# Domains without dense Steklov nodal sets

## TL;DR

This paper constructs a dense family of two-dimensional domains with analytic boundaries where Steklov eigenfunctions' nodal sets are not dense at the scale of their eigenvalues, challenging previous expectations about their geometric behavior.

## Contribution

It demonstrates the existence of domains with analytic boundaries where Steklov eigenfunctions have non-dense nodal sets at the eigenvalue scale, addressing an open problem in spectral geometry.

## Key findings

- Nodal sets are not dense at scale σ_j^{-1} in certain domains.
- Existence of points where eigenfunctions do not vanish within fixed radius.
- Contradicts prior assumptions about nodal set density at eigenvalue scale.

## Abstract

This article concerns the asymptotic geometric character of the nodal set of the eigenfunctions of the Steklov eigenvalue problem $$ -\Delta \phi_{\sigma_j}=0,\quad\text{ on }\Omega,\qquad\qquad \partial_\nu \phi_{\sigma_j}=\sigma_j \phi_{\sigma_j}\quad \text{ on }\partial\Omega $$ in two-dimensional domains $\Omega$. In particular, this paper presents a dense family $\mathcal{A}$ of simply-connected two-dimensional domains with analytic boundaries such that, for each $\Omega\in \mathcal{A}$, the nodal set of the eigenfunction $\phi_{\sigma_j}$ "is $not$ dense at scale $\sigma_j^{-1}$". This result addresses a question put forth under "Open Problem 10" in Girouard and Polterovich, J. Spectr. Theory, 321-359 (2017). In fact, the results in the present paper establish that, for domains $\Omega\in \mathcal{A}$, the nodal sets of the eigenfunctions $\phi_{\sigma_j}$ associated with the eigenvalue $\sigma_j$ have starkly different character than anticipated: they are not dense at any shrinking scale. More precisely, for each $\Omega\in \mathcal{A}$ there is a value $r_1>0$ such that for each $j$ there is $x_j\in \Omega$ such that $\phi_{\sigma_j}$ does not vanish on the ball of radius $r_1$ around $x_j$.