\Bibitem{Gru62} \by V.~V.~Grushin \paper On $Q$-hypo-elliptic equations \jour Sb. Math. \yr 1962 \vol 99 \issue 2 \mathnet{http://mi.mathnet.ru/eng/sm4648} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=144071} \zmath{https://zbmath.org/?q=an:0104.07302}