Monotonicity conjecture for the Cheeger constant of parallel bodies

Let ΩKn\Omega\in\mathcal{K}^n be a convex body, let r(Ω)r(\Omega) denote its inradius, and write Ωt=ΩtB1\Omega_t=\Omega\oplus tB_1 for its parallel body. The Cheeger constant of a set AA is h(A)=inf{P(E)/E:EA}h(A)=\inf\{P(E)/|E|:E\subset A\}, and CΩC^\Omega denotes a Cheeger set of Ω\Omega. Monotonicity conjecture. For every ΩKn\Omega\in\mathcal{K}^n, the function

tΩt1nh(Ωt)t\longmapsto |\Omega_t|^{\frac{1}{n}}h(\Omega_t)

is monotonically decreasing on (r(Ω),+)(-r(\Omega),+\infty). As a consequence,

Hn1(CΩΩ)Hn1(Ω)1nCΩΩ.\frac{\mathcal{H}^{n-1}(\partial C^\Omega\cap \partial \Omega)}{\mathcal{H}^{n-1}(\partial \Omega)}\ge \frac{1}{n}\cdot \frac{|C^\Omega|}{|\Omega|}.

This would extend the monotonicity result suggested by the planar theory to convex bodies in arbitrary dimensions and would improve the available estimate for the contact surface of a Cheeger set. The source provides no resolution of the conjecture.

Sources & referencesView supporting material

Primary source

Ilias Ftouhi, “The monotonicity of the Cheeger constant for parallel bodies”, arXiv:2412.20917 (2025).

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