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@article{MZ_1966__94_170616, author = {B.H. Neumann}, title = {On {Characteristic} {Subgroups} of {Free} {Groups.}}, journal = {Mathematische Zeitschrift}, pages = {143--151}, publisher = {mathdoc}, volume = {94}, year = {1966}, zbl = {0143.03603}, url = {http://geodesic.mathdoc.fr/item/MZ_1966__94_170616/} }
B.H. Neumann. On Characteristic Subgroups of Free Groups.. Mathematische Zeitschrift, Tome 94 (1966), pp. 143-151. http://geodesic.mathdoc.fr/item/MZ_1966__94_170616/