MathSciDoc: An Archive for Mathematician ∫

@inproceedings{lars1977free,
  title={Free systems of vector fields},
  author={Lars Hörmander, and Anders Melin},
  url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203502407745281},
  booktitle={Arkiv for Matematik},
  volume={16},
  number={1},
  pages={83-88},
  year={1977},
}