OnishchukV. A. “Groups Satisfying the Condition of Weak Minimality for Subgroups of Derived Length Two”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 9, Sept. 1994, pp. 1203–1207, https://umj.imath.kiev.ua/index.php/umj/article/view/5636.