1. Hormander, L.: Hypoelliptic second order differential equations. Acta Math. 119(1), 147–171 (1967)
2. Lanconelli, E., Polidoro, S.: On a class of hypoelliptic evolution operators. Rend. Sem. Mat. Univ. Politec. Torino 52(1), 29–63 (1994)
3. Kupcov, L.P.: The fundamental solutions of a certain class of elliptic-parabolic second order equations. Differ. Equ. 8, 1269–1278 (1972)
4. Feng, X., Niu, P., Zhu, M.: Global weak Morrey estimates for some ultraparabolic operators of Kolmogorov-Fokker-Planck type. Bull. Korean Math. Soc. 51(5), 1241–1257 (2014)
5. Polidoro, S.: On a class of ultraparabolic operators of Kolmogorov–Fokker–Planck type. Matematiche (Catania) 49(1), 53–105 (1994)