@inproceedings{gunnar1968on,
title={On the partial differential equation$u$_{$x$}^{2}$u$_{$xx$}+2$u$_{$x$}$u$_{$y$}$u$_{$xy$}+$u$_{$y$}^{2}$u$_{$yy$}=0},
author={Gunnar Aronsson},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203440420779095},
booktitle={Arkiv for Matematik},
volume={7},
number={5},
pages={395-425},
year={1968},
}
Gunnar Aronsson. On the partial differential equation$u$_{$x$}^{2}$u$_{$xx$}+2$u$_{$x$}$u$_{$y$}$u$_{$xy$}+$u$_{$y$}^{2}$u$_{$yy$}=0. 1968. Vol. 7. In Arkiv for Matematik. pp.395-425. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203440420779095.