On sumsets in ${\Bbb F}_2^n$

Abstract

Let ${\Bbb F}_2$ be the finite field of two elements, ${\Bbb F}_2^n$ be the vector space of dimension $n$ over ${\Bbb F}_2$. For sets $A,\,B\subseteq{\Bbb F}_2^n$, their sumset is defined as the set of all pairwise sums $a+b$ with $a\in A,\,b\in B$. Ben Green and Terence Tao proved that, let $K\geq 1$, if$A,\,B\subseteq{\Bbb F}_2^n$ and $|A+B|\leq K|A|^{1\over 2}|B|^{1\over 2}$, then there exists a subspace $H\subseteq{\Bbb F}_2^n$ with $$ |H|\gg\exp(-O(\sqrt{K}\log K))|A| $$ and $x,\,y\in{\Bbb F}_2^n$ such that $$ |A\cap(x+H)|^{1\over 2}|B\cap(y+H)|^{1\over 2}\geq{1\over 2K}|H|. $$ In this note, we shall use the method of Green and Tao with some modification to prove that if $$ |H|\gg\exp(-O(\sqrt{K}))|A|, $$ then the above conclusion still holds true.