Piskun, M. M., & Semko, N. N. (2008). On the application of some concepts of ring theory to the study of the influence of systems of subgroups of a group. Ukrains’kyi Matematychnyi Zhurnal, 60(5), 657–668. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3184