@article{GDZPPN002343665, title = {A basis for subgroups of free groups}, journal = {Mathematica Scandinavica}, volume = {3}, year = {1955}, pages = {31-43}, zbl = {0066.01202}, url = {http://dml.mathdoc.fr/item/GDZPPN002343665} }
NIELSEN, JACOB. A basis for subgroups of free groups. Mathematica Scandinavica, Tome 3 (1955) pp. 31-43. http://gdmltest.u-ga.fr/item/GDZPPN002343665/