Onishchuk , V. A., & Sysak , Y. P. . (1992). Local nilpotent groups satisfying a weak condition of minimality or maximality for subgroups of fixed degree of nilpotency . Ukrains’kyi Matematychnyi Zhurnal, 44(3), 384-389. https://umj.imath.kiev.ua/index.php/umj/article/view/8107