Mohamed Barraa, Mohamed Boumazgour, A Note on the Spectrum of an Upper Triangular Operator Matrix, Proceedings of the American Mathematical Society, Vol. 131, No. 10 (Oct., 2003), pp. 3083-3088 ...

Abstract. Let 𝓗 and 𝒦 be separable infinite dimensional Hilbert spaces. We denote by 𝑀𝐶 the 2×2 upper triangular operator matrix acting on 𝓗 ⊕ 𝒦 of the form M C =( A C 0 B ) . For given ...