@article{IM_1977__42_142497,
     author = {G. van der Geer and D. Zagier},
     title = {The {Hilbert} {Modular} {Group} for the {Field} ...(...13).},
     journal = {Inventiones mathematicae},
     pages = {93--134},
     publisher = {mathdoc},
     volume = {42},
     year = {1977},
     zbl = {0366.10024},
     url = {http://geodesic.mathdoc.fr/item/IM_1977__42_142497/}
}