Yury Kovalev. “Connections the Sobolev Spaces and the Hilbert Space ℓ2 and Their Application at Point Interaction Model ”. Ukrains’kyi Matematychnyi Zhurnal 69, no. 12 (July 19, 2023): 1615–1624. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/6012.