On the Hirota equation with a self-consistent source

We develop the formalism of the inverse scattering problem method for the Cauchy problem for the defocusing Hirota equation with a self-consistent source. The specific feature of the considered Cauchy problem is that the solution is assumed to approach nonzero limits as the spatial variable approaches the plus and minus infinities. The purpose of the paper is to present two main steps of the formalism: first, the inverse problem for the associated linear Zakharov–Shabat system and, second, the evolution of the associated scattering data. A theorem is proved on the evolution of scattering data of a self-adjoint Zakharov–Shabat system, with the potential given by a solution of the defocusing Hirota equation with a self-consistent source.