One line and ε

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 t(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.