Sharp extension inequalities on finite fields

TL;DR

This paper develops sharp restriction inequalities over finite fields, identifying maximizers for certain geometric surfaces and revealing new phenomena due to finite field arithmetic, advancing the understanding of Fourier extension problems in finite settings.

Contribution

It introduces the first sharp restriction inequalities on finite fields, characterizes maximizers for specific surfaces, and uncovers unique finite field phenomena not present in Euclidean cases.

Findings

Constant functions maximize the extension inequality on parabola and paraboloid at the Stein-Tomas endpoint.

Maximizers for the $L^2 o L^4$ inequality on $ ext{P}^2$ are fully characterized when $q mod 4=1$.

Constants maximize the inequality on the cone $ ext{Gamma}^3$ when $q mod 4=3$, but fail to be critical points on $ ext{Gamma}^3 ext{union} extbf{0}$.

Abstract

Sharp restriction theory and the finite field extension problem have both received a great deal of attention in the last two decades, but so far they have not intersected. In this paper, we initiate the study of sharp restriction theory on finite fields. We prove that constant functions maximize the Fourier extension inequality from the parabola $\mathbb{P}^1\subset \mathbb{F}^{2\ast}_q$ and the paraboloid $\mathbb{P}^2\subset \mathbb{F}_q^{3\ast}$ at the euclidean Stein-Tomas endpoint; here, $\mathbb{F}_q^{d\ast}$ denotes the (dual) $d$-dimensional vector space over the finite field $\mathbb F_q$ with $q=p^n$ elements, where $p$ is a prime number greater than $3$ or $2$, respectively. We fully characterize the maximizers for the $L^2\to L^4$ extension inequality from $\mathbb{P}^2$ whenever $q\equiv 1(\text{mod}\, 4)$. Our methods lead to analogous results on the hyperbolic paraboloid, whose corresponding euclidean problem remains open. We further establish that constants maximize the $L^2\to L^4$ extension inequality from the cone $\Gamma^3:=\{(\boldsymbol{\xi},\tau, \sigma)\in \mathbb{F}^{4\ast}_q: \tau\sigma=\boldsymbol{\xi}^2\}\setminus \{{\bf 0}\}$ whenever $q\equiv 3(\text{mod}\, 4)$. By contrast, we prove that constant functions fail to be critical points for the corresponding inequality on $\Gamma^3\cup \{{\bf 0}\}$ over $\mathbb{F}_p^4$. While some inspiration is drawn from the euclidean setting, entirely new phenomena emerge which are related to the underlying arithmetic and discrete structures.