S. A. Kuzhel', “On a form of the scattering matrix for ρ-perturbations of an abstract wave equations”, Ukr. Mat. Zhurn., vol. 51, no. 4, pp. 445–457, Apr. 1999, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4630