no. 2
An estimate of the gap of the first two eigenvalues in the Schrödinger operator
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 12 (1985) no. 2, pp. 319-333. 
@article{ASNSP_1985_4_12_2_319_0,
     author = {Singer, I. M. and Wong, Bun and Yau, Shing-Tung and Yau, Stephen S.-T.},
     title = {An estimate of the gap of the first two eigenvalues in the {Schr\"odinger} operator},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {319--333},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 12},
     number = {2},
     year = {1985},
     mrnumber = {829055},
     zbl = {0603.35070},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1985_4_12_2_319_0/}
}
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Singer, I. M.; Wong, Bun; Yau, Shing-Tung; Yau, Stephen S.-T. An estimate of the gap of the first two eigenvalues in the Schrödinger operator. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 12 (1985) no. 2, pp. 319-333. http://www.numdam.org/item/ASNSP_1985_4_12_2_319_0/

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