On Kaup and Newell's Method for Solving DNLS Equation

Kaup and Newell's revised inverse scattering transform for the derivative nonlinear Schrodinger (DNLS) equation is investigated. We compared it with a more reasonable approach proposed recently, which is rigorously proven by
the Liouville theorem. It is concluded that Kaup and Newell's revision is only suitable for giving single-soliton solution and can not be generalized to multi-soliton case.