@inproceedings{masaki1978on,
title={On a class of linear partial differential equations whose formal solutions always converge},
author={Masaki Kashiwara, Takahiro Kawai, and Johannes Sjöstrand},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203505143338307},
booktitle={Arkiv for Matematik},
volume={17},
number={1},
pages={83-91},
year={1978},
}
Masaki Kashiwara, Takahiro Kawai, and Johannes Sjöstrand. On a class of linear partial differential equations whose formal solutions always converge. 1978. Vol. 17. In Arkiv for Matematik. pp.83-91. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203505143338307.