@inproceedings{a.1974jordan,
title={Jordan decomposition for a class of singular differential operators},
author={A. H. M. Levelt},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203454928593218},
booktitle={Arkiv for Matematik},
volume={13},
number={1},
pages={1-27},
year={1974},
}
A. H. M. Levelt. Jordan decomposition for a class of singular differential operators. 1974. Vol. 13. In Arkiv for Matematik. pp.1-27. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203454928593218.