## Integrable Couplings: Generation, Hamiltonian Structures, Conservation Laws, and Applications

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# Binary Nonlinearization for AKNS-KN Coupling System

**Academic Editor:**Yufeng Zhang

#### Abstract

The AKNS-KN coupling system is obtained on the base of zero curvature equation by enlarging the spectral equation. Under the Bargmann symmetry constraint, the AKNS-KN coupling system is decomposed into two integrable Hamiltonian systems with the corresponding variables and the finite dimensional Hamiltonian systems are Liouville integrable.

#### 1. Introduction

Since the integrable coupling definition is proposed, we have got many integrable coupling systems. Furthermore, the exact solutions, Darboux transformation, and Hamiltonian structure of these coupling systems have been obtained [1â€“3]. In 2008, the AKNS-KN coupling system is obtained by the loop algebra whose Hamiltonian system is received with the variational identity [4]. In 1994, the binary nonlinearization method was put forward by Li and Ma [5], and then the technique of the binary nonlinearization has been successfully applied to many soliton equations, such as the AKNS hierarchy, the KdV hierarchy, and the super NLS-MKDV hierarchy, but there are few results on binary nonlinearization of the coupling system. In this paper, we design a proper spectrum equation and obtain the AKNS-KN coupling system under the zero curvature equation, but the recursive operator is different from the operator of [4].

This paper is organized as follows. In Section 2, we will consider the AKNS-KN coupling soliton hierarchy. Bargmann symmetry constraint for the AKNS-KN coupling system will be given in Section 3. Section 4 will be devoted to study the AKNS-KN coupling system by employing the binary nonlinearization technique which involves two sets of dependent variables and . We especially list the special cases, such as AKNS integrable coupling system and KN integrable coupling system.

#### 2. The AKNS-KN Coupling System

We design a spectral problem , where the spectral operator is as follows: and set where Under the zero curvature equation , we read Equation (4) is equivalent to where If we choose the initial conditions as then we can get all the other values according to (6). The first few sets are Let us associate (1) with the following problem: with The compatible condition of the spectral problem (1) and the auxiliary problem (10) is

After a direct calculation, we can get the AKNS-KN coupling system: Therefore, the AKNS-KN coupling system can be written as where the operator is determined by (6) and the Hamiltonian operator is as follows: wherewhere

To simplify (13), let ; then the AKNS integrable coupling system (14) can be written as where

Furthermore, let , and then is the AKNS system, where

On the other hand, when , then is the KN integrable coupling system, where

When , then is the KN system, where

#### 3. Bargmann Symmetry Constraint of AKNS-KN Coupling System

In order to get a Bargmann symmetry constraint, we can consider the Lax pairs and the adjoint Lax pairs of the AKNS-KN coupling system. The adjoint Lax pairs of the AKNS-KN coupling system arewhere means the transpose of matrix and . It follows from that where .

According to the zero boundary conditions , we can get where and are given by (6) and (26).

Now, let us discuss the spatial systems and the temporal systems where and are distinct parameters. From [6] and [7], the expression of the potential can be easily calculated:

#### 4. Binary Nonlinearization of AKNS-KN Coupling System

In order to perform binary nonlinearization of AKNS-KN coupling system, let us substitute (30) into the Lax pairs and adjoint Lax pairs (28) and (29); then we can get the following nonlinearized spatial Lax pairs and the adjoint Lax pairs: where means an expression of under the constraint equation (30). Clearly, (28) can be written: where . When , the coupling system equation is exactly system equation with . Obviously, system (28) can be written in the following form: where the Hamiltonian form is the following: When , then (33) can be written as and the Hamiltonian system is given by Furthermore, when , then (33) deduces to and the Hamiltonian form is given by

When , (33) is equivalent to the following: and the Hamiltonian system is given by

When , (33) is equivalent to the following system: