The Perturbed NLS Equation and Asymptotic Integrability
  The perturbed NLS equation is asymptotically integrable if the zero-order approximation to the solution is a single soliton. It is not integrable for any other zero-order solution. In the standard application of a Normal Form expansion, integrability is recovered if, from second order onwards, the perturbation obeys algebraic constraints. In general, these are not satisfied. As a result, the Normal Form is not asymptotically integrable; its solution, the zero-order approximation, loses the simple structure of the solution of the unperturbed equation. This work exploits the freedom in the perturbative expansion to propose an alternative, for which the loss of integrability only affects higher-order terms in the expansion of the solution. The Normal Form remains asymptotically integrable, and the zero-order approximation, not affected by the loss of integrability, has the same structure as the solution of the unperturbed equation. In the case of a multiple-soliton zero-order approximation, the loss of integrability affects the higher-order corrections to the solution in a finite domain around the origin in the x-t plane, the soliton interaction region. The effect decays exponentially in all directions in the plane as the distance from the origin grows. The computation is carried through second order. 
Yair Zarmi