Furstenberg Sets in Finite Fields: Explaining and Improving the Ellenberg–Erman Proof

Abstract

A (k, m)-Furstenberg set is a subset \(S \subset {\mathbb {F}}_q^n\) with the property that each k-dimensional subspace of \({\mathbb {F}}_q^n\) can be translated so that it intersects S in at least m points. Ellenberg and Erman (Algebra Number Theory 10(7), 1415–1436 (2016)) proved that (k, m)-Furstenberg sets must have size at least \(C_{n,k}m^{n/k}\), where \(C_{n,k}\) is a constant depending only n and k. In this paper, we adopt the same proof strategy as Ellenberg and Erman, but use more elementary techniques than their scheme-theoretic method. By modifying certain parts of the argument, we obtain an improved bound on \(C_{n,k}\), and our improved bound is nearly optimal for an algebraic generalization the main combinatorial result. We also extend our analysis to give lower bounds for sets that have large intersection with shifts of a specific family of higher-degree co-dimension \(n-k\) varieties, instead of just co-dimension \(n-k\) subspaces.