The Prelle-Singer method and Painlev\'e hierarchies

TL;DR

This paper explores the application of the Prelle-Singer method to systems of ODEs associated with Hamiltonian PDE hierarchies, aiming to find first integrals and reduce order, with new developments for coupled third order ODEs.

Contribution

It introduces a novel application of the Prelle-Singer method to coupled third order ODEs, expanding its use in analyzing integrable and nonintegrable systems related to Painlevé hierarchies.

Findings

First integrals can be obtained for systems based on dispersive water wave, Ito, and KdV structures.

The development of the PS method for coupled third order ODEs is new.

The method is useful for analyzing sequences of systems of increasing order.

Abstract

We consider systems of ordinary differential equations (ODEs) of the form ${\cal B}{\mathbf K}=0$, where $\cal B$ is a Hamiltonian operator of a completely integrable partial differential equation (PDE) hierarchy, and ${\mathbf K}=(K,L)^T$. Such systems, whilst of quite low order and linear in the components of $\mathbf K$, may represent higher-order nonlinear systems if we make a choice of $\mathbf K$ in terms of the coefficient functions of $\cal B$. Indeed, our original motivation for the study of such systems was their appearance in the study of Painlev\'e hierarchies, where the question of the reduction of order is of great importance. However, here we do not consider such particular cases; instead we study such systems for arbitrary $\mathbf K$, where they may represent both integrable and nonintegrable systems of ordinary differential equations. We consider the application of the Prelle-Singer (PS) method --- a method used to find first integrals --- to such systems in order to reduce their order. We consider the cases of coupled second order ODEs and coupled third order ODEs, as well as the special case of a scalar third order ODE; for the case of coupled third order ODEs, the development of the PS method presented here is new. We apply the PS method to examples of such systems, based on dispersive water wave, Ito and Korteweg-de Vries Hamiltonian structures, and show that first integrals can be obtained. It is important to remember that the equations in question may represent sequences of systems of increasing order. We thus see that the PS method is a further technique which we expect to be useful in our future work.