Hypercontractivity on HDX II: Symmetrization and q-Norms

Max Hopkins

Abstract: Bourgain's symmetrization theorem is a powerful technique reducing boolean analysis on product spaces to the cube. It states that for any product $\Omega_i^{\otimes d}$, function $f: \Omega_i^{\otimes d} \to \mathbb{R}$, and $q > 1$: $$||T_{\frac{1}{2}}f(x)||_q \leq ||\tilde{f}(r,x)||_{q} \leq ||T_{c_q}f(x)||_q$$ where $T_{\rho}f = \sum\limits \rho^Sf^{=S}$ is the noise operator and $\widetilde{f}(r,x) = \sum\limits r_Sf^{=S}(x)$ `symmetrizes' $f$ by convolving its Fourier components $\{f^{=S}\}_{S \subseteq [d]}$ with a random boolean string $r \in \{\pm 1\}^d$. In this work, we extend the symmetrization theorem to high dimensional expanders (HDX). Adapting work of O'Donnell and Zhao (2021), we give as a corollary a proof of optimal global hypercontractivity for partite HDX, resolving one of the main open questions of Gur, Lifshitz, and Liu (STOC 2022). Adapting work of Bourgain (JAMS 1999), we also give the first booster theorem for HDX, resolving a main open questions of Bafna, Hopkins, Kaufman, and Lovett (STOC 2022). Our proof is based on two elementary new ideas in the theory of high dimensional expansion. First we introduce `$q$-norm HDX', generalizing standard spectral notions to higher moments, and observe every spectral HDX is a $q$-norm HDX. Second, we introduce a simple method of coordinate-wise analysis on HDX which breaks high dimensional random walks into coordinate-wise components, and allows each component to be analyzed as a $1$-dimensional operator locally within $X$. This allows for application of standard tricks such as the replacement method, greatly simplifying prior analytic techniques.