On the classification of scalar evolutionary integrable equations in 2 + 1 dimensions

Abstract

We consider evolutionary equations of the form ut = F(u, w) where \documentclass[12pt]{minimal}\begin{document}$w=D_x^{-1}D_yu$\end{document}w=Dx−1Dyu is the nonlocality, and the right hand side F is polynomial in the derivatives of u and w. The recent paper [Ferapontov, Moro, and Novikov, J. Phys. A: Math. Theor. 52, 18 (2009)] provides a complete list of integrable third order equations of this kind. Here we extend the classification to fifth order equations. Besides the known examples of Kadomtsev–Petviashvili, Veselov–Novikov, and Harry Dym equations, as well as fifth order analogs and modifications thereof, our list contains a number of equations which are apparently new. We conjecture that our examples exhaust the list of scalar polynomial integrable equations with the nonlocality w. The classification procedure consists of two steps. First, we classify quasilinear systems which may (potentially) occur as dispersionless limits of integrable scalar evolutionary equations. After that we reconstruct dispersive terms based on the requirement of the inheritance of hydrodynamic reductions of the dispersionless limit by the full dispersive equation.