KONSTANTINOV, A. Yu.; STADNYUK, A. G. Eigenfunction expansion for symmetric multiparameter problems. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 8, p. 1166–1169, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5915. Acesso em: 7 apr. 2025.