Gromyak, M. I. “Construction of Periodical Solutions of the Wave Differential and Integral-Differential Second Order Equations ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 1, Dec. 1987, pp. 48-53, https://umj.imath.kiev.ua/index.php/umj/article/view/9053.