When Generalized Sumsets are Difference Dominated

Abstract

We study the relationship between the number of minus signs in a generalized sumset, $A+...+A-...-A$, and its cardinality; without loss of generality we may assume there are at least as many positive signs as negative signs. As addition is commutative and subtraction is not, we expect that for most $A$ a combination with more minus signs has more elements than one with fewer; however, recently Iyer, Lazarev, Miller and Zhang proved that a positive percentage of the time the combination with fewer minus signs can have more elements. Their analysis involves choosing sets $A$ uniformly at random from ${0,...,N}$; this is equivalent to independently choosing each element of ${0,...,N}$ to be in $A$ with probability 1/2. We investigate what happens when instead each element is chosen with probability $p(N)$, with $\lim_{N\to\infty} p(N) =0$. We prove that the set with more minus signs is larger with probability 1 as $N\to\infty$ if $p(N)=cN^{-\delta}$ for $\delta\ge\frac{h-1}{h}$, where $h$ is the number of total summands in $A+...+A-...-A$, and explicitly quantify their relative sizes. The results generalize earlier work of Hegarty and Miller, and we see a phase transition in the behavior of the cardinalities when $\delta = \frac{h-1}{h}$.