Poincaré series have always played a central rôle in the theory of automorphic functions and harmonic analysis on manifolds of constant negative curvature. The trace formulae approach was introduced by Selberg [20] in 1956 and has become the key to their analysis. However, with the advent of a more geometrical viewpoint (through the work of Patterson [15, 16], Sullivan [21, 22] and others) it b...
