Khmelnitskii, N. A., and N. S. Chernikov. “Locally Nilpotent Groups With Weak Conditions of π-Layer Minimality and π-Layer Maximality”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 7 (July 25, 2002): 991–997. Accessed April 7, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4135.