Sharp quantitative nonembeddability of the Heisenberg group into superreflexive Banach spaces

Abstract

Let $\H$ denote the discrete Heisenberg group, equipped with a word metric $d_W$ associated to some finite symmetric generating set. We show that if $(X,\Vert \cdot \Vert )$ is a $p$-convex Banach space then for any Lipschitz function $f:\stackrel{˝}{\to }X$ there exist $x,y\in \H$ with $d_W(x,y)$ arbitrarily large and \begin{equation}\label{eq:comp abs} \frac{\|f(x)-f(y)\|}{d_W(x,y)}\lesssim \left(\frac{\log\log d_W(x,y)}{\log d_W(x,y)}\right)^{1/p}. \end{equation} We also show that any embedding into $X$ of a ball of radius $R\ge 4$ in $\H$ incurs bi-Lipschitz distortion that grows at least as a constant multiple of \begin{equation}\label{eq:dist abs} \left(\frac{\log R}{\log\log R}\right)^{1/p}. \end{equation} Both~\eqref{eq:comp abs} and~\eqref{eq:dist abs} are sharp up to the iterated logarithm terms. When $X$ is Hilbert space we obtain a representation-theoretic proof yielding bounds corresponding to~\eqref{eq:comp abs} and~\eqref{eq:dist abs} which are sharp up to a universal constant.

References in corpus (2)

• Differentiating maps into L^1 and the geometry of BV functions
• Compression bounds for Lipschitz maps from the Heisenberg group to $L_1$

Cited by in corpus (1)

• Vertical versus horizontal Poincaré inequalities on the Heisenberg group