MathSciDoc: An Archive for Mathematician ∫

@inproceedings{b.1978approximation,
  title={Approximation d'un système de champs de vecteurs et applications à l'hypoellipticité},
  author={B. Helffer, and J. F. Nourrigat},
  url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203506711529319},
  booktitle={Arkiv for Matematik},
  volume={17},
  number={1},
  pages={237-254},
  year={1978},
}