PiskunM. M., and SemkoN. N. “On Some Generalizations of Nearly Normal Subgroups”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 10 (October 25, 2009): 1381-1395. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3108.