@article{MZ_1949__51_169073,
     author = {H. MAASS},
     title = {\"Uber die {Erweiterungsf\"ahigkeit} der {Hilbertschen} {Modulgruppe}},
     journal = {Mathematische Zeitschrift},
     pages = {255--261},
     publisher = {mathdoc},
     volume = {51},
     year = {1949},
     zbl = {0031.00701},
     url = {http://geodesic.mathdoc.fr/item/MZ_1949__51_169073/}
}