no. 1
Theorems of existence and multiplicity for nonlinear elliptic problems with noninvertible linear part
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 5 (1978) no. 1, pp. 15-28 
@article{ASNSP_1978_4_5_1_15_0,
     author = {Ambrosetti, Antonio and Mancini, Giovanni},
     title = {Theorems of existence and multiplicity for nonlinear elliptic problems with noninvertible linear part},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {15--28},
     year = {1978},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 5},
     number = {1},
     mrnumber = {487001},
     zbl = {0375.35024},
     language = {en},
     url = {https://www.numdam.org/item/ASNSP_1978_4_5_1_15_0/}
}
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Ambrosetti, Antonio; Mancini, Giovanni. Theorems of existence and multiplicity for nonlinear elliptic problems with noninvertible linear part. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 5 (1978) no. 1, pp. 15-28. https://www.numdam.org/item/ASNSP_1978_4_5_1_15_0/

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