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Mohammed Berkani

Publications and source records attributed to Mohammed Berkani.

11 recordsLinked to original sources
arXiv2024

On Left and Right Semi-B-Fredholm Operators

To complete the study of Fredholm type operators of [10] and [11], we define in this paper the classes of left and right semi-B-Fredholm operators (Definition 3.1). Then, we prove that an operator $T \in L(X), X$ being a Banach space, is a left( resp. right) semi-B-Fredholm operator if and only if $T$ is the direct sum of left(resp. right) semi-Fredholm operator and a nilpotent one. This result extend the earlier characterization of B-Fredholm operators as the direct sum of a Fredholm operator and a nilpotent one obtained in [4,Theorem 2.7].

math.FA↗
arXiv2024

On Classes of Fredholm Type Operators

Given an idempotent $p$ in a Banach algebra and following the study in \cite{P50} of p-invertibility, we consider here left p-invertibility, right p-invertibility and p-invertibility in the Calkin Algebra $\mathcal{C}(X),$ where $X$ is a Banach space. Then we define and study left and right generalized Drazin invertibility and we characterize left and right Drazin invertible elements in the Calkin algebra. Globally, this leads to define and characterize the classes of P-Fredholm, pseudo B-Fredholm and weak B-Fredholm operators.

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arXiv2024

Fredholm-type Operators and Index

While in \cite{HB} we studied classes of Fredholm-type operators defined by the homomorphism $Π$ from $L(X)$ onto the Calkin algebra $\mathcal{C}(X)$, $X$ being a Banach space, we study in this paper two classes of Fredholm-type operators defined by the homomorphism $π$ from $L(X)$ onto the algebra $\mathcal{C}_0(X)= L(X)/F_0(X),$ where $F_0(X)$ is the ideal of finite rank operators in $L(X).$ Then we define an index for Fredholm-type operators and we show that this new index satisfies similar properties as the usual Fredholm index.

math.FA↗
arXiv2020

A new approach in index theory

In this paper, we define an analytical index for a continuous family of Fredholm operators parameterized by a topological space $\mathbb{X}$ into a Hilbert space $H,$ as a sequence of integers, extending naturally the usual definition of the index and we prove the homotopy invariance of the index. We give also an extension of the Weyl theorem for normal continuous families and we prove that if $H$ is separable, then the space of B-Fredholm operators on $H$ is path connected.

math.SP↗
arXiv2019

On the B-discrete spectrum

In this paper, we introduce the B-discrete spectrum of an unbounded closed operator and we prove that a closed operator has a purely B-discrete spectrum if and only if it has a meromorphic resolvent. After that, we study the stability of the B-discrete spectrum under several type of perturbations and we establish that two closed invertible linear operators having quasisimilar totally paranormal inverses have equal spectra and B-discrete spectra.

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arXiv2018

Toplogical uniform descent, quasi-Fredholmness and operators originated from semi-B-Fredholm theory

In this paper we study operators originated from semi-B-Fredholm theory and as a consequence we get some results regarding boundaries and connected hulls of the corresponding spectra. In particular, we prove that a bounded linear operator $T$ acting on a Banach space, having topological uniform descent, is a {\bf BR} operator if and only if $0$ is not an accumulation point of the associated spectrum $σ_{\bf R}(T)=\{λ\in\CC:T-λI\notin {\bf R}\}$, where ${\bf R}$ denote any of the following classes: upper semi-Weyl operators, Weyl operators, upper semi-Fredholm operators, Fredholm operators, operators with finite (essential) descent and ${\bf BR}$ the B-regularity associated to ${\bf R}$ as in \cite{P8}. Under the stronger hypothesis of quasi-Fredholmness of $T,$ we obtain a similar characterization for $T$ being a {$\bf BR$} operator for much larger families of sets ${\bf R}.$

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arXiv2018

Pseudo-B-Fredholm operators, poles of the resolvent and mean convergence in the Calkin Algebra

We define here a pseudo B-Fredholm operator as an operator such that 0 is isolated in its essential spectrum, then we prove that an operator $T$ is pseudo- B-Fredholm if and only if $T = R + F$ where $R$ is a Riesz operator and $F$ is a B-Fredholm operator such that the commutator $[R,\, F]$ is compact. Moreover, we prove that 0 is a pole of the resolvent of an operator $T$ in the Calkin algebra if and only if $T= K+F$, where $K$ is a power compact operator and $F$ is a B-Fredholm operator, such that the commutator $[K,\, F]$ is compact. As an application, we characterize the mean convergence in the Calkin algebra.

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arXiv2016

A trace formula for the index of B-Fredholm operators

In this paper we define B-Fredholm elements in a Banach algebra $A$ modulo an ideal $J$ of $A.$ When a trace function is given on the ideal $J,$ it generate an index for B-Fredholm elements. In the case of a B-Fredholm operator $T$ acting on a Banach space, we prove that its usual index $ind(T)$ is equal to the trace of the commutator $ [T, T_0],$ where $T_0$ is a Drazin inverse of $T$ modulo the ideal of finite rank operators, extending a Fedosov's trace formula for Fredholm operators. In the case of a semi-simple Banach algebra, we prove a punctured neighborhood theorem for the index.

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arXiv2013

Abstract Weyl-type Theorems

In this paper, we give a new approach for the study of Weyl-type theorems. Precisely we introduce the concepts of spectral valued and spectral partitioning functions. Using two natural order relations on the set of spectral valued functions, we reduce the question of relationship between Weyl-type theorems to the study of the set difference between the parts of the spectrum that are involved. This study solves completely the question of relationship between two spectral valued functions, comparable for one or the other order relation. Then several known results about Weyl-type theorems becomes corollaries of the results obtained.

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