Sums along the edges of random regular graphs

Abstract: Let $G$ be a graph on $n$ vertices and $(H,+)$ be an abelian group. What is the minimum size ${\sf S}H(G)$ of the set of all sums $A(u)+A(v)$ over all injections $A:V(G)\to H$? In 2012, Alon, Angel, the first author, and Lubetzky proved that, for expander graphs and $H=\mathbb{Z}$, this minimum is at least $\Omega(\log n)$, and this bound is tight -- there exists a regular expander $G$ with ${\sf S}{\mathbb{Z}}(G)=O(\log n)$. We prove that, for every constant $d\geq 3$, the random $d$-regular graph $\mathcal{G}{n,d}$ has significantly larger sum-sets: with high probability, for every abelian group $H$, ${\sf S}_H(\mathcal{G}{n,d})=\Omega(n^{1-2/d})$. In particular, this proves that, for every $\varepsilon>0$, there exists a regular graph with $O(n)$ edges and with sum-sets of size at least $n^{1-\varepsilon}$, for all abelian groups. We also prove that, for $d\gg\ln^2 n$, with high probability, for every abelian group $H$, ${\sf S}H(\mathcal{G}{n,d})=n(1-o(1))$.