1. F. Hall, Matematika 12(1), 3–36 (1968).
2. M. I. Kargapolov, V. P. Remeslennikov, N. S. Romonovskii, et al., Algebra Logika 8(6), 643–659 (1969).
3. G. A. Myasnikov and V. A. Remeslennikov, Dokl. Akad. Nauk SSSR 258, 1056–1059 (1981).
4. Kh. Kh. Magomaev, Izv. Vyssh. Uchebn. Zaved., Mat., No. 4, 50–58 (1971).
5. A. D. Tavadze, Soobshch. Akad. Nauk Gruz. SSR 79(2), 301–304 (1975).