On the bichromatic k-set problem

  • Authors:
  • Timothy M. Chan

  • Affiliations:
  • University of Waterloo, Waterloo, Ontario, Canada

  • Venue:
  • ACM Transactions on Algorithms (TALG)
  • Year:
  • 2010

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Abstract

We study a bichromatic version of the well-known k-set problem: given two sets R and B of points of total size n and an integer k, how many subsets of the form (R∩ h) ∪ (B∖ h) can have size exactly k over all halfspaces h? In the dual, the problem is asymptotically equivalent to determining the worst-case combinatorial complexity of the k-level in an arrangement of n halfspaces. Disproving an earlier conjecture by Linhart [1993], we present the first nontrivial upper bound for all k≪ n in two dimensions: O(nk1/3 + n5/6−&epsis; k2/3+2 &epsis;+k2) for any fixed &epsis;O(nk3/2 + n0.5034k2.4932 + k3). Incidentally, this also implies a new upper bound for the original k-set problem in four dimensions: O(n2 k3/2 + n1.5034 k2.4932 + n k3), which improves the best previous result for all k≪ n0.923. Extensions to other cases, such as arrangements of disks, are also discussed.