A combinatorial version of Sylvester's four-point problem

J.J. Sylvester's four-point problem asks for the probability that four points chosen uniformly at random in the plane have a triangle as their convex hull. Using a combinatorial classification of points in the plane due to Goodman and Pollack, we generalize Sylvester's problem to one involving reduced expressions for the long word ill S(n). We conjecture an answer of 1/4 for this new version of the problem. (C) 2010 Elsevier Inc. All rights reserved.