“…At present, the Krein-de Branges (KdB) theory became a standard tool in the area of spectral problems. The theory experiences a new peak of popularity in the last 20 years due to its connections to other areas such as the theory of orthogonal polynomials, number theory, random matrices and the nonlinear Fourier transform [1,2,6,8,14,16,17,29,34]. In addition to the original book by de Branges [7] and a chapter in the book by Dym and McKean [10], more recent monographs by Remling [31] and Romanov [32] contain the basics of KdB-theory and further references.…”