@article{MZ_1966__92_170554,
     author = {Heinz Helling},
     title = {Bestimmung der {Kommensurabilit\"atsklasse} der {Hilbertschen} {Modulgruppe.}},
     journal = {Mathematische Zeitschrift},
     pages = {269--280},
     publisher = {mathdoc},
     volume = {92},
     year = {1966},
     zbl = {0143.30601},
     url = {http://geodesic.mathdoc.fr/item/MZ_1966__92_170554/}
}