@inproceedings{lars1967hypoelliptic,
title={Hypoelliptic second order differential equations},
author={Lars Hörmander},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203213279884044},
booktitle={Acta Mathematica},
volume={119},
number={1},
pages={147-171},
year={1967},
}