\Bibitem{Ryv68} \by V.~B.~Ryvkin \paper Letter to the editor \jour Differ. Uravn. \yr 1968 \vol 4 \issue 3 \pages 556 \mathnet{http://mi.mathnet.ru/de323} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=226215} \zmath{https://zbmath.org/?q=an:0163.34501}