Additive energy of polynomial images

Abstract

Given a monic polynomial $f(X)\in \mathbb{Z}_m[X]$ over a residue ring $\mathbb{Z}_m$ modulo an integer $m\ge 2$ and a discrete interval $\mathcal{I} = \{1, \ldots, H\}$ of $H \le m$ consecutive integers, considered as elements of $\mathbb{Z}_m$, we obtain a new upper bound for the additive energy of the set $f(\mathcal I)$, where $f(\mathcal I)$ denotes the image set $f(\mathcal I) = \{f(u):~u \in \mathcal I\}$. We give an application of our bounds to multiplicative character sums, improving some previous result of Shkredov and Shparlinski~(2018).