Structure forest and composition factors for small base groups in nearly linear time

  • Authors:
  • Robert Beals;Ákos Seress

  • Affiliations:
  • University of Chicago;Ohio State University

  • Venue:
  • STOC '92 Proceedings of the twenty-fourth annual ACM symposium on Theory of computing
  • Year:
  • 1992

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Abstract

A base of a permutation group G is a subset B of the permutation domain such that only the identity of G fixes B pointwise. The permutation representations of important classes of groups, including all finite simple groups other than the alternating groups, admit O(log n) size bases, where n is the size of the permutation domain. Groups with very small bases dominate the work on permutation groups within computational group theory.We use the “soft” version of the big-O notation introduced by [BLSI]: for two functions f(n), g(n), we write f(n)=O˜(g(n)) if for some constants c, C 0, we have f(n) ≤ Cg(n) logcn.We address the problems of finding structure trees and composition factors for permutation groups with small (O˜(1) size) bases. For general permutation groups, a method of Atkinson will find a structure tree in O(n2) time. We give an O˜(n) algorithm for the small base case. The composition factor problem was first shown to have a polynomial time solution by Luks [Lu], and recently Babai, Luks, Seress [BLS2] gave an O˜(n3) algorithm. The [BLS2] algorithm takes &THgr;(n3) time even in the small base case. We overcome several quadratic and cubic bottlenecks in the [BLS2] algorithm to give an O˜(n) Monte Carlo algorithm for the small base case. In addition, we show that the center of a small base group can be found in time O˜(n).