Almost-periodic solutions of second-order differential equations (Q1111745)

We give the fundamental properties of the infinitesimal generator A that corresponds to an almost-periodic cosine operator-function C(t). The cosine operator-function is related with an abstract linear second-order differential equation in the same way as the strongly continuous operator semigroups are related with an abstract first-order linear differential equation. Therefore, we may investigate the almost-periodicity property of the solution in terms of the infinitesimal generating operator.