@inproceedings{gudrun1976global,
title={Global properties of differential operators of constant strength},
author={Gudrun Gudmundsdottir},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203500712824268},
booktitle={Arkiv for Matematik},
volume={15},
number={1},
pages={169-198},
year={1976},
}
Gudrun Gudmundsdottir. Global properties of differential operators of constant strength. 1976. Vol. 15. In Arkiv for Matematik. pp.169-198. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203500712824268.