“…where A is an unbounded operator that is the generator of a C 0 semigroup in the Hilbert space H, B is a bounded operator from another Hilbert space X, and M , N are supposed to be bounded operators. Many problems from physics enter in this framework, let us mention dispersive medium models [32,34,55,43,46,31], the generalized telegraph equations [28,29,44,26], the heat equation with memory effects [20,50,35], and cascades of ODE-hyperbolic systems [30,36], see below for more explanations. Eliminating P by the formula P (t) = e Bt P 0 + t 0 e B(t−s) U (s) ds, we see that the first equation of (1.1) becomes U t = Au + M e Bt P 0 + t 0 M e B(t−s) U (s) ds.…”