The distribution of $απ$ modulo one in $\mathbb{F}q(T)$: beyond Matomäki's exponent

Abstract

Let $α$ be an element of $\Fq((1/T))$ which is not in $\Fq(T)$. We ask how small $\nrm{απ}$ can be made when $π$ runs over irreducible polynomials, and write $θ(α)$ for the largest exponent $θ$ such that $\nrm{απ}<|π|^{-θ}$ has infinitely many solutions. The analogue of Matomäki's exponent, $θ(α)\ge\frac13$, was proved by Baier and the author, and it is the natural limit of \chg{methods based on} the Riemann Hypothesis. In this paper we go beyond it. For odd characteristic $p$ and $q$ large in terms of $p$ and $θ$, we prove that $θ(α)\geθ$ for every $α$ and every $θ<\sqrt2-1=0.414\ldots$, and for every $θ<\frac12$ when $α$ has bounded partial quotients. The main tools are an exact count of points in affine subspaces of $\Fq[T]$, a splitting of these subspaces into long arithmetic progressions, and the results of Sawin and Shusterman on the Möbius function along such progressions.