@inproceedings{michael1980regularity,
title={Regularity theorems for nondiagonal elliptic systems},
author={Michael Wiegner},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203511581957360},
booktitle={Arkiv for Matematik},
volume={20},
number={1},
pages={1-13},
year={1980},
}