SkibaA. N., and ShemetkovL. A. 2000. “Multiply $\mathfrak{L}$ Formations of Finite Groupsformations of Finite Groups”. Ukrains’kyi Matematychnyi Zhurnal 52 (6), 783-87. https://umj.imath.kiev.ua/index.php/umj/article/view/4473.