@article{GDZPPN002398176, title = {On Characteristic Subgroups of Free Groups.}, journal = {Mathematische Zeitschrift}, volume = {93}, year = {1966}, pages = {143-151}, zbl = {0143.03603}, url = {http://dml.mathdoc.fr/item/GDZPPN002398176} }
Neumann, B.H. On Characteristic Subgroups of Free Groups.. Mathematische Zeitschrift, Tome 93 (1966) pp. 143-151. http://gdmltest.u-ga.fr/item/GDZPPN002398176/