@article{CMH_1968__43_139357,
     author = {U. Stammbach},
     title = {\"Uber freie {Untergruppen} gegebener {Gruppen.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {132--136},
     year = {1968},
     volume = {43},
     zbl = {0175.29503},
     url = {http://geodesic.mathdoc.fr/item/CMH_1968__43_139357/}
}