Chernikov , S. N., and N. S. Chernikov. “Complementability Criterion of Periodical Almost Soluble Subgroups in the Group Containing It”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 44, no. 6, July 1992, pp. 822-6,
https://umj.imath.kiev.ua/index.php/umj/article/view/8016.