@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/