Discrete Analogues in Harmonic Analysis: Directional Maximal Functions in ${\mathbb{Z}}^2$

Abstract

Let $V=\{v_1,\dots ,v_N\}$ be a collection of $N$ vectors that live near a discrete sphere. We consider discrete directional maximal functions on ${\mathbb{Z}}^2$ where the set of directions lies in $V$, given by \[ \sup_{v \in V, k \geq C \log N} \left| \sum_{n \in \mathbb{Z}} f(x-v\cdot n ) \cdot ϕ_k(n) \right|, \ f:\mathbb{Z}^2 \to \mathbb{C}, \] where and ${\varphi }_k(t):=2^{-k}\varphi (2^{-k}t)$ for some bump function $\varphi$. Interestingly, the study of these operators leads one to consider an "arithmetic version" of a Kakeya-type problem in the plane, which we approach using a combination of geometric and number-theoretic methods. Motivated by the Furstenberg problem from geometric measure theory, we also consider a discrete directional maximal operator along polynomial orbits, \[ \sup_{v \in V} \left| \sum_{n \in \mathbb{Z}} f(x-v\cdot P(n) ) \cdot ϕ_k(n) \right|, \ P \in \mathbb{Z}[-] \] for $k\ge C_d\log N$ sufficiently large.