Sharp Lipschitz estimates for operator dbar_M on a q-concave CR manifold
  We prove that the integral operators $R_r$ and $H_r$ constructed in \cite{P} and such that $$f = \bar\partial_{\bold M} R_r(f) + R_{r+1}(\bar\partial_{\bold M} f) + H_r(f),$$ for a differential form $f \in C_{(0,r)}^{\infty}({\bold M})$ on a regular q-concave CR manifold ${\bold M}$ admit sharp estimates in the Lipschitz scale. 
Peter L. Polyakov