1. K. W. Gruenberg, ‘‘Residual properties of infinite soluble groups,’’ Proc. London Math. Soc. 7, 29–62 (1957).
2. E. V. Sokolov, ‘‘A characterization of root classes of groups,’’ Commun. Algebra 43, 856–860 (2015).
3. D. N. Azarov and D. Tieudjo, ‘‘On the root-class residuality of a free product of groups with an amalgamated subgroup,’’ Nauch. Tr. Ivanovsk. Univ., Mat. 5, 6–10 (2002).
4. D. Tieudjo, ‘‘On root-class residuality of some free constructions,’’ JP J. Algebra, Number Theory Appl. 18, 125–143 (2010).
5. E. A. Tumanova, ‘‘Certain conditions of the root-class residuality of generalized free products with a normal amalgamated subgroup,’’ Chebyshev. Sb. 14, 134–141 (2013).