\Bibitem{Kal71} \by B.~S.~Kalitin \paper Letter to the editor \jour Differ. Uravn. \yr 1971 \vol 7 \issue 2 \pages 378 \mathnet{http://mi.mathnet.ru/de1221} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=280805}