FUSHCHICH , V. I.; SEROV , N. I. Conditional invariance of the nonlinear wave equation. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 43, n. 3, p. 394–399, 1991. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/9621. Acesso em: 12 jan. 2026.