Kuzhel’S. A. “On the Inverse Problem for Perturbations of an Abstract Wave Equation in the Lax-Phillips Scattering Scheme”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 52, no. 5, May 2000, pp. 638-47, https://umj.imath.kiev.ua/index.php/umj/article/view/4458.