Spherical higher order Fourier analysis over finite fields I: equidistribution for nilsequences

This paper is the first part of the series "Spherical higher order Fourier analysis over finite fields", aiming to develop the higher order Fourier analysis method along spheres over finite fields, and to solve the geometric Ramsey conjecture in the finite field setting. In this paper, we prove a quantitative equidistribution theorem for polynomial sequences in a nilmanifold, where the average is taken along spheres instead of cubes. To be more precise, let $\Omega\subseteq \mathbb{Z}^{d}$ be the preimage of a sphere $\mathbb{F}_{p}^{d}$ under the natural embedding from $\mathbb{Z}^{d}$ to $\mathbb{F}_{p}^{d}$. We showed that if a rational polynomial sequence $(g(n)\Gamma)_{n\in\Omega}$ is not equidistributed on a nilmanifold $G/\Gamma$, then there exists a nontrivial horizontal character $\eta$ of $G/\Gamma$ such that $\eta\circ g \mod \mathbb{Z}$ vanishes on $\Omega$. We also prove quantitative equidistribution and factorization theorems for more general sets arising from quadratic forms defined over $\mathbb{F}_{p}^{d}$. These results will serve as fundamental tools in later parts of the series to proof the spherical Gowers inverse theorem and the geometric Ramsey conjecture.