On Perturbations of Generators of $C_0$-Semigroups
  We present a perturbation result for generators of $C_0$-semigroups which can be considered as an operator theoretic version of the Weiss-Staffans perturbation theorem for abstract linear systems. The result are illustrated by applications to the Desch-Schappacher, the Miyadera Voigt perturbation theorems, and to unbounded perturbations of the boundary conditions of a generator. 
M. Adler, M. Bombieri and K.-J. Engel
math.FA