Essential approximate point spectra and Weyl's theorem for operator matrices (Q1772392)

The authors characterize the essential approximate point spectrum and Weyl spectrum of a \(2\times 2\) upper triangular operator matrix \(M_C\), where \(M_C=\left(\begin{smallmatrix} A&C\\ 0&B\end{smallmatrix}\right)\) is an operator acting on the Hilbert space \(\mathcal{H}\oplus \mathcal{K}\). In addition, the authors consider Weyl's theorem, Browder's theorem, a-Weyl's theorem and a-Browder's theorem for \(M_C\).