Kadomtsev–Petviashvili Equation Revisited and Integrability in 4 + 2 and 3 + 1

The Cauchy problem of Kadomtsev–Petviashvili I (KPI) was reduced to a nonlocal Riemann–Hilbert (RH) problem by the author and Ablowitz in 1983. This formulation was based on the introduction of two spectral functions (nonlinear Fourier transforms, FTs). This formalism was improved by Boiti et al. [ 1 ], where it was shown that the earlier nonlocal RH problem can be formulated in terms of a single spectral function (nonlinear FT). A different formalism was presented by Zhou [ 2 ], where the Cauchy problem was rigorously solved in terms of a linear integral equation involving a nonanalytic eigenfunction. Here, we first revisit the above results and then review some recent results about the derivation of integrable generalizations of KP in 4 + 2 (i.e., in four spatial and two temporal dimensions), as well as in 3 + 1 (i.e., in three spatial and one temporal dimensions).