Spectral theory for structured perturbations of linear operators
  We characterize the spectrum (and its parts) of operators which can be represented as G=A+BC for a simpler operator A and a structured perturbation BC. The interest in this kind of perturbations is motivated, e.g., by perturbations of the domain of an operator A but also arises in the theory of closed-loop systems in control theory. In many cases our results yield the spectral values of G as zeros of a "characteristic equation". 
Martin Adler, Klaus-Jochen Engel
math.SP