The discussion centers on the derivation of the 2D Korteweg-de Vries (KdV) equation, also referred to as the KP equation. The user has successfully completed the linear theory but is struggling with the weakly nonlinear theory, specifically in extending the derivation to include surface tension and an electrical field. They reference Johnson's 1980 paper on water waves and KdV equations as a valuable resource for overcoming their derivation challenges. Additionally, they note that while Johnson did not mention this paper in their prior communications, it has proven helpful in addressing their issues. The user has also found that the linear problem for the 3D case is comparable in difficulty to the 2D case, despite challenges in plotting solutions.