On Perturbations of Linear m-Accetive Operators on Reflexive Banach Spaces. Klaus-J. Engel — 1995 Monatshefte für Mathematik
Polynomial operator matrices as semigroup generators: the 2 x 2 case. Klaus-J. Engel — 1989 Mathematische Annalen
Cauchy Problems for Polynomial Operator Matrices on Abstract Energy Spaces. Klaus-J. Engel; Rainer Nagel — 1990 Forum mathematicum