@article{MZ_1978__159_172652,
     author = {Hans Rudolf Schneebeli},
     title = {On {Virtual} {Properties} and {Group} {Extensions.}},
     journal = {Mathematische Zeitschrift},
     pages = {159--168},
     publisher = {mathdoc},
     volume = {159},
     year = {1978},
     zbl = {0358.20048},
     url = {http://geodesic.mathdoc.fr/item/MZ_1978__159_172652/}
}