English

Generic failure of uniform separation in planar Dirichlet spectra

Analysis of PDEs 2026-07-28 v1 Spectral Theory

Abstract

Can a bounded planar domain have a simple Dirichlet spectrum with uniformly separated consecutive eigenvalues? Dimension two is critical: Weyl's law permits both uniform separation and arbitrarily small gaps. We prove that uniform separation is nevertheless exceptional in a natural rough-domain setting, even after multiplicities are removed. Let DR2D\subset\mathbb{R}^2 be a bounded domain and, for 1\ell\geq 1, let C(D)\mathcal{C}_\ell(D) be the space of nonempty connected open sets ΩD\Omega\subset D such that DΩ\overline{D}\setminus\Omega has at most \ell connected components, endowed with the complementary-Hausdorff topology. We prove that C(D)\mathcal{C}_\ell(D) is completely metrizable and Baire, and that smooth domains are dense in it. If 0<ν1(Ω)<ν2(Ω)<0<\nu_1(\Omega)<\nu_2(\Omega)<\cdots are the distinct Dirichlet eigenvalues, our main result states that {ΩC(D):infm1(νm+1(Ω)νm(Ω))=0} \left\{\Omega\in\mathcal{C}_\ell(D):\inf_{m\geq 1}\bigl(\nu_{m+1}(\Omega)-\nu_m(\Omega)\bigr)=0\right\} is residual. This statement requires no simplicity assumption. Combining it with our transfer of Micheletti's classical generic-simplicity theorem to C(D)\mathcal{C}_\ell(D) shows that a generic domain has simple spectrum and consecutive gaps with zero lower limit. The proof uses \v{S}ver\'ak's planar spectral continuity theorem and a local surgery that implants an arbitrarily high pair of close consecutive distinct eigenvalues.

Keywords

Cite

@article{arxiv.2607.26224,
  title  = {Generic failure of uniform separation in planar Dirichlet spectra},
  author = {Vincent Boulard},
  journal= {arXiv preprint arXiv:2607.26224},
  year   = {2026}
}