@inproceedings{norman1950perturbations,
title={Perturbations of discontinuous solutions of non-linear systems of differential equations},
author={Norman Levinson},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203126890853700},
booktitle={Acta Mathematica},
volume={82},
number={1},
pages={71-106},
year={1950},
}
Norman Levinson. Perturbations of discontinuous solutions of non-linear systems of differential equations. 1950. Vol. 82. In Acta Mathematica. pp.71-106. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203126890853700.