ALDASHEV, S. A. Darboux–Protter Spectral Problems for One Class of Multidimensional Hyperbolic Equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 55, n. 1, p. 100–107, 2003. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3891. Acesso em: 6 dec. 2025.