Variational methods for non-local operators of elliptic type

In this paper we study the existence of non-trivial solutions for equations driven by a non-local integrodifferential operator LK with homogeneous Dirichlet boundary conditions. More precisely, we consider the problem {LKu + \u3bbu + f(x, u) = 0 in \u3a9 u = 0 in \u211dn\\u3a9, where \u3bb is a real parameter and the nonlinear term f satisfies superlinear and subcritical growth conditions at zero and at infinity. This equation has a variational nature, and so its solutions can be found as critical points of the energy functional J\u3bb associated to the problem. Here we get such critical points using both the Mountain Pass Theorem and the Linking Theorem, respectively when \u3bb < \u3bb1 and \u3bb 65 \u3bb1, where \u3bb1 denotes the first eigenvalue of the operator-LK. As a particular case, we derive an existence theorem for the following equation driven by the fractional Laplacian {(-\u3b4)su - \u3bbu = f(x, u) in \u3a9 u = 0 in \u211dn\\u3a9. Thus, the results presented here may be seen as the extension of some classical nonlinear analysis theorems to the case of fractional operators