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@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/} }
John R. Stallings. A topological proof of Grushko's theorem on free products.. Mathematische Zeitschrift, Tome 90 (1965), pp. 1-8. http://geodesic.mathdoc.fr/item/MZ_1965__90_170458/