BozhokR. V., and KoshmanenkoV. D. 2005. “Singular Perturbations of Self-Adjoint Operators Associated With Rigged Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal 57 (5), 622–632. https://umj.imath.kiev.ua/index.php/umj/article/view/3628.