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| Format: | Preprint |
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2026
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| Online Access: | https://arxiv.org/abs/2603.14748 |
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| _version_ | 1866908932468899840 |
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| author | Fu, Siqi Pendleton, Andrew |
| author_facet | Fu, Siqi Pendleton, Andrew |
| contents | 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 $θ$, we show that $M$ is an infinite set if and only if both $r\cosθ$ 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 $r\cosθ$ and $r^2$. In the case when at least one of $r\cosθ$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_14748 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Multiplicities of eigenvalues and quadratic representations of integers Fu, Siqi Pendleton, Andrew Number Theory Spectral Theory 11E25, 35P15 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 $θ$, we show that $M$ is an infinite set if and only if both $r\cosθ$ 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 $r\cosθ$ and $r^2$. In the case when at least one of $r\cosθ$ 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. |
| title | Multiplicities of eigenvalues and quadratic representations of integers |
| topic | Number Theory Spectral Theory 11E25, 35P15 |
| url | https://arxiv.org/abs/2603.14748 |