@inproceedings{zeev1960characteristic,
title={Characteristic values associated with a class of nonlinear second-order differential equations},
author={Zeev Nehari},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203156604725918},
booktitle={Acta Mathematica},
volume={105},
number={3},
pages={141-175},
year={1960},
}
Zeev Nehari. Characteristic values associated with a class of nonlinear second-order differential equations. 1960. Vol. 105. In Acta Mathematica. pp.141-175. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203156604725918.