@inproceedings{stephan1958on,
title={On weak and strong extensions of partial differential operators with constant coefficients},
author={Stephan Schwarz},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203423385654949},
booktitle={Arkiv for Matematik},
volume={3},
number={6},
pages={515-526},
year={1958},
}
Stephan Schwarz. On weak and strong extensions of partial differential operators with constant coefficients. 1958. Vol. 3. In Arkiv for Matematik. pp.515-526. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203423385654949.