@inproceedings{walter1967polar,
title={Polar sets and removable singularities of partial differential equations},
author={Walter Littman},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203437354321068},
booktitle={Arkiv for Matematik},
volume={7},
number={1},
pages={1-9},
year={1967},
}
Walter Littman. Polar sets and removable singularities of partial differential equations. 1967. Vol. 7. In Arkiv for Matematik. pp.1-9. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203437354321068.