\Bibitem{Mys70} \by A.~D.~Myshkis \paper Letter to the editors \jour Differ. Uravn. \yr 1970 \vol 6 \issue 8 \pages 1532 \mathnet{http://mi.mathnet.ru/de1074} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=271439}