1. Ault, J. C., Watters, J. F.: Circle groups of nilpotent rings. Amer. math. Monthly80, 48?52 (1973)
2. Bachmann, F., Grünenfelder, L.: Homological methods and the third dimension subgroup. Commentarii math. Helvet.47, 526?531 (1972)
3. Bergman, G. M., Dicks, W.: On universal derivations. To appear in J. Algebra
4. Bovdi, A. A.: Group Rings (Russian) U?gorod: U?gorodskiî Gosudarstvennyî Universitet 1974
5. Eldridge, K. E.: On rings and groups of units. In: Coll. Math. Soc. János Bolyai6. Rings, Modules and Radicals. Keszthely (Hungary), 1971, pp. 171?191 (A. Kertész, ed.). Amsterdam: North Holland 1973