\Bibitem{Zub59} \by V.~I.~Zubov \paper Some problems in stability of motion \jour Sb. Math. \yr 1959 \vol 90 \issue 2 \mathnet{http://mi.mathnet.ru//eng/sm4892} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=104881} \zmath{https://zbmath.org/?q=an:0108.08601}