1. W. Gaschutz, ‘‘Zur Theorie der endlichen auflosbaren Gruppen,’’ Math. Z. 80, 300–305 (1963).
2. L. A. Shemetkov, ‘‘Stepped formations of groups,’’ Math. Sb. 94, 628–648 (1974).
3. B. Hartley, ‘‘On Fischer’s dualization of formation theory,’’ Proc. London Math. Soc. 3 (9), 193–207 (1969).
4. L. A. Shemetkov, Formations of Finite Groups (Nauka, Moscow, 1978) [in Russian].
5. L. A. Shemetkov and A. N. Skiba, Formations of Algebraic Systems (Nauka, Moscow, 1989) [in Russian].