Periodic solutions of a weakly nonlinear wave equation in $E_3$ in a spherically symmetrical case.
(English).Aplikace matematiky,
vol. 14
(1969),
issue 2,
pp. 160-167
[1] O. Vejvoda: Periodic solutions of a linear and weakly nonlinear wave equation in one dimension, I. Czechoslovak Math. Journal, 14 (89), 1964, 341-382. MR 0174872