@inproceedings{shing-tung1985an,
  title={An Estimate of the Gap of the First Two Eigenvalues in the Schr\" odinger Operator},
  author={Shing-Tung Yau},
  url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191224204207689593143},
  booktitle={arXiv preprint arXiv:0902.2250},
  year={1985},
}
             
            
                
Shing-Tung Yau. An Estimate of the Gap of the First Two Eigenvalues in the Schr\" odinger Operator. 1985. In arXiv preprint arXiv:0902.2250. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191224204207689593143.