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Othman Abad

Publications and source records attributed to Othman Abad.

3 recordsLinked to original sources

On the dynamics of Toeplitz operators over Bergman spaces

We investigate the hypercyclicity of Toeplitz operators on the Bergman space $L_{A}^{2}(\mathbb{D})$ with symbols of the form $Ψ(z) = γ\bar{z}+ψ(z)$, where $γ\in \mathbb{C} \setminus \{0\}$ and $ψ$ is analytic on an open neighborhood of the closed unit disc $\overline{\mathbb{D}}$. Our approach bypasses the classical Hardy space techniques (reproducing kernel linear combinations, Nevanlinna factorization) by directly solving the integro-differential resolvent equation arising from the Bergman projection. A key winding number argument shows that for sense-reversing symbols ($|γ| > \sup_{z \in \overline{\mathbb{D}}} |ψ'(z)|$), the symbol's image $Ψ(\mathbb{D})$ is contained in the point spectrum of $T_Ψ$. In the tridiagonal case $Ψ(z) = a\bar{z}+b+cz$, we fully resolve the longstanding eigenvector completeness problem by linking the recurrence coefficients to a rotated Favard spectral measure on the major axis of the symbol's ellipse. Combined with self-commutator positivity, this establishes an unconditional, exact necessary and sufficient characterization of hypercyclicity: $T_Ψ$ is hypercyclic if and only if $|a| > |c|$ and $Ψ(\mathbb{D})$ intersects both the unit disc and its exterior, completely eliminating the $(3+\sqrt{2})$ restriction of previous literature.

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Generalized Drazin-Riesz invertible elements in a semi-simple Banach algebra

We extend the notion of generalized Drazin-Riesz inverse introduced for bounded linear operators in \cite{Ziv} to elements in a complex unital semi-simple Banach algebra. Several characterizations and properties of generalized Drazin-Riesz invertible elements are given. In particular, we extend those of \cite{AbZg1,Djor,Ziv}.

math.FA↗

On the closed generalized Drazin-Riesz invertible operators and $C_{0}$-semigroups

This paper is a continuation of our paper [Med. J. Math 19, Article number: 31 (2022)] in which we extended the notion of generalized Drazin-Riesz invertible operators to closed operators. We establish here, results relating the notion of closed generalized Drazin-Riesz invertibility with the theory of $C_{0}$-semigroups. Firstly, we generalize results obtained in the bounded case [1] to the context of closed operators. Secondly, we investigate when an infinitesimal generator $A$ of a given $C_{0}$-semigroup is closed generalized Drazin-Riesz invertible. An application to $C_{0}$-groups and abstract second order differential equations is proposed, and an example of a $C_{0}$-group with closed generalized Drazin-Riesz invertible infinitesimal generator is given.

math.FA↗