On curves intersecting at most once

Abstract

We prove that on a closed surface of genus $g$, the cardinality of a set of simple closed curves in which any two are non-homotopic and intersect at most once is $\lesssim g^2 \log(g)$. This bound matches the largest known constructions to within a logarithmic factor. The proof uses a probabilistic argument in graph theory. It generalizes as well to the case of curves that intersect at most $k$ times in pairs.