Periodic solutions for a second order partial differential equation
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Serie 4, Volume 19 (1992) no. 4, p. 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},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 19},
     number = {4},
     year = {1992},
     pages = {493-506},
     zbl = {0778.35006},
     mrnumber = {1205881},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1992_4_19_4_493_0}
}

d'Ancona, Piero. Periodic solutions for a second order partial differential equation. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Serie 4, Volume 19 (1992) no. 4, pp. 493-506. http://www.numdam.org/item/ASNSP_1992_4_19_4_493_0/

[A] Sh. Agmon, Lectures on elliptic boundary value problems, Princeton, Van Nostrand 1965. | MR 178246 | Zbl 0142.37401

[B] H. Brézis, Analyse fonctionnelle, Masson, Paris, 1987. | MR 697382 | Zbl 0511.46001

[d] L. De Simon, Sull'equazione delle onde con termine noto periodico, Rend. Istit. Mat. Univ. Trieste 1 (1969), 150-162. | MR 283359 | Zbl 0186.42604

[GT] D. Gilbarg - N.S. Trudinger, Elliptic partial different equations of second order, Springer, Berlin, 1983. | MR 737190 | Zbl 0562.35001

[H] L. Hörmander, The analysis of partial differential operators III, Springer, Berlin, 1985. | Zbl 0601.35001

[K] T. Kato, Perturbation theory for linear operators, Springer, Berlin, 1980. | MR 407617 | Zbl 0435.47001

[Kh] A. Ya. Khinchin, Continued fractions, The University of Chicago Press, Chigago, 1964. | MR 161833 | Zbl 0117.28601

[LM] J.L. Lions - E. Magenes, Non-homogeneous boundary value problems and applications, Springer, Berlin, 1973. | MR 350179 | Zbl 0223.35039

[T] M. Taylor, Pseudodifferential operators, Princeton University Press, Princeton, 1981. | MR 618463 | Zbl 0453.47026