Fourth-order elliptic problems involving concave-superlinear nonlinearities

The existence of solutions for a huge class of superlinear elliptic problems involving fourth-order elliptic problems defined on bounded domains
under Navier boundary conditions is established. To this end we do not apply the well-known
Ambrosetti-Rabinowitz condition. Instead, we assume that the nonlinear term
is nonquadratic at infinity. Furthermore, the nonlinear term is a concave-superlinear function which can be indefinite in sign. 
In order to apply variational methods we employ some delicate arguments recovering some kind of compactness.