Skeleton conjecture for higher-order Beer indices

For a set XRdX\subseteq\mathbb{R}^d and an integer k0k\geq 0, define

Skelk(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 bk(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,dNk,d\in\mathbb{N} with 1kd1\leq k\leq d and every ε>0\varepsilon>0, there is a δ>0\delta>0 such that if SRdS\subseteq\mathbb{R}^d satisfies bk(S)ε\operatorname{b}_k(S)\geq\varepsilon, then there is a simplex TT with vertex set XX such that

λd(T)δλd(S)andSkelk(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.

Sources & referencesView supporting material

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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