1. R. H. Fox, “Free differential calculus. I,” Ann. Math. 57, 517–559 (1953).
2. W. Magnus, A. Karrass, and D. Solitar, Combinatorial Group Theory (Interscience, New York, 1966).
3. W. Magnus, “On a theorem of Marshall Hall,” Ann. Math. 40, 764–768 (1939).
4. W. Magnus, “Untersuchungen über einige unendliche diskontinuierliche Gruppen,” Math. Ann. 105, 52–74 (1931).
5. P. H. Kropholler, P. A. Linnell, and J. A. Moody, “Applications of a new K-theoretic theorem to soluble group rings,” Proc. Am. Math. Soc. 104, 675–684 (1988).