Complex vector fields and hypoelliptic partial differential operators
  We prove a subelliptic estimate for systems of complex vector fields under some assumptions that generalize the essential pseudoconcavity for $CR$ manifolds and H\"ormander's bracket condition for real vector fields. Applications are given to prove the hypoellipticity of first order systems and second order partial differential operators. Finally we describe a class of compact homogeneous CR manifolds for which the distribution of $(0,1)$ vector fields satisfies a subelliptic estimate. v2: minor revision, to appear in Ann. Inst. Fourier 
Andrea Altomani (Luxembourg), C. Denson Hill (Stony Brook), Mauro   Nacinovich (Rome 2), Egmont Porten (Sundsvall)