@article{MZ_1964__85_170282,
     author = {Ulrich Christian},
     title = {\"Uber die {Modulgruppe} zweiten {Grades} {I.}},
     journal = {Mathematische Zeitschrift},
     pages = {1--28},
     publisher = {mathdoc},
     volume = {85},
     year = {1964},
     zbl = {0143.30404},
     url = {http://geodesic.mathdoc.fr/item/MZ_1964__85_170282/}
}