no. 4
On the stability of the Dirichlet problem for the vibrating string equation
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 4, pp. 719-728. 
@article{ASNSP_1979_4_6_4_719_0,
     author = {Papi Frosali, Gloria},
     title = {On the stability of the Dirichlet problem for the vibrating string equation},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {719--728},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 6},
     number = {4},
     year = {1979},
     zbl = {0429.35043},
     mrnumber = {563341},
     language = {en},
     url = {www.numdam.org/item/ASNSP_1979_4_6_4_719_0/}
}
Papi Frosali, Gloria. On the stability of the Dirichlet problem for the vibrating string equation. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 4, pp. 719-728. http://www.numdam.org/item/ASNSP_1979_4_6_4_719_0/

[1] D.G. Bourgin - R. Duffin, The Dirichlet problem for the vibrating string equation, Bull. Amer. Math. Soc., 45 (1939), 851-858. | JFM 65.0438.01 | MR 729 | Zbl 0023.04201

[2] D.W. Fox - C. Pucci, The Dirichlet problem for the wave equation, Ann. Mat. Pura Appl., (IV), vol. XLVI (1958), 155-182. | MR 104902 | Zbl 0101.31001

[3] F. John, The Dirichlet problem for a hyperbolic equation, Amer. J. Math., 63 (1941), 141-154. | JFM 67.0362.02 | MR 3346 | Zbl 0024.20304

[4] C. Viola, Diophantine approximation in short intervals, Ann. Scuola Norm. Sup. Pisa, 6 (1979), pp. 703-717. | Numdam | MR 563340 | Zbl 0424.10022

[5] S. Dolecki, Observability for the one-dimensional heat equation, Studia Math., 48 (1973), 291-305. | MR 333444 | Zbl 0264.35036

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