PetrovskiiY. B. “Periodic Solutions of Quasilinear Hyperbolic Integro-Differential Equations of Second Order”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 11, Nov. 1996, pp. 1572-5, https://umj.imath.kiev.ua/index.php/umj/article/view/5211.