Properties of two dimensional sets with small sumset

Abstract

Let $A, B\subseteq \mathbb{R}^2$ be finite, nonempty subsets, let $s\geq 2$ be an integer, and let $h_1(A,B)$ denote the minimal number $t$ such that there exist $2t$ (not necessarily distinct) parallel lines, $\ell_1,...,\ell_{t},\ell'_1,...,\ell'_{t}$, with $A\subseteq \bigcup_{i=1}^{t}\ell_i$ and $B\subseteq\bigcup_{i=1}^{t}\ell'_i$. Suppose $h_1(A,B)\geq s$. Then we show that: (a) if $||A|-|B||\leq s$ and $|A|+|B|\geq 4s^2-6s+3$, then $$|A+B|\geq (2-\frac 1 s)(|A|+|B|)-2s+1;$$ (b) if $|A|\geq |B|+s$ and $|B|\geq 2s^2-{7/2}s+{3/2}$, then $$|A+B|\geq |A|+(3-\frac 2 s)|B|-s;$$ (c) if $|A|\geq {1/2}s(s-1)|B|+s$ and either $|A|> {1/8}(2s-1)^2|B|-{1/4}(2s-1)+\frac{(s-1)^2}{2(|B|-2)}$ or $|B|\geq \frac{2s+4}{3}$, then $$|A+B|\geq |A|+s(|B|-1).$$ This extends the 2-dimensional case of the Freiman $2^d$--Theorem to distinct sets $A$ and $B$, and, in the symmetric case $A=B$, improves the best prior known bound for $|A|+|B|$ (due to Stanchescu, and which was cubic in $s$) to an exact value. As part of the proof, we give general lower bounds for two dimensional subsets that improve the 2-dimensional case of estimates of Green and Tao and of Gardner and Gronchi, and that generalize the 2-dimensional case of the Brunn-Minkowski Theorem.