Conservation Laws for a Degasperis Procesi Equation and a Coupled Variable-Coefficient Modified Korteweg-de Vries System in a Two-Layer Fluid Model via the Multiplier Approach

We employ the multiplier approach (variational derivative method) to derive the conservation laws for the Degasperis Procesi equation and a coupled variable-coefficient modified Korteweg-de Vries system in a two-layer fluid model. Firstly, the multipliers are computed and then conserved vectors are obtained for each multiplier.


Introduction
The conservation laws are important in the solution and reductions of partial differential equations. Conservation law, also called law of conservation, in physics, is several principles that state that certain physical properties (i.e., measurable quantities) do not change in the course of time within an isolated physical system. In classical physics, laws of this type govern energy, momentum, angular momentum, mass, and electric charge. In particle physics, other conservation laws apply to properties of subatomic particles that are invariant during interactions. An important function of conservation laws is that they make it possible to predict the macroscopic behaviour of a system without having to consider the microscopic details of the course of a physical process or chemical reaction. Many powerful methods have been developed for the construction of conservation laws, such as The Laplace Direct method [1], multiplier approach [2,3], Kara and Mahomed symmetry condition [4], Wolf [5,6], Göktaş and Hereman [7], Hereman et al. [8][9][10], and Cheviakov [11] who developed powerful software packages to compute conservation laws for partial differential equations. Infinitely many conservation laws are obtained based on the Lax pair via the Hirota method and symbolic computation, bilinear forms, bilinear Backlund transformations, and one-and two-soliton-like solutions are also derived. With different coefficients, bell-shaped, periodic-changing, quadratic-varying, exponential-decreasing, and exponentialincreasing soliton-like profiles are obtained in [12]. Also by the spectral analysis the Hamiltonian and periodicity of the qZK equation are investigated by usig the Hirota method [13]. The nonautonomous matter waves with timedependent modulation in a one-dimensional trapped spin-1 Bose-Einstein condensate and the generalized three-coupled Gross-Pitaevskii equations by means of the Hirota bilinear method are studied in [14]. The multiplier approach (also known as variational derivative method) was proposed by Steudel [15] who wrote the conservation law in characteristic form as = . Later, Olver [16] modified the method of determining the characteristics (multipliers) by taking the variational derivative of = not only for the arbitrary functions, but also for solutions of system of partial differential equations. The outline of the paper is as follows. In Section 2, some definitions related to the multiplier approach are given. In Section 3, conservation laws for the Degasperis Procesi equation are derived by first computing the multipliers. The conservation laws for a coupled variable-coefficient modified Korteweg-de Vries system in a two-layer fluid model are derived in Section 4. Finally, conclusions are summarized in Section 5.
(1) The total derivative operator with respect to is where denotes the derivative of with respect to . Similarly denotes the derivative of with respect to and .
(2) The Euler operator is defined by Consider a th-order partial differential equation of independent and one dependent variable holds for all solutions of (3) is known as the conserved vector of (3).
Once the multipliers are computed from (6), the conserved vectors can be derived systematically using (5) as the determining equation. But in some problems it is not difficult to construct the conserved vectors by elementary manipulations after the determination of the multipliers.

Conservation Laws for the Degasperis Procesi Equation
The Degasperis Procesi equation [17] takes the form The Degasperis Procesi equation (7) is very interesting as it is an integrable shallow water equation and presents a quite rich structure. Also it can be used to model wave perturbations in relaxing media. We will derive the conservation laws for (7) by the multiplier approach. The determining equation for multiplier ( , , ), from (6), is The standard Euler operator / from (2) can be defined as and total derivative operators and using (1) are Equation (8) after expansion and simplification takes the following form: which yields From (5) and (12), we have International Journal of Partial Differential Equations 3 for arbitrary functions ( , ). When ( , ) is solutions of (7) then left hand side of (13) vanishes and we obtain Therefore the conserved vectors for the Degasperis Procesi equation (7) are The variational derivative approach for the Degasperis Procesi equation gives three multipliers of the form ( , , ) and hence three conserved vectors are obtained.

Conservation Laws for a Coupled Variable-Coefficient Modified Korteweg-de Vries System in a Two-Layer Fluid Model
In this section we recall some basic definitions related to the multiplier approach. Let ( , ) be two independent variables and let ( , V) be dependent variables.
(1) The total derivative operators and are (2) The standard Euler operators / and / V are Consider a th-order system of two partial differential equations of two independent and two dependent variables (3) A vector = ( 1 , 2 ) satisfying for all solutions of (19) is known as the conserved vector of (19).
(5) The determining equations for the multipliers are obtained by taking variational derivative of (21): Equation (22) holds for the arbitrary functions ( , ), V( , ) not only for the solutions of system (19). Equation (22) yields multipliers for all local conservation laws. Then conserved vectors can be derived systematically using (21) as the determining equation. But in some problems it is not difficult to construct the conserved vectors by elementary manipulations once the multiplier has been determined.

Example 1. Consider a coupled variable-coefficient modified
Korteweg-de Vries system in a two-layer fluid model System (23) was proposed in [18] as an important particular case of the formidable generalized coupled variablecoefficient modified Korteweg-de Vries (CVmKdV) system. The (CVmKdV) system was derived by Gao and Tang [19] as a two-layer model describing atmospheric and oceanic phenomena like interactions between the atmosphere and ocean, atmospheric blocking, oceanic circulations, hurricanes, typhoons, and so forth.
where the standard Euler operators / and / V are defined in (17) and (18), respectively. Expansion of (24) yields Equations (25) and (26) are separated according to different combinations of derivatives of and V and, after some simplification following system of equations for 1 , 2 is obtained: Solution of system (27) yields where 1 , 2 , 3 , and 4 are constants.

Conclusions
The conservation laws for the Degasperis Procesi equation and a coupled variable-coefficient modified Korteweg-de Vries system in a two-layer fluid model were established with the help of the multiplier approach. The multiplier approach on the Degasperis Procesi equation yielded three multipliers and thus three local conserved vectors were obtained in each case. The multiplier approach when applied to a coupled variable-coefficient modified Korteweg-de Vries system in a two-layer fluid model gave four multipliers of form ( , , , V). Each multiplier corresponds to a conserved vector and thus four local conserved vectors were obtained.