1. M. I. Kargapolov and Yu. I. Merzlyakov, Fundamentals of Group Theory [in Russian], 4th edn., Nauka, Moscow (1996).
2. M. Krasner and L. Kaloujnine, “Produit complet des groupes de permutation et le probléme d’extension de groups, III,” Acta Sci. Math., Szeged, 14, 69-82 (1951).
3. A. Yu. Olshanskii, “On Kaluzhnin–Krasner’s embedding of groups,” Alg. Discr. Math., 19, No. 1, 77-86 (2015).
4. H. Neumann, Varieties of Groups, Ergebn. Math. Grenzg., 37, Springer, Berlin (1967).
5. G. Higman, “Some remarks on varieties of groups,” Quart. J. Math. Oxford, II. Ser., 10, 165-178 (1959).