Fourier Growth of Communication Protocols for XOR Functions

TL;DR

This paper studies the Fourier growth of functions derived from communication protocols for XOR functions, providing new bounds that improve understanding of their complexity and implications for communication lower bounds.

Contribution

It introduces improved Fourier growth bounds for XOR-fibers of protocols with bounded communication, including tight bounds for the first and second levels, and connects Fourier growth to lifting theorems.

Findings

Tight $O(\sqrt{d})$ bound for the first level Fourier growth.

Polynomial improvement on the lower bounds for XOR-lift of Forrelation.

New method using Gaussian space partitioning and martingale arguments.

Abstract

The level-$k$ $\ell_1$-Fourier weight of a Boolean function refers to the sum of absolute values of its level-$k$ Fourier coefficients. Fourier growth refers to the growth of these weights as $k$ grows. It has been extensively studied for various computational models, and bounds on the Fourier growth, even for the first few levels, have proven useful in learning theory, circuit lower bounds, pseudorandomness, and quantum-classical separations. We investigate the Fourier growth of certain functions that naturally arise from communication protocols for XOR functions (partial functions evaluated on the bitwise XOR of the inputs to Alice and Bob). If a protocol $\mathcal C$ computes an XOR function, then $\mathcal C(x,y)$ is a function of the parity $x\oplus y$. This motivates us to analyze the XOR-fiber of $\mathcal C$, defined as $h(z):=\mathbb E_{x,y}[\mathcal C(x,y)|x\oplus y=z]$. We present improved Fourier growth bounds for the XOR-fibers of protocols that communicate $d$ bits. For the first level, we show a tight $O(\sqrt d)$ bound and obtain a new coin theorem, as well as an alternative proof for the tight randomized communication lower bound for Gap-Hamming. For the second level, we show an $d^{3/2}\cdot\mathrm{polylog}(n)$ bound, which improves the previous $O(d^2)$ bound by Girish, Raz, and Tal (ITCS 2021) and implies a polynomial improvement on the randomized communication lower bound for the XOR-lift of Forrelation, extending its quantum-classical gap. Our analysis is based on a new way of adaptively partitioning a relatively large set in Gaussian space to control its moments in all directions. We achieve this via martingale arguments and allowing protocols to transmit real values. We also show a connection between Fourier growth and lifting theorems with constant-sized gadgets as a potential approach to prove optimal bounds for the second level and beyond.