@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.