Continuous binary Darboux transformation as an abstract framework for KdV soliton gases

Abstract

We present a unified operator-theoretic framework for constructing deterministic KdV soliton gases and step-type KdV solutions. Starting from Dyson's determinantal formula, we obtain a broad class of reflectionless solutions and describe their basic spectral and analytic properties, including their interpretation as deterministic soliton gases. We then introduce a continuous binary Darboux transformation that acts directly on the scattering data and generates general step-type solutions, with particular emphasis on reflectionless hydraulic-jump-type profiles modelling a soliton condensate on the left and vacuum on the right. The paper is methodological in nature: our goal is not to develop a full kinetic or probabilistic theory, but to show how classical tools from spectral and scattering theory can be combined into a conceptually simple framework that accommodates both reflectionless and non-reflectionless soliton gas configurations, including step-like backgrounds.