@article{MS2_1963__12_165846,
     author = {Roger C. Lyndon},
     title = {Length {Functions} in {Groups.}},
     journal = {Mathematica Scandinavica},
     pages = {209--234},
     publisher = {mathdoc},
     volume = {12},
     year = {1963},
     zbl = {0119.26402},
     url = {http://geodesic.mathdoc.fr/item/MS2_1963__12_165846/}
}