@inproceedings{james1959perturbations,
title={Perturbations of nonlinear systems},
author={James C. Lillo},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203154125039901},
booktitle={Acta Mathematica},
volume={103},
number={1},
pages={123-138},
year={1959},
}