Tridimensional character sums with polynomial arguments and applications

Source 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 $Z\le z p^{\frac{1}{8}+\varepsilon}$, (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.