Basic aspects of soliton theory

Abstract

This is a review of the main ideas of the inverse scattering method (ISM) for solving nonlinear evolution equations (NLEE), known as soliton equations. As a basic tool we use the fundamental analytic solutions (FAS) of the Lax operator L. Then the inverse scattering problem for L reduces to a Riemann-Hilbert problem. Such construction cab be applied to wide class of Lax operators, related to the simple Lie algebras g. We construct the kernel of the resolvent of L in terms of FAS and derive the spectral decompositions of L. Thus we can solve the relevant classes of NLEE which include the NLS eq. and its multi-component generalizations, the N-wave equations etc. Using the dressing method of Zakharov and Shabat we derive the N-soliton solutions of these equations. We explain that the ISM is a natural generalization of the Fourier transform method. As appropriate generalizations of the usual exponential function we use the so-called "squared solutions" which are constructed again in terms of FAS and the Cartan-Weyl basis of the relevant Lie algebra. One can prove the completeness relations for the "squared solutions" which in fact provide the spectral decompositions of the recursion operator Λ. These decompositions can be used to derive all fundamental properties of the corresponding NLEE in terms of Λ: i) the explicit form of the class of integrable NLEE; ii) the generating functionals of integrals of motion; iii) the hierarchies of Hamiltonian structures. We outline the importance of the classical R-matrices for extracting the involutive integrals of motion.