\Bibitem{Kam73} \by A.~M.~Kamachkin \paper Letters to the editors \jour Differ. Uravn. \yr 1973 \vol 9 \issue 1 \pages 192 \mathnet{http://mi.mathnet.ru/de1798} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=328213} \zmath{https://zbmath.org/?q=an:0254.93018}