We survey recent results on the existence and uniqueness of the weak time-periodic solutions of some semilinear wave equations or systems submitted to classical boundary conditions on intervals, balls or spheres. We show that those results sharply depend upon some arithmetical properties of the period and the size of the space domains.
Mawhin, J. (1995). Periodic-solutions of Some Semilinear Wave-equations and Systems - a Survey. Chaos, Solitons & Fractals, 5(9), 1651-1669. https://doi.org/10.1016/0960-0779(94)00169-Q (Original work published 1995)