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Structure of n-quasi left m-invertible and related classes of operators

Duggal Bhagwati Prashad, Kim In Hyun

Demonstratio Mathematica

2020

Vol. 53, nr 1

249--268

Given Hilbert space operators T, S ∈ B(H), let Δ and δ ∈ B(B (H)) denote the elementary operators ΔT,S(X) = (LT RS − I) (X) = TXS - X and δT,S(X) = (LT – RS)(X) = TX - XS. Let d = Δ or δ. Assuming T commutes with S∗, and choosing X to be the positive operator S∗nSn for some positive integer n, this paper exploits properties of elementary operators to study the structure of n-quasi [m, d]-operators dm T,S (X) = 0 to bring together, and improve upon, extant results for a number of classes of operators, such as n-quasi left m-invertible operators, n-quasi m-isometric operators, n-quasi m-self-adjoint operators and n-quasi (m, C) symmetric operators (for some conjugation C of H). It is proved that Sn is the perturbation by a nilpotent of the direct sum of an operator Sn1 = (…)n satisfying dmT1S1(I1) = 0 , T1 = (…) , with the 0 operator; if S is also left invertible, then Sn is similar to an operator B such that dmB∗,B(I) = 0. For power bounded S and T such that ST∗ - T∗S = 0 and ΔTS(S∗nSn) = 0, S is polaroid (i.e., isolated points of the spectrum are poles). The product property, and the perturbation by a commuting nilpotent property, of operators T, S satisfying dmT,S (I) = 0, given certain commutativity properties, transfers to operators satisfying S∗ndmT,S (I)Sn = 0.

Generalizations of Kaplansky's Theorem Involving Unbounded Linear Operators

Benali A., Mortad M. H.

Bulletin of the Polish Academy of Sciences. Mathematics

2014

Vol. 62, no 2

181--186

We are mainly concerned with the result of Kaplansky on the composition of two normal operators in the case in which at least one of the operators is unbounded.

On the closedness, the self-adjointness and the normality of the product of two unbounded operators

Mortad M. H.

Demonstratio Mathematica

2012

Vol. 45, nr 1

161-167

Let A and B be two unbounded densely defined operators on a Hilbert space H . The purpose of this work is to give simple conditions that make the product AB closed, self-adjoint and normal provided the two operators are so.