Multiple solutions for nonlinear Neumann problems driven by a nonhomogeneous differential operator.

We consider a nonlinear Neumann problem driven by a nonhomogeneous quasilinear degenerate elliptic differential operator $ \operatorname{div} a(x,\nabla u)$-Laplacian. The reaction term is a Carathéodory function $ f(x,s)$. Using variational methods based on the mountain pass and second deformation theorems, together with truncation and minimization techniques, we show that the problem has three nontrivial smooth solutions, two of which have constant sign (one positive, the other negative). A crucial tool in our analysis is a result of independent interest which we prove here and which relates $ W^{1,p}$ local minimizers of a $ C^1$ $ \operatorname{div} a(x,\nabla u)$ [ABSTRACT FROM AUTHOR]