@article{IM_1972__17_142170,
     author = {H. Helling},
     title = {Diskrete {Untergruppen} von {SL2} {(IR).}},
     journal = {Inventiones mathematicae},
     pages = {217--230},
     publisher = {mathdoc},
     volume = {17},
     year = {1972},
     url = {http://geodesic.mathdoc.fr/item/IM_1972__17_142170/}
}