1. The orders of relatively free groups;Higman, Graham,1967
2. A remark on finitely generated nilpotent groups;Higman, Graham;Proc. Amer. Math. Soc.,1955
3. Varieties of groups;Neumann, Hanna,1967
4. Basic commutators;Ward, M. A.;Philos. Trans. Roy. Soc. London Ser. A,1969