Bozhok, R. V., and V. D. Koshmanenko. “Singular Perturbations of Self-Adjoint Operators Associated With Rigged Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 5 (May 25, 2005): 622–632. Accessed May 6, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3628.