P. M. Cohn, Free Rings and Their Relations, 1985.

S. Fröhler and U. Oberst, Continuous time-varying linear systems, Systems & Control Letters, vol.35, pp.97-110, 1998.

E. R. Gentile, On rings with one-sided field of quotients, Proc. Amer. Math. Soc, vol.11, issue.13, pp.380-384, 1960.

T. Y. Lam, Lectures on Modules and Rings, 1999.

J. C. Mcconnell and J. C. Robson, Noncommutative Noetherian Rings, cor, 2001.