@article{MZ_1952__56_169297,
     author = {Hermann Haken},
     title = {Zum {Identit\"atsproblem} bei {Gruppen.}},
     journal = {Mathematische Zeitschrift},
     pages = {335--362},
     publisher = {mathdoc},
     volume = {56},
     year = {1952},
     zbl = {0048.01301},
     url = {http://geodesic.mathdoc.fr/item/MZ_1952__56_169297/}
}