1. Mal’tsev A. I., “On free solvable groups,” Dokl. Akad. Nauk SSSR, 130, No. 3, 495–498 (1960).
2. Timoshenko E. I., Endomorphisms and Universal Theories of Soluble Groups [in Russian], Izdat. NGTU, Novosibirsk (2011).
3. Gupta Ch. K. and Timoshenko E. I., “Criterion for invertibility of endomorphisms and test rank of metabelian products of abelian groups,” Algebra and Logic, 43, No. 5, 316–326 (2004).
4. Romanovskiĭ N. S., “A Freiheitssatz for products of groups,” Algebra and Logic, 38, No. 3, 193–201 (1999).