@article{MAN_1931__105_159515,
     author = {F. Klein},
     title = {Zur {Theorie} der abstrakten {Verkn\"upfungen}},
     journal = {Mathematische Annalen},
     pages = {308--324},
     publisher = {mathdoc},
     volume = {105},
     year = {1931},
     zbl = {57.0057.01},
     url = {http://geodesic.mathdoc.fr/item/MAN_1931__105_159515/}
}