Onishchuk , V. A., and Ya. P. Sysak. 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): 384-89. https://umj.imath.kiev.ua/index.php/umj/article/view/8107.