1. Bergman, G.M.: Modules over coproducts of rings. University of California (Berkeley) preprint.
2. Cohn, P.M.: The embedding of firs in skew fields. Proc. London Math. Soc. III. Ser.23, 193?213 (1971).
3. Cohn, P.M.: Free rings and their relations. London-New York: Academic Press, 1971.
4. Cohn, P. M.: Algebra I. London-New York: J. Wiley and Sons 1973/74.
5. Jacobson, N.: Theory of rings. Providence, Amer. Math. Soc. 1943.