A refined and unified version of the inverse scattering method for the Ablowitz-Ladik lattice and derivative NLS lattices

We refine and develop the inverse scattering theory on a lattice in such a way that the Ablowitz-Ladik lattice and derivative NLS lattices as well as their matrix analogs can be solved in a unified way. The inverse scattering method for the (matrix analog of the) Ablowitz-Ladik lattice is simplified to the same level as that for the continuous NLS system. Using the linear eigenfunctions of the Lax pair for the Ablowitz-Ladik lattice, we can construct solutions of the derivative NLS lattices such as the discrete Gerdjikov-Ivanov (also known as Ablowitz-Ramani-Segur) system and the discrete Kaup-Newell system. Thus, explicit solutions such as the multisoliton solutions for these systems can be obtained by solving linear summation equations of the Gel'fand-Levitan-Marchenko type. The derivation of the discrete Kaup-Newell system from the Ablowitz-Ladik lattice is based on a new method that allows us to generate new integrable systems from known systems in a systematic manner. In an appendix, we describe the reduction of the matrix Ablowitz-Ladik lattice to a vector analog of the modified Volterra lattice from the point of view of the inverse scattering method.