@article{MZ_1988__197_1_183720,
     author = {Heiner Zieschang and Donald J. Collins},
     title = {On the {Nielsen} {Method} in {Free} {Products} with {Amalgamated} {Subgroups.}},
     journal = {Mathematische Zeitschrift},
     pages = {97--118},
     publisher = {mathdoc},
     volume = {197},
     number = {1},
     year = {1988},
     zbl = {0617.20011},
     url = {http://geodesic.mathdoc.fr/item/MZ_1988__197_1_183720/}
}