@article{chen58,
 ISSN = {0003486X},
 URL = {http://www.jstor.org/stable/1970044},
 author = {K. T. Chen and R. H. Fox and R. C. Lyndon},
 journal = {Annals of Mathematics},
 number = {1},
 pages = {81--95},
 publisher = {Annals of Mathematics},
 title = {Free Differential Calculus, IV. The Quotient Groups of the Lower Central Series},
 volume = {68},
 year = {1958},
 doi = {https://doi.org/10.2307/1970044}
}