A bilinear Bogolyubov-Ruzsa lemma with poly-logarithmic bounds (1808.04965v2)

Abstract: The Bogolyubov-Ruzsa lemma, in particular the quantitative bounds obtained by Sanders, plays a central role in obtaining effective bounds for the inverse $U^3$ theorem for the Gowers norms. Recently, Gowers and Mili\'cevi\'c applied a bilinear Bogolyubov-Ruzsa lemma as part of a proof of the inverse $U^4$ theorem with effective bounds. The goal of this note is to obtain quantitative bounds for the bilinear Bogolyubov-Ruzsa lemma which are similar to those obtained by Sanders for the Bogolyubov-Ruzsa lemma. We show that if a set $A \subset \mathbb{F}_p^n \times \mathbb{F}_p^n$ has density $\alpha$, then after a constant number of horizontal and vertical sums, the set $A$ would contain a bilinear structure of co-dimension $r=\log^{O(1)} \alpha^{-1}$. This improves the results of Gowers and Mili\'cevi\'c which obtained similar results with a weaker bound of $r=\exp(\exp(\log^{O(1)} \alpha^{-1}))$ and by Bienvenu and L^e which obtained $r=\exp(\exp(\exp(\log^{O(1)} \alpha^{-1})))$.