@inproceedings{j.1927le,
title={Le principe de huyghens dans le cas de quatre variables indépendantes},
author={J. Hadamard},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203046523180391},
booktitle={Acta Mathematica},
volume={49},
number={3},
pages={203-244},
year={1927},
}