In this paper we study the existence of periodic solutions of the fourth-order equations u − pu′′ − a x u + b x u3 = 0 and u − pu′′ + a x u − b x u3 = 0, where p is a positive constant, and a x and b x are continuous positive 2Lperiodic functions. The boundary value problems P1 and P2 for these equations are considered respectively with the boundary conditions u 0 = u L = u′′ 0 = u′′ L = 0. Exi...

where Ω ⊂ R is a bounded domain with Lipschitz boundary and f : R → R is of class C with f(0) = 0. Thus u0 ≡ 0 is a trivial solution of (D) and we are interested in finding and studying nontrivial solutions. One way of obtaining these is to compare the behavior of f near the origin and near infinity. We shall always assume that f grows subcritically at infinity so that variational methods can b...

We consider the motion of n particles under the action of a potential F . Imposing appropriate conditions on F we obtain quasiperiodic solutions using variational methods. A Diophantine condition on the frequency similar to those encountered in KAM theory allows us to establish the necessary properties of the corresponding functional. The solutions are then obtained by means of the mountain pas...