BILUSYAK, N.I.; PTASHNIK, B. I. Boundary-Value Problem for Weakly Nonlinear Hyperbolic Equations with Variable Coefficients. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 53, n. 9, p. 1281–1286, 2001. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4347. Acesso em: 5 dec. 2025.