@inproceedings{nils1981on,
title={On the propagation of singularities for pseudo-differential operators of principal type},
author={Nils Dencker},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203511828400362},
booktitle={Arkiv for Matematik},
volume={20},
number={1},
pages={23-60},
year={1981},
}
Nils Dencker. On the propagation of singularities for pseudo-differential operators of principal type. 1981. Vol. 20. In Arkiv for Matematik. pp.23-60. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203511828400362.