# Neumann eigenvalue problems on the exterior domains

## TL;DR

This paper investigates the spectrum of weighted Neumann eigenvalue problems involving the p-Laplace operator on exterior domains, establishing existence, unboundedness, and properties of eigenvalues under various function space conditions.

## Contribution

It proves the existence of an unbounded sequence of positive eigenvalues, including a unique principal eigenvalue, for weighted Neumann problems on exterior domains, using compact embedding techniques.

## Key findings

- Existence of an unbounded sequence of eigenvalues.
- Identification of a unique principal eigenvalue.
- Compact embeddings of Sobolev spaces into weighted Lebesgue spaces.

## Abstract

For $ p\in (1, \infty)$, we consider the following weighted Neumann eigenvalue problem on $B_1^c$, the exterior of the closed unit ball in $R^N$: \begin{equation}\label{Neumann eqn} \begin{aligned} -\Delta_p \phi & = \lambda g |\phi|^{p-2} \phi \ \text{in}\ B^c_1, \\ \displaystyle\frac{\partial \phi}{\partial \nu} &= 0 \ \text{on} \ \partial B_1, \end{aligned} \end{equation} where $\Delta_p$ is the $p$-Laplace operator and $g \in L^1_{loc}(B^c_1)$ is an indefinite weight function. Depending on the values of $p$ and the dimension $N$, we take $g$ in certain Lorentz spaces or weighted Lebesgue spaces and show that the above eigenvalue problem admits an unbounded sequence of positive eigenvalues that includes a unique principal eigenvalue. For this purpose, we establish the compact embeddings of $W^{1,p}(B^c_1)$ into $L^p(B^c_1, |g|)$ for $g$ in certain weighted Lebesgue spaces. For $N>p$, we also provide an alternate proof for the embedding of $W^{1,p}(B^c_1)$ into $L^{p^*,p}(B^c_1)$. Further, we show that the set of all eigenvalues is closed.