On the Minimum Size of Signed Sumsets in Elementary Abelian Groups

For a finite abelian group $G$ and positive integers $m$ and $h$, we let $ρ(G, m, h) = \min \{|hA| \; : \; A \subseteq G, |A|=m\}$ and $ρ_{\pm} (G, m, h) = \min \{|h_{\pm} A| \; : \; A \subseteq G, |A|=m\},$ where $hA$ and $h_{\pm} A$ denote the $h$-fold sumset and the $h$-fold signed sumset of $A$, respectively. The study of $ρ(G, m, h)$ has a 200-year-old history and is now known for all $G$, $m$, and $h$. In previous work we provided an upper bound for $ρ_{\pm} (G, m, h)$ that we believe is exact, and proved that $ρ_{\pm} (G, m, h)$ agrees with $ρ(G, m, h)$ when $G$ is cyclic. Here we study $ρ_{\pm} (G, m, h)$ for elementary abelian groups $G$; in particular, we determine all values of $m$ for which $ρ_{\pm} (\mathbb{Z}_p^2, m, 2)$ equals $ρ(\mathbb{Z}_p^2, m, 2)$ for a given prime $p$.

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