@article{MAN_1971__191_162122,
     author = {J.S. BIRMAN},
     title = {On {Siegel's} {Modular} {Group.}},
     journal = {Mathematische Annalen},
     pages = {59--68},
     publisher = {mathdoc},
     volume = {191},
     year = {1971},
     zbl = {0208.10601},
     url = {http://geodesic.mathdoc.fr/item/MAN_1971__191_162122/}
}