FUSHCHICH, V. I.; REVENKO, I. V.; ZHDANOV, R. Z. Separation of variables in two-dimensional wave equations with potential. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 10, p. 1343–1361, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5613. Acesso em: 6 dec. 2025.