1. Hawkes T. O., “On formation subgroups of a finite soluble group,” J. London Math. Soc., 4, 243–250 (1969).
2. Shemetkov L. A., Formations of Finite Groups [in Russian], Nauka, Moscow (1978).
3. Kegel O. H., “Untergruppenverbande endlicher Gruppen, die den Subnormalteilerverband echt enthalten,” Arch. Math., 30, No. 3, 225–228 (1978).
4. Kamornikov S. F. and Sel’kin V. M., Subgroup Functors and Classes of Finite Groups [in Russian], Belarusskaya Nauka, Minsk (2003).
5. Ballester-Bolinches A. and Ezquerro L. M., Classes of Finite Groups, Springer-Verlag, Dordrecht (2006).