Radial rapid decay does not imply rapid decay

TL;DR

This paper introduces a new dynamical criterion for the radial rapid decay property, demonstrating that certain groups have RRD without having RD, and explores implications for lattices in semisimple Lie groups.

Contribution

It provides a novel criterion for radial rapid decay and applies it to specific groups, showing RRD does not imply RD and that RRD isn't inherited by open subgroups.

Findings

Group $ ext{SL}_2(A)$ has RRD but not RD.

The criterion applies to lattices in semisimple Lie groups.

RRD property isn't inherited by open subgroups.

Abstract

We provide a new, dynamical criterion for the radial rapid decay property. We work out in detail the special case of the group $\Gamma := \mathbf{SL}_2(A)$, where $A := \mathbb{F}_q[X,X^{-1}]$ is the ring of Laurent polynomials with coefficients in $\mathbb{F}_q$, endowed with the length function coming from a natural action of $\Gamma$ on a product of two trees, to show that is has the radial rapid decay (RRD) property and doesn't have the rapid decay (RD) property. The criterion also applies to irreducible lattices in semisimple Lie groups with finite center endowed with a length function defined with the help of a Finsler metric. These examples answer a question asked by Chatterji and moreover show that, unlike the RD property, the RRD property isn't inherited by open subgroups.