The asymptotic distribution of orbits for discrete subgroups of motions in Euclidean and non-Euclidean spaces are found; our principal tool is the wave equation. The results are new for the crystallographic groups in Euclidean space and for those groups in non-Euclidean spaces which have fundamental domains of infinite volume. In the latter case we show that the only point spectrum of the Laplace-Beltrami operator lies in the interval (-( (m - 1) 2)2,0]; furthermore we show that when the subgroup is nonelementary and the fundamental domain has a cusp, then there is at least one eigenvalue in this interval. © 1982.
Lax, P. D., & Phillips, R. S. (1982). The asymptotic distribution of lattice points in Euclidean and non-Euclidean spaces. Journal of Functional Analysis, 46(3), 280–350. https://doi.org/10.1016/0022-1236(82)90050-7