\Bibitem{VolGin67} \by L.~R.~Volevich, S.~G.~Gindikin \paper Pseudo-differential operators and Cauchy problem for differential equations with variable coefficients \jour Funktsional. Anal. i Prilozhen. \yr 1967 \vol 1 \issue 4 \pages 8--25 \mathnet{http://mi.mathnet.ru/faa2840} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=223721} \zmath{https://zbmath.org/?q=an:0164.11703} \transl \jour Funct. Anal. Appl. \yr 1967 \vol 1 \issue 4 \pages 262--277 \crossref{https://doi.org/10.1007/BF01076006}