Uniform estimates for spherical functions on complex semisimple Lie groups

Abstract

We prove new estimates for spherical functions and their derivatives on complex semisimple Lie groups, establishing uniform polynomial decay in the spectral parameter. This improves the customary estimate arising from Harish-Chandra's series expansion, which gives only a polynomial growth estimate in the spectral parameter. In particular, for arbitrary positive-definite spherical functions \( \varphi_\lambda \) on higher rank complex simple groups, we establish estimates for \( |d^k\varphi_\lambda(a_t)/dt^k| \) which are of the form \( (1\,+ \parallel \lambda \parallel)^{k-\gamma} \) in the spectral parameter \( \lambda \) and have uniform exponential decay in regular directions in the group variable a t . Here \( \gamma \) is an explicit constant depending on G, and \( \lambda \) may be singular, for instance.¶The uniformity of the estimates is the crucial ingredient needed in order to apply classical spectral methods and Littlewood—Paley—Stein square functions to the analysis of singular integrals in this context. To illustrate their utility, we prove maximal inequalities in L p for singular sphere averages on complex semisimple Lie groups for all p in \( (1\,+ {1 \over 2\gamma}, \infty) \). We use these to establish singular differentiation theorems and pointwise ergodic theorems in L p for the corresponding singular spherical averages on locally symmetric spaces, as well as for more general measure preserving actions.