Abstract
Let S be a set of n points in the plane in general position (no three points from S are collinear). For a positive integer k, a k-hole in S is a convex polygon with k vertices from S and no points of S in its interior. For a positive integer l, a simple polygon P is l-convex if no straight line intersects the interior of P in more than l connected components. A point set S is l-convex if there exists an l-convex polygonization of S.
Considering a typical Erdős–Szekeres-type problem, we show that every 2-convex point set of size n contains an \(\varOmega (\log n)\)-hole. In comparison, it is well known that there exist arbitrarily large point sets in general position with no 7-hole. Further, we show that our bound is tight by constructing 2-convex point sets with holes of size at most \(O(\log n)\).
Research supported by OEAD project CZ 18/2015 and by project no. 7AMB15A T023 of the Ministry of Education of the Czech Republic. O. Aichholzer and B. Vogtenhuber supported by ESF EUROCORES programme Euro-GIGA - ComPoSe, Austrian Science Fund (FWF): I648-N18. M. Balko and P. Valtr supported by grant GAUK 690214, by project CE-ITI no. P202/12/G061 of the Czech Science Foundation GAČR, and by ERC Advanced Research Grant no 267165 (DISCONV). T. Hackl supported by Austrian Science Fund (FWF): P23629-N18. A. Pilz supported by an Erwin Schrödinger fellowship, Austrian Science Fund (FWF): J-3847-N35. P. Ramos supported by MINECO project MTM2014-54207, and ESF EUROCORES programme EuroGIGA, CRP ComPoSe: MICINN Project EUI-EURC-2011-4306, for Spain.
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Acknowledgments
This work was initiated during the ComPoSe Workshop on Algorithms using the Point Set Order Type held in March/April 2014 in Ratsch, Austria.
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Aichholzer, O. et al. (2018). Holes in 2-Convex Point Sets. In: Brankovic, L., Ryan, J., Smyth, W. (eds) Combinatorial Algorithms. IWOCA 2017. Lecture Notes in Computer Science(), vol 10765. Springer, Cham. https://doi.org/10.1007/978-3-319-78825-8_14
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