Skiba, A. N., and L. A. Shemetkov. “Multiply $\mathfrak{L}$ Formations of Finite Groupsformations of Finite Groups”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 6 (June 25, 2000): 783-787. Accessed May 10, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4473.