\Bibitem{Plo61} \by B.~I.~Plotkin \paper Radicals in group pairs \jour Dokl. Akad. Nauk SSSR \yr 1961 \vol 140 \issue 5 \pages 1019--1022 \mathnet{http://mi.mathnet.ru/dan25626} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=0151523} \zmath{https://zbmath.org/?q=an:0199.05801}