@inproceedings{ulf1954the,
title={The reality of the eigenvalues of certain integral equations},
author={Ulf Hellsten},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203418421873905},
booktitle={Arkiv for Matematik},
volume={3},
number={1},
pages={79-87},
year={1954},
}