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DOI: 10.4230/LIPIcs.SoCG.2023.8
On Helly Numbers of Exponential Lattices

Authors: Gergely Ambrus, Martin Balko, Nóra Frankl, Attila Jung, and Márton Naszódi

Published in: LIPIcs, Volume 258, 39th International Symposium on Computational Geometry (SoCG 2023)

Abstract
Given a set S ⊆ ℝ², define the Helly number of S, denoted by H(S), as the smallest positive integer N, if it exists, for which the following statement is true: for any finite family ℱ of convex sets in ℝ² such that the intersection of any N or fewer members of ℱ contains at least one point of S, there is a point of S common to all members of ℱ. We prove that the Helly numbers of exponential lattices {αⁿ : n ∈ ℕ₀}² are finite for every α > 1 and we determine their exact values in some instances. In particular, we obtain H({2ⁿ : n ∈ ℕ₀}²) = 5, solving a problem posed by Dillon (2021). For real numbers α, β > 1, we also fully characterize exponential lattices L(α,β) = {αⁿ : n ∈ ℕ₀} × {βⁿ : n ∈ ℕ₀} with finite Helly numbers by showing that H(L(α,β)) is finite if and only if log_α(β) is rational.
Cite as

Gergely Ambrus, Martin Balko, Nóra Frankl, Attila Jung, and Márton Naszódi. On Helly Numbers of Exponential Lattices. In 39th International Symposium on Computational Geometry (SoCG 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 258, pp. 8:1-8:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2023)

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@InProceedings{ambrus_et_al:LIPIcs.SoCG.2023.8,
  author =	{Ambrus, Gergely and Balko, Martin and Frankl, N\'{o}ra and Jung, Attila and Nasz\'{o}di, M\'{a}rton},
  title =	{{On Helly Numbers of Exponential Lattices}},
  booktitle =	{39th International Symposium on Computational Geometry (SoCG 2023)},
  pages =	{8:1--8:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-273-0},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{258},
  editor =	{Chambers, Erin W. and Gudmundsson, Joachim},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2023.8},
  URN =		{urn:nbn:de:0030-drops-178584},
  doi =		{10.4230/LIPIcs.SoCG.2023.8},
  annote =	{Keywords: Helly numbers, exponential lattices, Diophantine approximation}
}
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