Ablowitz, Chakravarty, and Halburd (2003) explain how a variety of completely integrable equations such as the Korteweg–De Vries equation (KdV) equation, the Kadomtsev–Petviashvili equation (KP) equation, the nonlinear Schrödinger equation, the sine-Gordon equation, the Ernst equation and the Painlevé equations all arise as reductions or other simplifications of the self-dual Yang–Mills equations :