Resumen de Upper triangular operator matrices and limit points of the essential spectrum

Mohammed Karmouni, Abdelaziz Tajmouati, Abdeslam El Bakkali

• In this paper, we investigate the limit points set of essential spectrum of upper triangular operator matrices We prove that accσe(MC) ∪ W = accσe(A) ∪ accσe(B) where W is the union of certain holes in accσe(MC), which happen to be subsets of accσe(B) ∩ accσe(A). Also, several sufficient conditions for accσe(MC) = accσe(A) ∪ accσe(B) holds are given.