@inproceedings{johannes1973parametrices,
title={Parametrices for pseudodifferential operators with multiple characteristics},
author={Johannes Sjöstrand},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203453169853202},
booktitle={Arkiv for Matematik},
volume={12},
number={1},
pages={85-130},
year={1973},
}
Johannes Sjöstrand. Parametrices for pseudodifferential operators with multiple characteristics. 1973. Vol. 12. In Arkiv for Matematik. pp.85-130. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203453169853202.