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 ...

Of particular interest are canonical forms which, by definition, have a D-matrix consisting of the nontrivial invariants. Among these is a family of triangular forms, which, besides being canonical, ...