Petrovskii, Ya. B. “Periodic Solutions of Quasilinear Hyperbolic Integro-Differential Equations of Second Order”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 11 (November 25, 1996): 1572–1575. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5211.