1. Algebraic division ring extensions;Faith,1960
2. Non-commutative Rings;Herstein,1968
3. I. N. Herstein, On the hypercenter of a ring, J. Algebra, to appear.
4. Structure of Rings;Jacobson,1964
5. Rings that are radical over commutative subrings;Lihtman;Mat. Sb.,1970