@inproceedings{johan1974hybrids,
title={Hybrids between hyperbolic and elliptic differential operators with constant coefficients},
author={Johan Fehrman},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203456087096228},
booktitle={Arkiv for Matematik},
volume={13},
number={1},
pages={209-235},
year={1974},
}
Johan Fehrman. Hybrids between hyperbolic and elliptic differential operators with constant coefficients. 1974. Vol. 13. In Arkiv for Matematik. pp.209-235. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203456087096228.