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Article . 2001
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Norm of a Derivation and Hyponormal Operators

Norm of a derivation and hyponormal operators
Authors: Barraa, Mohamed; Boumazgour, Mohamed;

Norm of a Derivation and Hyponormal Operators

Abstract

Let \(({\mathcal J}, \|\cdot\|_{{\mathcal J}})\) be a two-sided norm ideal in the algebra \({\mathcal L}(H)\) of all bounded linear operators on a complex Hilbert space \(H\). For \(A\in{\mathcal L}(H)\), the inner derivation induced by \(A\) is the operator \(\delta_A\) defined on \({\mathcal L}(H)\) by \(\delta_A(X)= AX-XA\), \(X\in {\mathcal L}(H)\). The restriction \(\delta_{{\mathcal J},A}\) of \(\delta_A\) to \({\mathcal J}\) is a bounded linear operator on \({\mathcal J}\), and \(\|\delta_{{\mathcal J},A}\|\leq\|\delta_A\|=2d(A)\), where \(d(A)=\inf\{\|A-\lambda\|: \lambda\in{\mathbb C}\}\). An operator \(A\in{\mathcal L}(H)\) is called \(S\)-universal if \(\|\delta_{{\mathcal J},A}\|=2d(A)\) for each norm ideal \({\mathcal J}\) in \({\mathcal L}(H)\). \textit{L. A. Fialkow} [Isr. J. Math. 32, 331-348 (1979; Zbl 0434.47006)] showed that a subnormal operator is \(S\)-universal if and only if the diameter of the spectrum is equal to the diameter of the smallest disk containing it. Continuing this investigation, the authors prove that the same conclusion holds true for an arbitrary hyponormal operator.

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Spain
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Keywords

1202.01 Álgebra de Operadores, numerical range, Espacios de Hilbert, Hiponormalidad, 510, spectrum, Derivación, Hyponormality, Hilbert spaces, Linear operators belonging to operator ideals (nuclear, \(p\)-summing, in the Schatten-von Neumann classes, etc.), hyponormal operators, subnormal operators, 12 Matemáticas, inner derivation, Operator algebra, Linear operators, Álgebra de operadores, Operadores lineales, 1202.14 Espacio de Hilbert, Numerical range, numerical radius, Derivation, Hilbert-Schmidt operators, Subnormal operators, hyponormal operators, etc., operator ideals

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
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