@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/}
}