@inproceedings{r.1967uniqueness,
title={Uniqueness in cauchy's problem for elliptic equations with double characteristics},
author={R. N. Pederson},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203436934352064},
booktitle={Arkiv for Matematik},
volume={6},
number={6},
pages={535-549},
year={1967},
}
R. N. Pederson. Uniqueness in cauchy's problem for elliptic equations with double characteristics. 1967. Vol. 6. In Arkiv for Matematik. pp.535-549. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203436934352064.