Almost periodic solutions of nonlinear second order differential equations (Q1193220)

Existence results are proved for almost periodic solutions of nonlinear second order differential equations of the form \(x''=f(t,x)\). The results extend existence results for periodic solutions of periodic equations, where the existence of periodic sub- and supersolutions implied the existence of periodic solutions. The main condition on \(f\) is that it should be quasimonotone increasing.