1. G. Glauberman, “Central elements in core-free groups,” J. Alg., 4, No. 3, 403-420 (1966).
2. D. Gorenstein, Finite Simple Groups, An Introduction to Their Classification, Plenum, New York (1982).
3. Unsolved Problems in Group Theory, The Kourovka Notebook, 17th edn., Institute of Mathematics SO RAN, Novosibirsk (2010), http://www.math.nsc.ru/~alglog/17kt.pdf .
4. S. I. Adyan, The Burnside Problem and Identities in Groups [in Russian], Nauka, Moscow (1975).
5. A. Yu. Ol’shanskii, The Geometry of Defining Relations in Groups [in Russian], Nauka, Moscow (1989).