Abstract
In this paper, the conservation laws for a generalized Itotype coupled Kortewegde Vries (KdV) system are constructed by increasing the order of the partial differential equations. The generalized Itotype coupled KdV system is a thirdorder system of two partial differential equations and does not have a Lagrangian. The transformation , converts the generalized Itotype coupled KdV system into a system of fourthorder partial differential equations in U and V variables, which has a Lagrangian. Noether’s approach is then used to construct the conservation laws. Finally, the conservation laws are expressed in the original variables u and v. Some local and infinitely many nonlocal conserved quantities are found for the generalized Itotyped coupled KdV system.
1 Introduction
In this paper, we consider the generalized Itotype coupled Kortewegde Vries (KdV) system [1]
It is well known that coupled nonlinear systems in which a KdV structure is embedded occur naturally in shallow water wave problems [2,3]. When , and , the system (1a) and (1b) is called Ito’s system and it describes the interaction process of two internal long waves [4,5]. It should be noted that in the absence of the effect of v, the system (1a) and (1b) reduces to the ordinary KdV equation. In [6] it has been shown that this Ito’s system can be a member of a biHamiltonian integrable hierarchy. The numerical methods for this system are very limited [1]. However, Xu and Shu [3] developed local discontinuous Galerkin methods for Ito’s system and proved the stability of these methods and, as a result, showed some good numerical results. Recently, in [1], the generalized Itotype coupled KdV system (1a) and (1b) was constructed as a multisymplectic Hamiltonian partial differential equation by introducing some new variables, and multisymplectic numerical methods were applied to investigate this system.
In this paper, we derive conservation laws for the generalized Itotype coupled KdV system (1a) and (1b). It is well known that the conservation laws play a central role in the solution and reduction of partial differential equations. Conservation laws are mathematical expressions of the physical laws, such as conservation of energy, mass, momentum and so on. In the literature, conservation laws have been extensively used in studying the existence, uniqueness and stability of solutions of nonlinear partial differential equations (see, for example, [79]). Conservation laws have also been applied in the development and use of numerical methods (see for example, [10,11]). Recently, conserved vectors associated with Lie point symmetries have been used to find exact solutions (by exploiting a double reduction method) of some classical partial differential equations [1214]. Thus, it is important to derive all the conservation laws for a given differential equation.
For variational problems, the celebrated Noether theorem [15] provides an elegant way to construct conservation laws. In fact, it gives an explicit formula for determining a conservation law once a Noether symmetry associated with a Lagrangian is known for an EulerLagrange equation. Thus, the knowledge of a Lagrangian is essential in this case. However, there are differential equations, such as scalar evolution differential equations, which do not have a Lagrangian. In such cases, several methods [1627] have been developed by researchers about the construction of conserved quantities. Comparison of several different methods for computing conservation laws can be found in [21,27].
2 Conservation laws for the Itotype coupled KdV system
In this section, we construct conservation laws for the Itotype KdV system (1a) and (1b). We note that this system does not have a Lagrangian. However, we can put the system into a variational form by letting , . Then the Itotype coupled KdV system (1a) and (1b) transforms into a fourthorder system, viz.,
It can be readily verified that L given by
is a Lagrangian for the system (2a) and (2b). This is because L satisfies
where and are the EulerLagrange operators defined by
and
Consider the vector field
which has the second extension
where
and
We recall that the vector field X, of the form (7), is called a Noether point symmetry generator corresponding to the Lagrangian L if there exists gauge functions and such that
The insertion of L from (3) into Eq. (9) yields
The analysis of Eq. (10) prompts the following two cases.
2.1 Case 1
In this case, the splitting of (10) with respect to different combinations of the derivatives of U and V gives an overdetermined system of linear partial differential equations for , , , , and . Solving this system yields the following Noether symmetries and gauge terms:
The above results will now be used to find the components of the conserved vectors for the secondorder Lagrangian L. Here we can choose , as they contribute to the trivial part of the conserved vector. We recall that the conserved vectors for the secondorder Lagrangian L are given by [15,27]
where and are the Lie characteristic functions. Thus, using (12) and (13) together with (11) and , yields the following independent conserved vectors for our system (1a) and (1b):
It should be noted that the conserved vectors (14) and (16) are nonlocal conserved vectors and (15) is a local conserved vector for the system (1a) and (1b). We now extract two special cases from the conserved vector (16) by letting and , which gives a nonlocal conserved vector
and by choosing and , which gives a nonlocal conserved vector
We note that since the functions and are arbitrary, one obtains infinitely many nonlocal conservation laws for the system (1a) and (1b). It should also be noted that for the special values of α, β and γ, namely, , and , we retrieve the three constants of the motion , and obtained in [4].
2.2 Case 2
In this case, we obtain the following Noether symmetries and gauge terms:
Again, we can set , as they contribute to the trivial part of the conserved vector. The independent conserved vectors for the system (1a) and (1b) in this case are as follows:
Note that the conserved vectors (19), (21) and (22) are nonlocal conserved vectors whereas the conserved vector (20) is a local conserved vector for the system (1a) and (1b). The conserved vector (22), for and , gives a nonlocal conserved vector
and for and , it gives a nonlocal conserved vector
We see that for the arbitrary values of and , infinitely many nonlocal conservation laws exist for the system (1a) and (1b).
3 Concluding remarks
In this paper, we studied the generalized Itotype coupled Kortewegde Vries system (1a) and (1b). This system does not have a Lagrangian. In order to apply Noether’s theorem, we transformed the system into the fourthorder system (2a) and (2b), which admitted a standard Lagrangian (3). Then Noether’s approach was used to derive the conservation laws in U and V variables. Finally, by reverting back to our original variables u and v, we obtained the conservation laws for the thirdorder generalized Itotype coupled KdV system (1a) and (1b). The conservation laws for the generalized Itotype coupled KdV system consisted of some local and infinite number of nonlocal conserved vectors.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
ETM, BM and CMK worked together in the derivation of the mathematical results. All authors read and approved the final manuscript.
Acknowledgements
This paper is dedicated to Prof. Ravi P. Agarwal on his 65th birth anniversary. BM and CMK would like to thank the Organizing Committee of ‘International Conference on Applied Analysis and Algebra’, (ICAAA 2012) Conference for their kind hospitality during the conference. BM also thanks the Faculty Research Committee of FAST, NorthWest University, and ETM thanks SANHARP for financial support.
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