@article{MZ_1950__53_169184,
     author = {G. BIRKHOFF},
     title = {Extensions of {Lie} {Groups}},
     journal = {Mathematische Zeitschrift},
     pages = {226--235},
     year = {1950},
     volume = {53},
     zbl = {0039.02003},
     url = {http://geodesic.mathdoc.fr/item/MZ_1950__53_169184/}
}