Radial rapid decay does not imply rapid decay

Abstract

We provide a new, dynamical criterion for the radial rapid decay property. We work out in detail the special case of the group Γ := SL 2 (A), where A := F q [X, X −1 ] is the ring of Laurent polynomials with coefficients in F q , endowed with the length function coming from a natural action of Γ 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. Contents 12 5.1. Structure of the proof 12 5.2. The Koopman representation on the product of the boundaries 12 5.3. Estimates on the growth of Γ 13 5.4. Estimates on the Harish-Chandra function 15 5.5. Comparing the decay of the Harish-Chandra function with the volume growth in the group SL 2 (F q [X, X −1 ]) 16 6. Proof of Corollary 2 17 6.1. RD and amenable subgroups of exponential growth 17 6.2. The lamplighter subgroup 17 References 18