Fourier level inequalities on the discrete cube via complex hypercontractivity and Riesz transforms

Abstract

Eldan's level-$1{:}3$ inequality is a central tool in his second-order correlation inequalities for increasing Boolean functions. His proof is going to Gaussian space and uses an entropy estimate for second Hermite tensors. We give a direct proof on the discrete hypercube. For increasing real-valued functions $f,g:\{0,1\}^n\to\mathbb R$, we prove $$ W_3[f,g]\ge -2W_1[f,g]\log\!\left(\frac{\sqrt{\mathrm{Var}(f)\mathrm{Var}(g)}}{W_1[f,g]}\right). $$ The proof combines complex hypercontractivity with the discrete Riesz transforms. We also give a short proof of a level-$1:d$ inequality obtained by Keller and Kindler. As an application, we recover Eldan's correlation inequality with a slight improvement. If $f,g:\{0,1\}^n\to\{0,1\}$ are increasing and $g$ is antipodal, then $$ \mathrm{Cov}(f,g)\ge \frac1{24}\,\frac{\sum_i\mathrm{Inf}_i[f]\mathrm{Inf}_i[g]}{\sqrt{\log\!\left(4e\sqrt{\mathrm{Var}(f)\mathrm{Var}(g)}/(\sum_i\mathrm{Inf}_i[f]\mathrm{Inf}_i[g])\right)}}, $$ where the square-root logarithm cannot be improved as shown in the OR--majority example.