@article{MAN_1979__245_163311,
     author = {James Lepowsky and Robert V. Moody},
     title = {Hyperbolic {Lie} {Algebras} and {Quasi-Regular} {Cusps} on {Hilbert}  {Modular} {Surfaces.}},
     journal = {Mathematische Annalen},
     pages = {63--88},
     publisher = {mathdoc},
     volume = {245},
     year = {1979},
     zbl = {0396.17007},
     url = {http://geodesic.mathdoc.fr/item/MAN_1979__245_163311/}
}