\Bibitem{Lop45} \by J.~Lopatinsky \paper Linear differential operators \jour Sb. Math. \yr 1945 \vol 59 \issue 2 \mathnet{http://mi.mathnet.ru//eng/sm6353} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=16845} \zmath{https://zbmath.org/?q=an:0061.20603}