@inproceedings{kjell-ove1967on,
title={On the boundary behavior of solutions to a class of elliptic partial differential equations},
author={Kjell-Ove Widman},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203436796781063},
booktitle={Arkiv for Matematik},
volume={6},
number={6},
pages={485-533},
year={1967},
}
Kjell-Ove Widman. On the boundary behavior of solutions to a class of elliptic partial differential equations. 1967. Vol. 6. In Arkiv for Matematik. pp.485-533. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203436796781063.