seminorm is given. Moving $\lambda$ along the interval $[0,1]$, a wide spectrum of seminorms are obtained, having the $A$-numerical radius $w_A(\cdot)$ at the beginning (associated with $\lambda=0$) and the $A$-operator seminorm ${\|\cdot\|}_A$ at the end (associated with $\lambda=1$). Moreover, if $A=I$ the identity operator, the classical operator norm and numerical radius are obtained. Therefore, the results in this paper are significant extensions and generalizations of known results in this area.