1. V. V. Kabanov, A. A. Makhnev, and A. I. Starostin, Algebra i Logika 15(6), 655 (1976).
2. Unsolved Problems of Group Theory. The Kourovka Notebook. Issue 15, Ed. by V. D. Mazurov and E. I. Khukhro (Novosib. Gos. Univ., Novosibirsk, 2002) [in Russian].
3. T. Yu. Voitenko and V. M. Levchuk, in Symmetry and Differential Equations (Inst. of Comp. Modeling, Krasnoyarsk, 2000), pp. 71–74 [in Russian].
4. T. Yu. Voitenko and V. M. Levchuk. Ukr. Mat. Zh. 54(7), 749 (2002).
5. V. M. Levchuk, Algebra i Logika 41(6), 730 (2002).