A. Y. Konstantinov and A. G. Stadnyuk, “Eigenfunction expansion for symmetric multiparameter problems”, Ukr. Mat. Zhurn., vol. 45, no. 8, pp. 1166–1169, Aug. 1993, Accessed: Apr. 07, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5915