1. V. P. Zarovnyi, “Automaton substitutions and interlacements of groups,” Dokl. Akad. Nauk SSSR, No. 3, 562–565 (1965).
2. V. I. Sushchanskii and V. S. Sikora, Operations over Substitution Groups [in Ukrainian], Ruta, Chernovtsy (2003).
3. M. Hall, The Theory of Groups [Russian translation], Izd. Inostr. Lit., Moscow (1962).