@inproceedings{nils1971some,
title={Some estimates for spectral functions connected with formally hypoelliptic differential operators},
author={Nils Nilsson},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203450019615176},
booktitle={Arkiv for Matematik},
volume={10},
number={1},
pages={251-275},
year={1971},
}
Nils Nilsson. Some estimates for spectral functions connected with formally hypoelliptic differential operators. 1971. Vol. 10. In Arkiv for Matematik. pp.251-275. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203450019615176.