@article{MZ_1930__32_168228,
     author = {F., Levi},
     title = {\"Uber die {Untergruppen} freier {Gruppen}},
     journal = {Mathematische Zeitschrift},
     pages = {315--318},
     publisher = {mathdoc},
     volume = {32},
     year = {1930},
     zbl = {56.0134.01},
     url = {http://geodesic.mathdoc.fr/item/MZ_1930__32_168228/}
}