On the number of ordinary conics

Abstract

We prove a lower bound on the number of ordinary conics determined by a finite point set in ${\mathbb{R}}^2$. An ordinary conic for a subset $S$ of ${\mathbb{R}}^2$ is a conic that is determined by five points of $S$, and contains no other points of $S$. Wiseman and Wilson proved the Sylvester-Gallai-type statement that if a finite point set is not contained in a conic, then it determines at least one ordinary conic. We give a simpler proof of their result and then combine it with a result of Green and Tao to prove our main result: If $S$ is not contained in a conic and has at most $c|S|$ points on a line, then $S$ determines ${\Omega }_c(|S{|}^4)$ ordinary conics. We also give a construction, based on the group structure of elliptic curves, that shows that the exponent in our bound is best possible.

References in corpus (1)

• On the number of ordinary circles