Multi-Lagrangians for Integrable Systems

Abstract

We propose a general scheme to construct multiple Lagrangians for completely integrable non-linear evolution equations that admit multi- Hamiltonian structure. The recursion operator plays a fundamental role in this construction. We use a conserved quantity higher/lower than the Hamiltonian in the potential part of the new Lagrangian and determine the corresponding kinetic terms by generating the appropriate momentum map. This leads to some remarkable new developments. We show that nonlinear evolutionary systems that admit $N$-fold first order local Hamiltonian structure can be cast into variational form with $2N-1$ Lagrangians which will be local functionals of Clebsch potentials. This number increases to $3N-2$ when the Miura transformation is invertible. Furthermore we construct a new Lagrangian for polytropic gas dynamics in $1+1$ dimensions which is a {\it local} functional of the physical field variables, namely density and velocity, thus dispensing with the necessity of introducing Clebsch potentials entirely. This is a consequence of bi-Hamiltonian structure with a compatible pair of first and third order Hamiltonian operators derived from Sheftel's recursion operator.

typos corrected and a reference added

Cited by in corpus (14)

• Gurevich-Zybin system
• Lagrangian multiforms and multidimensional consistency
• On Hamiltonian flows on Euler-type equations
• Gardner's deformations of the N=2 supersymmetric a=4-KdV equation
• Recursion operators and the hierarchies of MKdV equations related to $D_4^{(1)}$, $D_4^{(2)}$ and $D_4^{(3)}$ Kac-Moody algebras
• On the water-bag model of dispersionless KP hierarchy (II)
• On the water-bag model of dispersionless KP hierarchy
• Remarks on the Lagrangian representation of bi-Hamiltonian equations
• On the Geometry of Extended Self-Similar Solutions of the Airy Shallow Water Equations
• Multi-Lagrangians, Hereditary Operators and Lax Pairs for the Korteweg-de Vries Positive and Negative Hierarchies
• Non polynomial conservation law densities generated by the symmetry operators in some hydrodynamical models
• Hamiltonian formalism for nonlinear Schrödinger equations
• Elliptic Euler-Poisson-Darboux equation, critical points and integrable systems
• Rational Approximate Symmetries of KdV Equation