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A Theorem à la Fatou for the Square Root of Poisson Kernel in \(\delta \)-Hyperbolic Spaces and Decay of Matrix Coefficients of Boundary Representations

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Abstract

In this note, we prove a theorem à la Fatou for the square root of Poisson Kernel in the context of quasi-convex cocompact discrete groups of isometries of \(\delta \)-hyperbolic spaces. As a corollary we show that some matrix coefficients of boundary representations cannot satisfy the weak inequality of Harish-Chandra. Nevertheless, such matrix coefficients satisfy an inequality which can be viewed as a particular case of the inequality coming from property RD for boundary representations. The inequality established in this paper is based on a uniform bound which appears in the proof of the irreducibility of boundary representations. Moreover this uniform bound can be used to prove that the Harish-Chandra’s Schwartz space associated with some discrete groups of isometries of \(\delta \)-hyperbolic spaces carries a natural structure of a convolution algebra. Then in the context of CAT(−1) spaces we show how our elementary techniques enable us to apply an equidistribution theorem of Roblin to obtain information about the decay of matrix coefficient of boundary representations associated with continuous functions.

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Acknowledgements

I would like to thank Nigel Higson who suggested that I investigate the structure of convolution algebra of Harish-Chandra’s Schwartz space of discrete groups. I would like also to thank Uri Bader and Amos Nevo for valuable discussions. And I am grateful to Felix Pogorzelski and Dustin Mayeda for their remarks and comments on this note. This work is supported by ERC Grant 306706.

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  1. Technion, Haifa, Israel

    Adrien Boyer

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  1. Adrien Boyer
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Boyer, A. A Theorem à la Fatou for the Square Root of Poisson Kernel in \(\delta \)-Hyperbolic Spaces and Decay of Matrix Coefficients of Boundary Representations. J Geom Anal 28, 284–316 (2018). https://doi.org/10.1007/s12220-017-9820-5

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