Tridimensional character sums with polynomial arguments and applications

Abstract

Let $p$ be a large prime and $χ$ a non-trivial Dirichlet character modulo $p$. We study the character sum \[ \sum_{a \sim A} \sum_{b \sim B} \sum_{c \sim C} α(a,b) β(c)χ(f(a) + bc), \] where $z \sim Z$ means $Z\le z < 2Z$, $f \in \mathbb{Z}[X]$ is of small degree $k$, and $\boldsymbolα=(α(a,b))_{a\sim A,b\sim B}$ and $\boldsymbolβ=(β(c))_{c\sim C}$ are two complex coefficients. We prove non-trivial upper bounds for this sum in either of the two cases: (1) $\boldsymbolβ\equiv1$, $k=2,3,4,5$ and $A,B,C>p^{\frac{1}{8}+\varepsilon}$, (2) $\boldsymbolβ$ general, $k=2,3$ and $A,B,C>p^{\frac{1}{6}+\varepsilon}$, where $\varepsilon>0$ is fixed. This work was originally motivated by an intermediate result of Ganguly and Rajan (2023) on counting $2\times2$ matrices over $\mathbb{F}_p$ with irreducible characteristic polynomials, where the entries are in short segments. The new bounds here allow us to count such matrices in much shorter segments.