Berezansky Y. М. and A. Y. . Konstantinov, “Multiparametric spectral problems as expanded by the author’s own vectors ”, Ukr. Mat. Zhurn., vol. 44, no. 7, pp. 901–913, Aug. 1992, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/8122