Where g ( r , r ′ ′ ) is a symmetric kernel, such that g ( r , r ′ ′ ) = g ( r ′ ′ , r ) , which is computed from the scattering data. Solving the Marchenko equation, one obtains the kernel of the transformation operator K ( r , r ′ ′ ) from which the potential can be read off. This equation is derived from the Gelfand–Levitan integral equation, using the Povzner–Levitan representation.