Multiplicities of eigenvalues and quadratic representations of integers

Abstract

We study the set $M$ of all multiplicities of non-zero eigenvalues for the Laplace operator on a two-dimensional rectangle or torus. We show that for a rectangle with the side length ratio $r$, $M=\mathbb{N}$, the set of all positive integers, if and only if $r^2$ is rational. For a torus whose generating vectors have a length ratio $r$ and the angle between them $\theta$, we show that $M$ is an infinite set if and only if both $r\cos\theta$ and $r^2$ are rational. In this case, $M=2\mathbb{N}$, $4\mathbb{N}$, or $6\mathbb{N}$, and we obtain a characterization for each of these cases in term of $r\cos\theta$ and $r^2$. In the case when at least one of $r\cos\theta$ or $r^2$ is irrational, we show that $M=\{2\}$ or $\{2, 4\}$, and obtain a characterization for these cases. We prove these results by studying the number of integral lattice points on dilated ellipses.