1. M. Christ, A remark on sums of squares of complex vector fields, 2004. Author’s web page.
2. On a criterion for hypoellipticity;Fediĭ;Mat. Sb.,1971
3. C. Fefferman, D.H. Phong, Subelliptic eigenvalue problems, in: Conf. in Honor of A. Zygmund, Wadsworth Math. Series, 1981.
4. Hypoelliptic second order differential equations;Hörmander;Acta Math.,1967
5. Hypoellipticity of some degenerate subelliptic operators;Kohn;J. Funct. Anal.,1998