PISKUN, M. M.; SEMKO, N. N. On some generalizations of nearly normal subgroups. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 61, n. 10, p. 1381–1395, 2009. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3108. Acesso em: 14 apr. 2025.