General-position sets and linear hypergraphs

Source abstract

We prove that every linear hypergraph on $N$ vertices with edge sizes $O(\sqrt N)$ and $O(N^2\log N)$ triples contained in its edges has a subset of size $Ω(\sqrt N)$ meeting each edge in at most two vertices. As an application, every set of $N$ planar points with at most $\sqrt N$ collinear points contains $Ω(\sqrt N)$ points in general position. This proves a conjecture of Payne and Wood and gives the optimal quadratic bound for a geometric Ramsey problem posed by Gowers.