Konstantinov, A. Yu. “Spectral and Scattering Theory of Some Matrix Operators Related to the Schrödinger Equations With Potentials Depending Rationally on Spectral Parameter”. Methods of Functional Analysis and Topology, vol. 2, no. 3, Dec. 1996, pp. 69-77, https://zen.imath.kiev.ua/index.php/mfat/article/view/21.