@inproceedings{l.1980necessary,
title={Necessary conditions for local solvability of homogeneous left invariant differential operators on nilpotent lie groups},
author={L. Corwin, and L. P. Rothschild},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203255980330307},
booktitle={Acta Mathematica},
volume={147},
number={1},
pages={265-288},
year={1980},
}
L. Corwin, and L. P. Rothschild. Necessary conditions for local solvability of homogeneous left invariant differential operators on nilpotent lie groups. 1980. Vol. 147. In Acta Mathematica. pp.265-288. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203255980330307.