  This note will prove a discreteness criterion for groups of
orientation-preserving isometries of the hyperbolic space which contain a
parabolic element. It can be viewed as a generalization of the well-known
results of Shimizu-Leutbecher and Jorgensen in dimensions 2 and 3, and is
closely related to Waterman's inequality in higher dimensions. Unlike his
algebraic method, the argument presented here is geometric and yields an
improved asymptotic bound.
