@inproceedings{johannes1972operators,
title={Operators of principal type with interior boundary conditions},
author={Johannes Sjöstrand},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203226813935141},
booktitle={Acta Mathematica},
volume={130},
number={1},
pages={1-51},
year={1972},
}
Johannes Sjöstrand. Operators of principal type with interior boundary conditions. 1972. Vol. 130. In Acta Mathematica. pp.1-51. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203226813935141.