The weak Pleijel theorem with geometric control

TL;DR

This paper establishes an upper bound on Courant-sharp eigenvalues of the Dirichlet Laplacian in bounded domains, linking spectral properties with geometric invariants, advancing understanding of nodal domain counts.

Contribution

It provides a new geometric bound on Courant-sharp eigenvalues, connecting spectral theory with geometric invariants of the domain.

Findings

Finite number of Courant-sharp eigenvalues proven.

Upper bounds expressed in terms of geometric invariants.

Connection with Safarov's problem highlighted.

Abstract

Let $\Omega\subset \mathbb R^d\,, d\geq 2$, be a bounded open set, and denote by $\lambda\_j(\Omega), j\geq 1$, the eigenvalues of the Dirichlet Laplacian arranged in nondecreasing order, with multiplicities. The weak form of Pleijel's theorem states that the number of eigenvalues $\lambda\_j(\Omega)$, for which there exists an associated eigenfunction with precisely $j$ nodal domains (Courant-sharp eigenvalues), is finite. The purpose of this note is to determine an upper bound for Courant-sharp eigenvalues, expressed in terms of simple geometric invariants of $\Omega$. We will see that this is connected with one of the favorite problems considered by Y. Safarov.