Banuelos and Burdzy's width conjecture for multiple Neumann eigenvalues

Let ΩR2\Omega\subseteq\mathbb{R}^2 be a convex domain. Let μ1(Ω)\mu_1(\Omega) denote its first nonzero Neumann eigenvalue, let D(Ω)\mathcal{D}(\Omega) be its diameter, and let W(Ω)\mathcal{W}(\Omega) be its width, defined by

W(Ω)=minvS1D(Pv(Ω)),\mathcal{W}(\Omega)=\min_{v\in\mathbb{S}^{1}}\mathcal{D}(\mathscr{P}_{v}(\Omega)),

where

Pv(Ω)={xR2:xv=0, x=y+sv for some yΩ, sR}.\mathscr{P}_{v}(\Omega)=\{x\in\mathbb{R}^2:x\cdot v=0,\ x=y+s\cdot v\ \text{for some}\ y\in\Omega,\ s\in\mathbb{R}\}.

Ba~{n}uelos and Burdzy's conjecture. If μ1(Ω)\mu_1(\Omega) has multiplicity 22, then

W(Ω)D(Ω)12.\frac{\mathcal{W}(\Omega)}{\mathcal{D}(\Omega)}\geq\frac{1}{\sqrt{2}}.

Nadirashvili proved that the multiplicity of the first nonzero Neumann eigenvalue is at most 22, and Ba~{n}uelos and Burdzy proved simplicity when the width-to-diameter ratio is at most π4j0,1\frac{\pi}{4j_{0,1}}. Their conjecture proposes a stronger necessary lower bound for the ratio whenever the first eigenvalue is not simple.

Sources & referencesView supporting material

Primary source

Qixuan Hu, “Multiple eigenvalues and the width”, arXiv:2512.15050 (2025).

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