@inproceedings{peter1972on,
title={On the adjoint of an elliptic linear differential operator and its potential theory},
author={Peter Sjögren},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203451466881187},
booktitle={Arkiv for Matematik},
volume={11},
number={1},
pages={153-165},
year={1972},
}
Peter Sjögren. On the adjoint of an elliptic linear differential operator and its potential theory. 1972. Vol. 11. In Arkiv for Matematik. pp.153-165. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203451466881187.