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

TL;DR

This paper extends the classification of scalar polynomial integrable equations in 2+1 dimensions to fifth order, identifying known and new equations, and proposes a conjecture that these exhaust all such equations with a specific nonlocality.

Contribution

It provides a comprehensive classification of fifth order integrable equations with a nonlocal term, including new equations, and introduces a two-step classification method.

Findings

Includes known KP, VN, HD equations and their fifth order analogues

Identifies several apparently new integrable equations

Proposes that the list likely exhausts all scalar polynomial integrable equations with the given nonlocality

Abstract

We consider evolutionary equations of the form $u_t=F(u, w)$ where $w=D_x^{-1}D_yu$ is the nonlocality, and the right hand side $F$ is polynomial in the derivatives of $u$ and $w$. The recent paper \cite{FMN} 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 (KP), Veselov-Novikov (VN) and Harry Dym (HD) equations, as well as fifth order analogues 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.