Piskun, M. M., and N. N. Semko. “On Some Generalizations of Nearly Normal Subgroups”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 10 (October 25, 2009): 1381–1395. Accessed April 14, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/3108.