Onishchuk , V. A., and Ya. P. Sysak. “Local Nilpotent Groups Satisfying a Weak Condition of Minimality or Maximality for Subgroups of Fixed Degree of Nilpotency ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 44, no. 3, Apr. 1992, pp. 384-9, https://umj.imath.kiev.ua/index.php/umj/article/view/8107.