Sharp refined-direction Kakeya estimates in finite Heisenberg groups

Abstract

Let $n\ge 2$ and let $q$ be an odd prime power. The first aim of this paper is to prove that, for every $E\subset {\mathbb{H}}_n({\mathbb{F}}_q)$ and every $\lambda >0$, the following sharp rich-direction estimate holds \[ \left| \left\{ \vartheta\in D_n: M^{\mathrm{rd}}_{\mathbb{H}_n}\mathbf{1}_E(\vartheta)\geqλ \right\} \right| \lesssim_n q^{2n-1}|E|λ^{-2n}. \] The second aim is to determine, for every $1\le u,v\le \infty$, the sharp exponent of $q$ in the corresponding ${\ell }^u\to {\ell }^v$ estimate. More precisely, we prove that \[ A_n^{\mathrm{rd}}(u,v) = \max\left\{ \frac{2n-1}{v},\ 1-\frac1u,\ \frac{2n}{v}-\frac1u,\ 1+\frac{2n}{v}-\frac{2n+1}{u} \right\}. \] The proof combines the polynomial method with multiplicities and a probabilistic covering argument based on the action of the affine symplectic group.

37 pages