@inproceedings{arne1951on,
title={On a closure problem},
author={Arne Beurling},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203411596509851},
booktitle={Arkiv for Matematik},
volume={1},
number={4},
pages={301-303},
year={1951},
}