Congruence Classes in $\mathbb{F}_p^2$: Sharp Results via an Energy Approach

Abstract: Let $T_k(E)$ denote the set of congruence classes of ordered $k$-tuples of pairwise distinct points of $E$. Let $p\equiv3\pmod4$ be prime. For $E\subset\mathbb{F}_p<sup>2$ with $3\leq|E|\leq p<sup>{3/4}$, we prove that $|T_3(E)|\gg|E|<sup>{11/6}$; for $4\leq|E|\leq p<sup>{3/4}$, we prove that $|T_4(E)|\gg|E|<sup>3/\log|E|$. For every fixed $k\geq5$ and $k\leq|E|\leq p<sup>{3(k-2)/(3k-4)}$, we prove that $|T_k(E)|\gg_k|E|<sup>{k-1}$. The proofs proceed by bounding the moments of the overlap function of rigid motions. The main inputs are an exact identity involving the distance energy and an incidence bound obtained by viewing rigid motions as lines over $\mathbb{F}_p(i)$. The bound for $k=4$ is sharp up to a logarithmic factor, while the bounds for $k\geq5$ are sharp up to constants.