Periodic solutions to the inhomogeneous sine-Gordon equation.
(English).Commentationes Mathematicae Universitatis Carolinae,
vol. 20
(1979),
issue 2,
pp. 329-333
[1] O. VEJVODA: Periodic solutions of a linear and weakly nonlinear wave equation in one dimension, I. Czechoslovak Math. J. 14 (89) (1964), 341-382. MR 0174872