1. S. Oates and M. B. Powell, “Identical relations in finite groups,” J. Algebra,1, 11–39 (1964).
2. H. Neumann, Varieties of Groups, Springer-Verlag (1967).
3. N. Jacobson, Structure of Rings, Am. Math. Soc. (1964).
4. A. I. Mal'tsev, Algebraic Systems [in Russian], Nauka, Moscow (1970).
5. A. G. Kurosh, "On the multiplication of classes of algebraic systems," Sibirsk. Matem. Zh.