@inproceedings{nils1965asymptotic,
title={Asymptotic estimates for spectral functions connected with hypoelliptic differential operators},
author={Nils Nilsson},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203433302524035},
booktitle={Arkiv for Matematik},
volume={5},
number={6},
pages={527-540},
year={1965},
}
Nils Nilsson. Asymptotic estimates for spectral functions connected with hypoelliptic differential operators. 1965. Vol. 5. In Arkiv for Matematik. pp.527-540. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203433302524035.