On Extending Pollard's Theorem for t-Representable Sums

Abstract

Let $t\ge 1$, let $A$ and $B$ be finite, nonempty subsets of an abelian group $G$, and let $A\pp{i} B$ denote all the elements $c$ with at least $i$ representations of the form $c=a+b$, with $a\in A$ and $b\in B$. For $|A|,|B|\ge t$, we show that either \be\label{almost}\Sum{i=1}{t}|A\pp{i} B|\geq t|A|+t|B|-2t^2+1,\ee or else there exist $A^{\prime }\subseteq A$ and $B^{\prime }\subseteq B$ with \ber \nn l&:=&|A\setminus A'|+|B\setminus B'|\leq t-1, \nn A'\pp{t}B'&=&A'+B'=A\pp{t}B,{and} \nn \Sum{i=1}{t}|A\pp{i}B|&\geq& t|A|+t|B|-(t-l)(|H|-ρ)-tl\geq t|A|+t|B|-t|H|,\eer where $H$ is the (nontrivial) stabilizer of $A\pp{t} B$ and $\rho =|A^{\prime }+H|-|A^{\prime }|+|B^{\prime }+H|-|B^{\prime }|$. In the case $t=2$, we improve (\ref{almost}) to $|A\pp{1}B|+|A\pp{2}B|\geq 2|A|+2|B|-4$.