@article{IM_1990__101_2_143820,
     author = {J. Mennicke and J. Elstrodt and F. Grunewald},
     title = {Kloosterman sums for {Clifford} algebras and a lower bound for the positive eigenvalues of the {Laplacian} for congruence subgroups acting on hyperbolic spaces.},
     journal = {Inventiones mathematicae},
     pages = {641--686},
     publisher = {mathdoc},
     volume = {101},
     number = {2},
     year = {1990},
     zbl = {0737.11013},
     url = {http://geodesic.mathdoc.fr/item/IM_1990__101_2_143820/}
}