Abstract
The colourful simplicial depth (CSD) of a point relative to a configuration \(P=(P^1, P^2, \ldots , P^k)\) of n points in k colour classes is exactly the number of closed simplices (triangles) with vertices from 3 different colour classes that contain \(x\) in their convex hull. We consider the problems of efficiently computing the colourful simplicial depth of a point x, and of finding a point in , called a median, that maximizes colourful simplicial depth.
For computing the colourful simplicial depth of \(x\), our algorithm runs in time \(O\left( n \log {n} + k n \right) \) in general, and O(kn) if the points are sorted around \(x\). For finding the colourful median, we get a time of \(O(n^4)\). For comparison, the running times of the best known algorithm for the monochrome version of these problems are \(O\left( n \log {n} \right) \) in general, improving to O(n) if the points are sorted around \(x\) for monochrome depth, and \(O(n^4)\) for finding a monochrome median.
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Some points of conv(P) may fall outside any cell.
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Acknowledgments
This research was partially supported by an NSERC Discovery Grant to T. Stephen and by an SFU Graduate Fellowships to O. Zasenko. We thank A. Deza for comments on the presentation.
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Zasenko, O., Stephen, T. (2016). Algorithms for Colourful Simplicial Depth and Medians in the Plane. In: Chan, TH., Li, M., Wang, L. (eds) Combinatorial Optimization and Applications. COCOA 2016. Lecture Notes in Computer Science(), vol 10043. Springer, Cham. https://doi.org/10.1007/978-3-319-48749-6_28
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