Konstantinov, A. Yu., and A. G. Stadnyuk. “Eigenfunction Expansion for Symmetric Multiparameter Problems”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 8 (August 25, 1993): 1166–1169. Accessed June 2, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5915.