Szeg\H{o}-Weinberger type inequalities for symmetric domains with holes

TL;DR

This paper extends classical inequalities for the first positive Neumann Laplacian eigenvalue to symmetric domains with holes, providing sharper bounds and higher eigenvalue inequalities under symmetry assumptions, with counterexamples and domain properties discussed.

Contribution

It refines Szeg\

Findings

Eigenvalue bounds for symmetric domains with holes are established.

Higher eigenvalue inequalities are proved under additional symmetry assumptions.

Counterexamples demonstrate limitations outside symmetry classes.

Abstract

Let $\mu_2(\Omega)$ be the first positive eigenvalue of the Neumann Laplacian in a bounded domain $\Omega\subset\mathbb{R}^N$. It was proved by Szeg\H{o} for $N=2$ and by Weinberger for $N \geq 2$ that among all equimeasurable domains $\mu_2(\Omega)$ attains its global maximum if $\Omega$ is a ball. In the present work, we develop the approach of Weinberger in two directions. Firstly, we refine the Szeg\H{o}-Weinberger result for a class of domains of the form $\Omega_{\text{out}}\setminus\overline{\Omega}_{\text{in}}$ which are either centrally symmetric or symmetric of order $2$ (with respect to every coordinate plane $(x_i,x_j)$) by showing that $\mu_{2}(\Omega_{\text{out}}\setminus\overline{\Omega}_{\text{in}})\leq\mu_2(B_\beta\setminus\overline{B}_\alpha)$, where $B_\alpha, B_\beta$ are balls centered at the origin such that $B_\alpha\subset\Omega_{\text{in}}$ and $|\Omega_{\text{out}}\setminus\overline{\Omega}_{\text{in}}|=|B_\beta\setminus\overline{B}_\alpha|$. Secondly, we provide Szeg\H{o}-Weinberger type inequalities for higher eigenvalues by imposing additional symmetry assumptions on the domain. Namely, if $\Omega_{\text{out}}\setminus\overline{\Omega}_{\text{in}}$ is symmetric of order $4$, then we prove $\mu_{i}(\Omega_{\text{out}}\setminus\overline{\Omega}_{\text{in}})\leq\mu_i(B_\beta\setminus\overline{B}_\alpha)$ for $i=3,\dots,N+2$, where we also allow $\Omega_{\text{in}}$ and $B_\alpha$ to be empty. If $N=2$ and the domain is symmetric of order $8$, then the latter inequality persists for $i=5$. Counterexamples to the obtained inequalities for domains outside of the considered symmetry classes are given. The existence and properties of nonradial domains with required symmetries in higher dimensions are discussed. As an auxiliary result, we obtain the non-radiality of the eigenfunctions associated to $\mu_{N+2}(B_\beta\setminus\overline{B}_\alpha)$.