@article{MAN_1976__221_162834,
     author = {Paul E. Schupp},
     title = {A {Strengthened} {Freiheitssatz.}},
     journal = {Mathematische Annalen},
     pages = {73--80},
     year = {1976},
     volume = {221},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/MAN_1976__221_162834/}
}