@article{MZ_1965__90_170458,
     author = {John R. Stallings},
     title = {A topological proof of {Grushko's} theorem on free products.},
     journal = {Mathematische Zeitschrift},
     pages = {1--8},
     publisher = {mathdoc},
     volume = {90},
     year = {1965},
     zbl = {0135.04603},
     url = {http://geodesic.mathdoc.fr/item/MZ_1965__90_170458/}
}