@article{MZ_1962__78_170040,
     author = {Gilbert Baumslag},
     title = {On generalised free products.},
     journal = {Mathematische Zeitschrift},
     pages = {423--438},
     publisher = {mathdoc},
     volume = {78},
     year = {1962},
     zbl = {0104.24402},
     url = {http://geodesic.mathdoc.fr/item/MZ_1962__78_170040/}
}