On hypoellipticity of second order operators. (English) Zbl 0321.35025
Commun. partial differ. Equations 1, 73-85 (1976).
MSC:
| 35H10 | Hypoelliptic equations |
| 35D10 | Regularity of generalized solutions of PDE (MSC2000) |
| 35M99 | Partial differential equations of mixed type and mixed-type systems of partial differential equations |
