The Riemann Hilbert Problem for degenerate elliptic vector fields
Solvability and hypoellipticity of first order partial differential operators and ...
Solvability for a class of first-order partial differential operators
Solvability and hypoellipticity of first order partial differential operators and...
Existence of periodic solutions for first-order partial differential equations
Semiglobal solvability for classes of non singular vector fields