Multiform description of the AKNS hierarchy and classical r-matrix

Abstract

In recent years, new properties of space-time duality in the Hamiltonian formalism of certain integrable classical field theories have been discovered and have led to their reformulation using ideas from covariant Hamiltonian field theory: in this sense, the covariant nature of their classical $r$-matrix structure was unraveled. Here, we solve the open question of extending these results to a whole hierarchy. We choose the Ablowitz-Kaup-Newell-Segur (AKNS) hierarchy. To do so, we introduce for the first time a Lagrangian multiform for the entire AKNS hierarchy. We use it to construct explicitly the necessary objects introduced previously by us: a symplectic multiform, a multi-time Poisson bracket and a Hamiltonian multiform. Equipped with these, we prove the following results: $(i)$ the Lax form containing the whole sequence of Lax matrices of the hierarchy possesses the rational classical $r$-matrix structure; $(ii)$ The zero curvature equations of the AKNS hierarchy are multiform Hamilton equations associated to our Hamiltonian multiform and multi-time Poisson bracket; $(iii)$ The Hamiltonian multiform provides a way to characterise the infinite set of conservation laws of the hierarchy reminiscent of the familiar criterion $\{I,H\}=0$ for a first integral $I$.

34 pages; accepted version

References in corpus (4)

• A multisymplectic approach to defects in integrable classical field theory
• A Variational Approach to Lax Representations
• Multisymplectic approach to integrable defects in the sine-Gordon model
• Multi-time Lagrangian 1-forms for families of Bäcklund transformations. Relativistic Toda-type systems

Cited by in corpus (7)

• On the Zakharov-Mikhailov action: $4$d Chern-Simons origin and covariant Poisson algebra of the Lax connection
• Classical Yang-Baxter equation, Lagrangian multiforms and ultralocal integrable hierarchies
• Lagrangian multiforms on coadjoint orbits for finite-dimensional integrable systems
• Lagrangian multiforms on Lie groups and non-commuting flows
• Hamiltonian formulation and aspects of integrability of generalised hydrodynamics
• Quantum integrability: Lagrangian 1-form case
• On the geometry of Lagrangian one-forms