@inproceedings{ferruccio1981an,
title={An example of a weakly hyperbolic Cauchy problem not well posed in$C$^{$∞$}},
author={Ferruccio Colombini, and Sergio Spagnolo},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203257219058317},
booktitle={Acta Mathematica},
volume={148},
number={1},
pages={243-253},
year={1981},
}
Ferruccio Colombini, and Sergio Spagnolo. An example of a weakly hyperbolic Cauchy problem not well posed in$C$^{$∞$}. 1981. Vol. 148. In Acta Mathematica. pp.243-253. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203257219058317.