Kuzenny , N. F., Levishchenko , S. S., & Semko , N. N. . (1988). On groups with invariant infinite non-Abelian subgroups. Ukrains’kyi Matematychnyi Zhurnal, 40(3), 314-321. https://umj.imath.kiev.ua/index.php/umj/article/view/9141