Mil’man, A. L. “Scattering Matrix for the Wave Equation With Finite Radial Potential in the Two-Dimensional Space”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 45, no. 6, June 1993, pp. 791–802, https://umj.imath.kiev.ua/index.php/umj/article/view/5868.