no. 4
Periodic solutions for a second order partial differential equation
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 19 (1992) no. 4, pp. 493-506 
@article{ASNSP_1992_4_19_4_493_0,
     author = {d'Ancona, Piero},
     title = {Periodic solutions for a second order partial differential equation},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {493--506},
     year = {1992},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 19},
     number = {4},
     mrnumber = {1205881},
     zbl = {0778.35006},
     language = {en},
     url = {https://www.numdam.org/item/ASNSP_1992_4_19_4_493_0/}
}
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d'Ancona, Piero. Periodic solutions for a second order partial differential equation. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 19 (1992) no. 4, pp. 493-506. https://www.numdam.org/item/ASNSP_1992_4_19_4_493_0/

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