@article{1077374788, author = {Crowell, R. H.}, title = {The group $G'/G''$ of a knot group $G$}, journal = {Duke Math. J.}, volume = {30}, number = {1}, year = {1963}, pages = { 349-354}, language = {en}, url = {http://dml.mathdoc.fr/item/1077374788} }
Crowell, R. H. The group $G'/G''$ of a knot group $G$. Duke Math. J., Tome 30 (1963) no. 1, pp. 349-354. http://gdmltest.u-ga.fr/item/1077374788/