Vasil’ev, V. A., and A. N. Skiba. “On One Generalization of Modular Subgroups”. Ukrains’kyi Matematychnyi Zhurnal 63, no. 10 (October 25, 2011): 1314-1325. Accessed April 25, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2808.