The KKL inequality and Rademacher type 2

Abstract

We show that a vector-valued Kahn--Kalai--Linial inequality holds in every Banach space of Rademacher type 2. We also show that for any nondecreasing function $h\geq 0$ with $0<\int_{1}^{\infty}\frac{h(t)}{t^{2}}\mathrm{dt}<\infty$ we have the inequality \begin{align*} \|f - \mathbb{E}f\|_2 \leq 12 \, T_{2}(X) \left(\int_{1}^{\infty}\frac{h(t)}{t^{2}} \mathrm{dt} \right)^{1/2} \, \left(\sum_{j=1}^n \frac{\|D_j f\|^{2}_2}{h\left( \log \frac{\|D_j f\|_2}{\|D_j f\|_1} \right)}\right)^{1/2} \end{align*} for all $f :\{-1,1\}^{n} \to X$ and all $n\geq 1$, where $X$ is a normed space and $T_{2}(X)$ is the associated type 2 constant.