Least energy solutions for a quasilinear Neumann problem

Abstract

We study the existence of nonconstant weak solutions and the qualitative behavior of the least energy level for a class of quasilinear elliptic problems with Neumann boundary conditions of the form \[ -\sum_{i,j=1}^N D_{x_j}(a_{ij}(x,u)D_{x_i}u) +\frac12\sum_{i,j=1}^N D_s a_{ij}(x,u)D_{x_i}uD_{x_j}u =g(u)-λu \quad \text{in }Ω. \] Since the associated energy functional is neither of class $C^1$ nor locally Lipschitz, the classical Nehari manifold method cannot be applied directly. We therefore rely on variational techniques combined with nonsmooth critical point theory. For $λ>0$, we prove the existence of least energy solutions and show that, under suitable assumptions, they are nonconstant for large $λ$ and constant for sufficiently small $λ$. Moreover, the least energy level tends to $+\infty$ as $λ\to+\infty$. In the indefinite regime, a key ingredient is a spectral decomposition of $H^1(Ω)$ associated with the linear operator \[ \mathcal L_0u :=-\sum_{i,j=1}^N D_{x_j}\bigl(a_{ij}(x,0)D_{x_i}u\bigr). \]