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Matching colored points with rectangles

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Abstract

Let S be a point set in the plane such that each of its elements is colored either red or blue. A matching of S with rectangles is any set of pairwise-disjoint axis-aligned closed rectangles such that each rectangle contains exactly two points of S. Such a matching is monochromatic if every rectangle contains points of the same color, and is bichromatic if every rectangle contains points of different colors. We study the following two problems: (1) Find a maximum monochromatic matching of S with rectangles. (2) Find a maximum bichromatic matching of S with rectangles. For each problem we provide a polynomial-time approximation algorithm that constructs a matching with at least 1 / 4 of the number of rectangles of an optimal matching. We show that the first problem is \(\mathsf {NP}\)-hard even if either the matching rectangles are restricted to axis-aligned segments or S is in general position, that is, no two points of S share the same x or y coordinate. We further show that the second problem is also \(\mathsf {NP}\)-hard, even if S is in general position. These \(\mathsf {NP}\)-hardness results follow by showing that deciding the existence of a matching that covers all points is \(\mathsf {NP}\)-complete in each case. Additionally, we prove that it is \(\mathsf {NP}\)-complete to decide the existence of a matching with rectangles that cover all points in the case where all the points have the same color, solving an open problem of Bereg et al. (Comput Geom 42(2):93–108, 2009).

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Notes

  1. The associated graph is the bipartite graph with vertices the variables and the clauses, and there exists an edge between a variable and a clause if and only if the variable participates in the clause.

  2. If D(ab) is a box, then its interior is the interior of the box. Otherwise, if D(ab) is a segment, then its interior is the set \(D(a,b){\setminus }\{a,b\}\).

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Acknowledgments

C. Ochoa is supported by CONICYT-PCHA/Doctorado Nacional/ 2013-63130161 (Chile). P. Pérez-Lantero is supported by project Millennium Nucleus Information and Coordination in Networks ICM/FIC RC130003 (Chile). J. Rojas-Ledesma is supported by CONICYT-PCHA/Doctorado Nacional/2013-63130209 (Chile). The authors would like to thank anonymous referees for helpful comments and suggestions.

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Caraballo, L.E., Ochoa, C., Pérez-Lantero, P. et al. Matching colored points with rectangles. J Comb Optim 33, 403–421 (2017). https://doi.org/10.1007/s10878-015-9971-x

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