The absence of a universal upper bound for the first eigenvalue in terms of inradius

From papers

Let G\mathcal{G} be a unilateral metric graph with Dirichlet set VD{\mathsf{V}^{\mathrm D}}, let λ1(G;VD)\lambda_1(\mathcal{G};{\mathsf{V}^{\mathrm D}}) denote its first eigenvalue, and define the inradius relative to the Dirichlet set by

inr(G;VD):=maxxGdist(x,VD).\operatorname{inr}(\mathcal{G};{\mathsf{V}^{\mathrm D}}):=\max_{x\in\mathcal{G}}\operatorname{dist}(x,{\mathsf{V}^{\mathrm D}}).

Inradius upper-bound conjecture. There is no universal constant C>0C>0 such that

λ1(G;VD)Cinr(G;VD)2\lambda_1(\mathcal{G};{\mathsf{V}^{\mathrm D}})\leq \frac{C}{\operatorname{inr}(\mathcal{G};{\mathsf{V}^{\mathrm D}})^2}

for all unilateral metric graphs. This is presented as a weaker version of the mean-distance conjecture. The source explains that a uniform lower bound in terms of inradius alone cannot hold, while the proposed uniform upper bound is left as an open conjecture.

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Sources & referencesView supporting material

Primary source

Delio Mugnolo, “The role of expanders in the spectral geometry of metric graphs”, arXiv:2607.14312 (2026).

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