On Sets of Points in General Position That Lie on a Cubic Curve in the Plane

Let P be a set of n points in general position in the plane. Let R be a set of points disjoint from P such that for every x, y € P the line through x and y contains a point in R . We show that if is contained in a cubic curve c in the plane, then P has a special property with respect to the natural group structure on c . That is, P is contained in a coset of a subgroup H of c of cardinality at most | R |. We use the same approach to show a similar result in the case where each of B and G is a set of n points in general position in the plane and every line through a point in B and a point in G passes through a point in R . This provides a partial answer to a problem of Karasev. The bound is best possible at least for part of our results. Our extremal constructions provide a counterexample to an old conjecture attributed to Jamison about point sets that determine few directions.