Abstract
We consider the problem of 2-coloring geometric hypergraphs. Specifically, we show that there is a constant m such that any finite set of points in the plane \({\mathcal {S}} \subset {\mathbb {R}}^2\) can be 2-colored such that every axis-parallel square that contains at least m points from \({\mathcal {S}}\) contains points of both colors. Our proof is constructive, that is, it provides a polynomial-time algorithm for obtaining such a 2-coloring. By affine transformations this result immediately applies also when considering 2-coloring points with respect to homothets of a fixed parallelogram.










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Notes
In a multisubfamily we allow taking multiple copies of the same set.
In fact in early papers that considered only coverings by translates, all mentioned variants of cover-decomposability were defined for sets instead of families, where a set is cover-decomposable if the family of its translates is cover-decomposable in the way we define it in this paper.
An octant is the set of points \(\{(x,y,z): x<a,y<b,z<c\}\) in the space for some a, b and c.
Unless stated otherwise, logarithms in this paper are base 2.
In a family of pseudo-disks the boundaries of every two regions cross at most twice.
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Acknowledgements
E. Ackerman’s research partially supported by ERC Advanced Research Grant No. 267165 (DISCONV). B. Keszegh’s research supported by the National Research, Development and Innovation Office – NKFIH under the Grant PD 108406 and K 116769 and by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. M. Vizer’s research supported by the National Research, Development and Innovation Office – NKFIH under the Grant SNN 116095. We thank the reviewers for reading our paper and for their valuable remarks. In particular, suggestions of one of reviewers helped to simplify some basic lemmas and improve the constant in our main theorem.
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A preliminary version of this paper was presented at the 32nd International Symposium on Computational Geometry (SoCG 2016).
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Ackerman, E., Keszegh, B. & Vizer, M. Coloring Points with Respect to Squares. Discrete Comput Geom 58, 757–784 (2017). https://doi.org/10.1007/s00454-017-9902-y
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DOI: https://doi.org/10.1007/s00454-017-9902-y