One line and &egr;

  • Authors:
  • Bernd Gärtner;József Solymosi;Falk Tschirschnitz;Emo Welzl;Pavel Valtr

  • Affiliations:
  • ETH Zürich;ETH Zürich;ETH Zürich;ETH Zürich;Charles University, Prague

  • Venue:
  • STOC '01 Proceedings of the thirty-third annual ACM symposium on Theory of computing
  • Year:
  • 2001

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Abstract

We analyze a randomized pivoting process involving one line and n points in the plane. The process models the behavior of the Random-Edge simplex algorithm on simple polytopes with n facets in dimension n-2. We obtain a tight O(\log^2 n) bound for the expected number of pivot steps. This is the first nontrivial bound for Random-Edge which goes beyond bounds for specific polytopes. The process itself can be interpreted as a simple algorithm for certain 2-variable linear programming problems, and we prove a tight &thgr;(n) bound for its expected runtime.The combinatorial structure behind the process is a directed graph over pairs of points, with arc orientations induced by the pivot steps. We characterize the class of graphs arising from one line and n points, up to oriented matroid realizability.