@inproceedings{im1985an,
title={An estimate of the gap of the first two eigenvalues in the Schrdinger operator},
author={IM Singer, Bun Wong, Shing-Tung Yau, and Stephen S-T Yau},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191224203539461906049},
booktitle={Annali della Scuola Normale Superiore di Pisa-Classe di Scienze},
volume={12},
number={2},
pages={319-333},
year={1985},
}

IM Singer, Bun Wong, Shing-Tung Yau, and Stephen S-T Yau. An estimate of the gap of the first two eigenvalues in the Schrdinger operator. 1985. Vol. 12. In Annali della Scuola Normale Superiore di Pisa-Classe di Scienze. pp.319-333. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191224203539461906049.