Nilpotent groups and biLipschitz embeddings into $L^1$

TL;DR

This paper proves that simply connected nilpotent Lie groups that biLipschitz embed into L^1 must be abelian, extending to Carnot groups, by analyzing cut measures and tangent structures to rule out non-abelian embeddings.

Contribution

It establishes that non-abelian nilpotent groups cannot biLipschitz embed into L^1, showing all such embeddable groups are necessarily abelian, using a novel blow-up and tangent analysis approach.

Findings

Non-abelian nilpotent groups do not embed into L^1 biLipschitzly.

BiLipschitz embeddability into L^1 implies the group is abelian.

The proof uses cut measures and tangent space analysis to reach these conclusions.

Abstract

We prove that if a simply connected nilpotent Lie group quasi-isometrically embeds into an $L^1$ space, then it is abelian. We reach this conclusion by proving that every Carnot group that biLipschitz embeds into $L^1$ is abelian. Our proof follows the work of Cheeger and Kleiner, by considering the pull-back distance of a Lipschitz map into $L^1$ and representing it using a cut measure. We show that such cut measures, and the induced distances, can be blown up and the blown-up cut measure is supported on "generic" tangents of the original sets. By repeating such a blow-up procedure, one obtains a cut measure supported on half-spaces. This differentiation result then is used to prove that bi-Lipschitz embeddings can not exist in the non-abelian settings.