Generic failure of uniform separation in planar Dirichlet spectra
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 be a bounded domain and, for , let be the space of nonempty connected open sets such that has at most connected components, endowed with the complementary-Hausdorff topology. We prove that is completely metrizable and Baire, and that smooth domains are dense in it. If are the distinct Dirichlet eigenvalues, our main result states that is residual. This statement requires no simplicity assumption. Combining it with our transfer of Micheletti's classical generic-simplicity theorem to 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}
}