1. Finite presentations of groups and 3-manifolds;Epstein, D. B. A.;Quart. J. Math. Oxford Ser. (2),1961
2. Free differential calculus. I. Derivation in the free group ring;Fox, Ralph H.;Ann. of Math. (2),1953
3. A quick trip through knot theory;Fox, R. H.,1962
4. K. A. Kurosh, The theory of groups, Vol. 2, Chelsea, New York, 1955.
5. The center of a group with a single defining relation;Murasugi, Kunio;Math. Ann.,1964