Spectral properties for polynomial and matrix operators involving demicompactness classes

Abstract

The first aim of this paper is to show that a polynomially demicompact operator satisfying certain conditions is demicompact. Furthermore, we give a refinement of the Schmoëger and the Rakocević essential spectra of a closed linear operator involving the class of demicompact ones. The second aim of this work is devoted to provide some sufficient conditions on the inputs of a closable block operator matrix to ensure the demicompactness of its closure. An example involving the Caputo derivative of fractional of order α is provided. Moreover, a study of the essential spectra and an investigation of some perturbation results.

Keywords

• Matrix operator
• Demicompact linear operator
• Fredholm and semi-Fredholm operators
• Perturbation theory
• Essential spectra