Research Article | Open Access
A New Soliton Hierarchy Associated with and Its Conservation Laws
Based on the three-dimensional real special orthogonal Lie algebra , we construct a new hierarchy of soliton equations by zero curvature equations and show that each equation in the resulting hierarchy has a bi-Hamiltonian structure and thus integrable in the Liouville sense. Furthermore, we present the infinitely many conservation laws for the new soliton hierarchy.
Conservation laws are widespread in applied mathematics, which reflect a phenomenon that some physical quantities do not change with time. In soliton theory, they play an important role in the study of integrability of soliton equations. An infinite number of conservation laws for KdV equation was first discovered by Miura in 1968 ; then many methods have been developed to find them. This may be mainly due to the contributions of Wadati et al. [2–4]. Conservation laws also play an important role in mathematics and engineering. Many papers dealing with symmetries and conservation laws have been presented. The direct construction method of multipliers for the conservation laws was presented in ; the Lagrangian approach for evolution equations was considered in [6, 7].
It is known that soliton hierarchies such as the Ablowitz-Kaup-Newell-Segur hierarchy and the Kaup-Newell hierarchy are generated from spectral problems associated with matrix Lie algebras . The trace identity is used to find Hamiltonian structures of soliton hierarchies . When associated matrix Lie algebras are non-semisimple, we can obtain integrable couplings [10, 11] and generate their Hamiltonian structures by the variational identity [12, 13].
We first recall a standard procedure for building soliton hierarchies. Starting from a spectral problem where is the spectral parameter and is a dependent variable, based on a matrix loop algebra associated with a given matrix Lie algebra , often being simple. We take solution to the stationary zero curvature equation We introduce the Lax matrices where denotes the polynomial part of in and is the modification terms, to formulate the temporal spectral problems The zero curvature equations can engender a hierarchy of soliton equations with Hamiltonian structures Those Hamiltonian functionals can often be generated by applying the trace identity [9, 14] where the constant is determined by
We will make use of the three-dimensional real special orthogonal Lie algebra ; this Lie algebra is simple and has a basis  along with the communicative operation The derived algebra is itself, and is one of the only two three-dimensional real Lie algebras with a three-dimensional derived algebra.
The matrix loop algebra we will adopt in what follows is The algebra contains matrices of the form with arbitrary integers . This matrix loop algebra lays a foundation for our study of soliton equations.
In this paper, we would like to from a spectral problem, based on the matrix loop algebra , and construct a new soliton hierarchy from associated zero curvature equations. The trace identity is used to furnish the corresponding Hamiltonian structures and so all equations in the resulting soliton hierarchy are Liouville integrable. Additionally, we introduce two variables and to construct conservation laws of the equation hierarchy and the first two conserved densities and fluxes are listed.
2. A New Spectral Problem and Soliton Hierarchy
To construct a soliton hierarchy from the matrix loop algebra , we consider an isospectral problem where This isospectral problem is of the same type as the Broer-Kaup-Kupershmidt one , but its underlying loop algebra is different.
Starting from the stationary zero curvature equation we have Setting further letting and choosing the initial data thus, substituting (18) into system (16), we obtain We impose the following conditions on constants of integration From the recursion relations in (20), the first few results can be obtained as follows:
Then, we consider the auxiliary spectral problem where Substituting (23) into the zero curvature equation we get a new soliton hierarchy
When , hierarchy (26) can be reduced to the second-order nonlinear integrable equations
As , hierarchy (26) is reduced to the third-order nonlinear integrable equations
3. Hamiltonian Structures of the New Soliton Hierarchy
A direct calculation reads Substitute the above formula into the trace identity (29) yields Comparing the coefficients of on both sides of (31) gives rise to By employing the computing formula (9) on the constant , we obtain . Therefore, we conclude that
So, the new soliton hierarchy (26) possesses the following Hamiltonian structures:
From the recursion relations in (20), we can obtain the hereditary recursion operator which satisfies thatwhere
4. Liouville Integrability
It is a direct but lengthy computation to verify that defined by (26) and constitute a Hamiltonian pair [17, 18]; that is, any linear combination of and satisfies for all vector fields , and . This implies that operator defined by (36) is hereditary , which satisfies for all vector fields and .
The hereditary property (39) is equivalent to where is an arbitrary vector field. The Lie derivative above is defined by where is the Lie bracket of vector fields. It is known that an autonomous operator is a recursion operator of an evolution equation if operator needs to satisfy Because operator (36) satisfies thuswhere are defined by (26). This implies that operator defined by (36) is a common hereditary recursion operator for the soliton hierarchy (26).
The new soliton hierarchy (26) has the following bi-Hamiltonian structures [17, 20]: where , , and are defined by (26), (37), and (33), respectively, and thus, the hierarchy is Liouville integrable; that is, it possesses infinitely many commuting symmetries and conservation laws. In particular, we have the Abelian symmetry algebra and the Abelian algebras of conserved functionals
The first two nonlinear integrable systems in hierarchy (26) are as follows: where the Hamiltonian functionals , , and are given by
5. Conservation Laws of the New Soliton Hierarchy
In the following, we will construct conservation laws of the new soliton hierarchy. We introduce the variables From (13), we have Expand in the power of : Substituting (52) into (51) and comparing the coefficients of the same power of , we obtain and a recursion formula for and :
in order to obtain the conservation laws for integrable hierarchy, we define Then (55) can be written as , which is just the formal definition of conservation laws. We expand and as series in powers of with the coefficients, which are called conserved densities and fluxes, respectively: the first two conserved densities and fluxes are read as follows:
The recursion relation for and is where and can be calculated from (54). We can display the first two conservation laws as where , , , and are defined in (59). Then the infinitely many conservation laws of (26) can be easily obtained from (50)–(61), respectively.
Based on the real loop algebra , we introduced a spectral problem and generated a new hierarchy of soliton equations from the associated zero curvature equations. The Liouville integrability of the resulting soliton equations has been shown upon furnishing a bi-Hamiltonian formulation by the trace identity. Finally, the conservation laws of the new equation hierarchy are also obtained. As its reduction, we gain the nonlinear integrable equations. The solution of reduced equations is a very important and difficult work, we will plan to study and discuss this problem in the near future.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by the National Natural Science Foundation of China (11547175, 11271008, and 11501526) and the Key Scientific Research Projects of Henan Province (16A110026).
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