1. Rings without nilpotent elements;Abian;Mat. Časopis Sloven Akad. Vied,1975
2. Nil semigroups of rings with a polynomial identity;Amitsur;Nagoya Math. J.,1966
3. Zero-divisors in tensor products;Bergman,1976
4. On a question of Mal'cev;Bowtell;J. Algebra,1967
5. Free Rings and Their Relations;Cohn,1971