@inproceedings{christer1975spectral,
title={Spectral theory for pairs of differential operators},
author={Christer Bennewitz},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203500046480262},
booktitle={Arkiv for Matematik},
volume={15},
number={1},
pages={33-61},
year={1975},
}