Kazunaga Tanaka "Infinitely many periodic solutions for the equation: $u_{tt} - u_{xx} \pm \left| u \right|^{s - 1} u = f\left( {x,t} \right)$," Proceedings of the Japan Academy, Series A, Mathematical Sciences, Proc. Japan Acad. Ser. A Math. Sci. 61(3), 70-73, (1985)