On resonant Neumann problems

Abstract

We consider semilinear Neumann problems at resonance and prove existence and multiplicity theorems. The existence theorems allow resonance with respect to any eigenvalue of the negative Neumann Laplacian. The multiplicity theorems concern problems resonant at 0 (the principal eigenvalue) or at the first nonzero eigenvalue. Our approach uses tools from critical point theory and from Morse theory.