@article{MZ_1991__207_2_174291,
     author = {Ulrich Christian},
     title = {Arithmetische {Eigenschaften} hyperbolischer {Klassen} elliptischer {Modulgruppen.}},
     journal = {Mathematische Zeitschrift},
     pages = {597--618},
     publisher = {mathdoc},
     volume = {207},
     number = {2},
     year = {1991},
     zbl = {0741.11027},
     url = {http://geodesic.mathdoc.fr/item/MZ_1991__207_2_174291/}
}