On sets of points in general position that lie on a cubic curve in the plane and determine lines that can be pierced by few points

Abstract

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 $x,y \in P$ the line through $x$ and $y$ contains a point in $R$. We show that if $|R| < \frac{3}{2}n$ and $P \cup R$ is contained in a cubic curve $c$ in the plane, then $P$ has a special property with respect to the natural group action 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 $|R| < \frac{3}{2}n$ 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. Jamison conjectured that if $P$ is a set of $n$ points in general position in the plane that determines at most $2n-c$ distinct directions, then $P$ is contained in an affine image of the set of vertices of a regular $m$-gon. This conjecture of Jamison is strongly related to our results in the case the cubic curve $c$ is reducible and our results can be used to prove Jamison's conjecture at least when $m-n$ is in the order of magnitude of $O(\sqrt{n})$.