Skiba, A. N., and L. A. Shemetkov. “Multiply $\mathfrak{L}$ Formations of Finite Groupsformations of Finite Groups”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 52, no. 6, June 2000, pp. 783-7, https://umj.imath.kiev.ua/index.php/umj/article/view/4473.