A Quadratic Vinogradov Mean Value Theorem in Finite Fields

Abstract: Let $p$ be a prime, let $s \geq 3$ be a natural number and let $A \subseteq \mathbb{F}p$ be a non-empty set satisfying $|A| \ll p^{1/2}$. Denoting $J_s(A)$ to be the number of solutions to the system of equations [ \sum{i=1}^{s} (x_i - x_{i+s}) = \sum_{i=1}^{s} (x_i^2 - x_{i+s}^2) = 0, ] with $x_1, \dots, x_{2s} \in A$, our main result implies that [ J_s(A) \ll |A|^{2s - 2 - 1/9}. ] This can be seen as a finite field analogue of the quadratic Vinogradov mean value theorem. Our techniques involve a variety of combinatorial geometric estimates, including studying incidences between cartesian products $A\times A$ and a special family of modular hyperbolae.