Three-wave equation
In nonlinear systems, the three-wave equations, sometimes called the three-wave resonant interaction equations or triad resonances, describe small-amplitude waves in a variety of nonlinear media, including water waves in shallow water, capillary waves, the coupling of acoustic waves in the littoral zone, acoustic waves in plasma, oscillations in electrical circuits and in non-linear optics. They are a set of three completely integrable nonlinear partial differential equations.