1. Neumann H., Varieties of Groups, Springer-Verlag, Berlin, Heidelberg, and New York (1967).
2. Mal’cev A. I., Algebraic Systems, Springer-Verlag and Akademie-Verlag, Berlin, Heidelberg, and New York (1973).
3. Smirnov D. M., “Varieties and quasivarieties of algebras,” in: Proc. Colloq. Universal Algebra (Esztergom, Hungary, 1977). Colloq. Math. Soc. János Bolyai, North-Holland, Amsterdam and New York, 1982, vol. 29, 745–751.
4. Budkin A. I. and Gorbunov V. A., “Implicative classes of algebras,” Algebra and Logic, vol. 12, no. 3, 139–149 (1973).
5. Mazurov V. D. and Khukhro E. I. (Eds.), The Kourovka Notebook: Unsolved Problems in Group Theory, vol. 18, Sobolev Inst. Math., Novosibirsk (2014).