@inproceedings{linda1975hypoelliptic,
title={Hypoelliptic differential operators and nilpotent groups},
author={Linda Preiss Rothschild, and E. M. Stein},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203241518294213},
booktitle={Acta Mathematica},
volume={137},
number={1},
pages={247-320},
year={1975},
}
Linda Preiss Rothschild, and E. M. Stein. Hypoelliptic differential operators and nilpotent groups. 1975. Vol. 137. In Acta Mathematica. pp.247-320. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203241518294213.