A nonvariational Neumann problem for the Helmholtz equation

Abstract

We consider a bounded open subset $\Omega$ of ${\mathbb{R}}^n$ of class $C^{1,\alpha}$ for some $\alpha\in]0,1[$ and we solve the Neumann problem for the Helmholtz equation both in $\Omega$ and in the exterior of $\Omega$. We look for solutions in the space for $\alpha$-H\"{o}lder continuous functions that may not have a classical normal derivative at the boundary points of $\Omega$ and that may have an infinite Dirichlet integral around the boundary of $\Omega$. Namely for solutions that do not belong to the classical variational setting.