@inproceedings{lars1958differentiability,
title={Differentiability properties of solutions of systems of differential equations},
author={Lars Hörmander},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203423490843950},
booktitle={Arkiv for Matematik},
volume={3},
number={6},
pages={527-535},
year={1958},
}
Lars Hörmander. Differentiability properties of solutions of systems of differential equations. 1958. Vol. 3. In Arkiv for Matematik. pp.527-535. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203423490843950.