The bounded search tree algorithm for the closest string problem has quadratic smoothed complexity

  • Authors:
  • Christina Boucher

  • Affiliations:
  • Department of Computer Science and Engineering, University of California, San Diego

  • Venue:
  • MFCS'11 Proceedings of the 36th international conference on Mathematical foundations of computer science
  • Year:
  • 2011

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Abstract

Given a set S of n strings, each of length l, and a nonnegative value d, we define a center string as a string of length l that has Hamming distance at most d from each string in S. The Closest String problem aims to determine whether there exists a center string for a given set of strings S and input parameters n, l, and d. When n is relatively large with respect to l then the basic majority algorithm solves the CLOSEST STRING problem efficiently, and the problem can also be solved efficiently when either n, l or d is reasonably small [12]. Hence, the only case for which there is no known efficient algorithm is when n is between log l/ log log l and log l. Using smoothed analysis, we prove that such CLOSEST STRING instances can be solved efficiently by the O(nl + ndċdd)-time algorithm by Gramm et al. [13]. In particular, we show that for any given CLOSEST STRING instance I, the expected running time of this algorithm on a small perturbation of I is O(nl + ndċd2+o(1)).