A note on the spectrum of an upper triangular operator matrix

摘要

Let M-C= [(A)(0) (C)(B)] be a 2x2 upper triangular operator matrix acting on the Banach space Ecircle plusF. We investigate the set of the operators C for which sigma(M-C)=sigma(A) boolean OR sigma(B), where sigma(.) denotes the spectrum. [References: 4]