@article{GDZPPN002347326, title = {Length Functions in Groups.}, journal = {Mathematica Scandinavica}, volume = {13}, year = {1963}, pages = {209-234}, zbl = {0119.26402}, url = {http://dml.mathdoc.fr/item/GDZPPN002347326} }
Lyndon, Roger C. Length Functions in Groups.. Mathematica Scandinavica, Tome 13 (1963) pp. 209-234. http://gdmltest.u-ga.fr/item/GDZPPN002347326/