Skeleton conjecture for higher-order Beer indices

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For a set X⊆RdX\subseteq\mathbb{R}^d and an integer k≥0k\geq 0, define

Skel⁡k(X)=⋃Y∈(Xk+1)Conv⁡(Y),\operatorname{Skel}_k(X)=\bigcup_{Y\in\binom{X}{k+1}}\operatorname{Conv}(Y),

where (Xk+1)\binom{X}{k+1} is the set of (k+1)(k+1)-element subsets of XX. Let b⁡k(S)\operatorname{b}_k(S) be the kk-th Beer index of a set SS, and let λd\lambda_d denote dd-dimensional Lebesgue measure. Skeleton conjecture. For every k,d∈Nk,d\in\mathbb{N} with 1≤k≤d1\leq k\leq d and every ε>0\varepsilon>0, there is a δ>0\delta>0 such that if S⊆RdS\subseteq\mathbb{R}^d satisfies b⁡k(S)≥ε\operatorname{b}_k(S)\geq\varepsilon, then there is a simplex TT with vertex set XX such that

λd(T)≥δλd(S)andSkel⁡k(X)⊆S.\lambda_d(T)\geq\delta\lambda_d(S)\qquad\text{and}\qquad\operatorname{Skel}_k(X)\subseteq S.

The conjecture is known when k=dk=d, since the dd-skeleton is the whole simplex, and in the stated restricted one-dimensional simply connected case. Its validity in general remains open.

References

Primary source

Martin Balko, Vít Jelínek, Pavel Valtr and Bartosz Walczak, “On the Beer index of convexity and its variants”, arXiv:1412.1769 (2016).

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