1. 1. M. I. Graev, "Free topological groups," Izvestiya Akad. Nauk SSSR. Ser. Mat., 12, No. 3, 279-324 (1948) (in Russian).
2. 2. I. Yo. Guran, M. M. Zarichnyi, Elements of the Theory of Topological Groups [in Ukrainian], NMK VO, Kyiv (1991).
3. 3. A. Markoff, "On free topological groups," C. R. (Doklady) Acad. Sci. URSS (N. S.), 31, No. 4, 299-301 (1941) (in Russian).
4. 4. O. G. Okunev, "M-equivalence of products," Trudy Moskov. Mat. Obshch., 56, 192-205 (1995) (in Russian).
5. 5. N. M. Pyrch, "Constructions preserving M-equivalence," Visn. Nats. Univ. "L'viv Polytechnic". Fiz.-Mat. Nauky, 625, 48-53 (2008) (in Ukrainian).