\Bibitem{Vin80} \by A.~M.~Vinogradov \paper The geometry of nonlinear differential equations \serial Itogi Nauki i Tekhniki. Ser. Probl. Geom. \yr 1980 \vol 11 \pages 89--134 \publ VINITI \publaddr Moscow \mathnet{http://mi.mathnet.ru/intg121} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=579929} \zmath{https://zbmath.org/?q=an:0475.58025|0461.58012} \transl \jour J. Soviet Math. \yr 1981 \vol 17 \issue 1 \pages 1624--1649 \crossref{https://doi.org/10.1007/BF01084594}