Kuzhel’, S. A. “On a Form of the Scattering Matrix for ρ-Perturbations of an Abstract Wave Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 4, Apr. 1999, pp. 445–457, https://umj.imath.kiev.ua/index.php/umj/article/view/4630.