Causality theory and multiplicative transformators

The concept of a multiplicative transformator formalized by Brodskii, Gokhberg, and Krein in a by now classical 1967 paper can play in causality theory a role similar to that played by the more familiar additive transformator concept. Among its advantages, one finds an alternative point of view (a system cascade connection representation in lieu of a parallel connection), a more compact formulation of some proofs and results, a natural tool for a generalization of these results into a nonlinear setting.