@article{SF_1992__45_3_135200,
     author = {G. Duchamp and D. Krob},
     title = {The lower central series of the free partially commutative  group.},
     journal = {Semigroup forum},
     pages = {385--394},
     year = {1992},
     volume = {45},
     number = {3},
     zbl = {0814.20025},
     url = {http://geodesic.mathdoc.fr/item/SF_1992__45_3_135200/}
}