Let be the probability that four points chosen at random in a region have a Convex Hull which is a
Quadrilateral. For an open, convex subset of the Plane of finite Area,
Schneinerman, E. and Wilf, H. S. ``The Rectilinear Crossing Number of a Complete Graph and Sylvester's
`Four Point' Problem of Geometric Probability.'' Amer. Math. Monthly 101, 939-943, 1994.