Variational Iteration Method for Solving the Generalized Degasperis-Procesi Equation

and Applied Analysis 3 Step 1. Make the independent variable transformation: ξ = k (x − ωt) + ξ 0 . (13) Here, ξ 0 ∈ C is an arbitrary complex number. k is wave number;ω is wave velocity. Substituting (13) into (12), we have −ωu 󸀠 + k 2 ωu 󸀠󸀠󸀠 + 4uu 󸀠 − 3kuu 󸀠󸀠 − k 2 uu 󸀠󸀠󸀠 = f 1 . (14) Here, u󸀠 is the derivative of u with respect to ξ; that is, u󸀠 = du/dξ. f 1 = f 1 (u, u 󸀠 , u 󸀠󸀠 , u 󸀠󸀠󸀠 ). Step 2. From [34], we find the special solution, when f is identical to 0:


Introduction
The theory of soliton has extensive applications in physics, mechanics, and combustion science.In recent years, many researchers studied the soliton theory in the fields of shock wave [1,2], light scattering, quantum mechanics, atmospheric physics, neural networks, explosion, and combustion [3].There are many new methods for searching the soliton solution of nonlinear evolution equations such as hyperbolic tangent function method [4], the homogeneous balance methods [5], Jacobi elliptic function expansion method [3], and pseudo-spectral method [6].
The variational iteration method (VIM) was developed, in 1999, by He [7][8][9][10][11][12][13]. The VIM gives rapidly convergent successive approximations of the exact solution if such a solution exists; otherwise, a few approximations can be used for numerical purposes.The Adomian decomposition method suffers from the complicated computational work needed for the derivation of Adomian polynomials for nonlinear terms.The VIM has no specific requirements, such as linearization, small parameters for nonlinear operators.Therefore, the VIM can overcome the foregoing restrictions and limitations of perturbation techniques, so that it provides us with a possibility to analyze strongly nonlinear problems.
On the other hand, the VIM is capable of greatly reducing the size of calculation while still maintaining high accuracy of the numerical solution [14].Moreover, the power of the method gives it a wider applicability in handling a huge number of analytical and numerical applications.The VIM was successfully applied to study a variety of differential equations.It is based on Lagrange multiplier, and it has the merits of simplicity and easy execution.As a result, it has been proved by many authors to be a powerful mathematical tool for addressing various kinds of linear and nonlinear problem.For example, this method was used for solving nonlinear wave equations and the Laplace equation by Wazwaz [14].The VIM for solving linear systems of ODEs with constant coefficients was studied by Khojasteh Salkuyeh [15].Helmholtz equation was researched by Momani and Abuasad [16].Geng [17] introduced the piecewise VIM for solving Riccati differential equation.Fractional vibration equation was researched by Das [18].Furthermore, higher order boundary value problems were researched by Xu [19],Noor, and Mohyud-Din [20].Noor et al. [21] applied a modified He's variational iteration method for solving singular fourthorder parabolic partial differential equations.The proposed modification is made by introducing He's polynomials in the correction functional.Ghorbani and Saberi-Nadjafi [22] modified the VIM by constructing an initial trial function without unknown parameters.Sevimlican [23] constructed approximate Green's function for a vector equation for the electric field by using VIM.
In this paper, we are concerned with the variational iterations method for solving the generalized Degasperis-Procesi equation.As a review, we will recall the VIM briefly in Section 2.

Variational Iteration Method
In this section, the basic concepts of variational iteration method (VIM) are introduced.Here, a description of method [7][8][9][10][11][12][13][14][15] is given to handle the general nonlinear problem.Consider the differential equation of the form  (, ) +  (, ) =  (, ) , where  is a linear operator,  is a nonlinear operator, and (, ) is the inhomogeneous term.According to He's variational iteration method, we can construct a correction functional for (1) as follows: where  is a general Lagrange multiplier, which can be identified optimally via variational theory [12,24].Here ũ is considered as a restricted variation [14,25] which means ũ  = 0; the subscript  denotes the th approximations.The successive approximations  +1 (, ), of the solution  (, ), can be obtained after using the obtained Lagrange multiplier and the zeroth approximation  0 (, ), which are selected from any function that satisfies the initial conditions.With  determined, several approximations  +1 (, ),  ⩾ 0 follow.Consequently, the exaction solution may be obtained as In fact, the VIM depends on the suitable selection of the initial approximation  0 (, ).Moreover, we use a well-known, powerful tool to prove the convergence of the sequence obtained via the VIM and its rate.It is the Banach's fixed point theorem that follows.
Theorem 1 (Banach's fixed point theorem).Assume that  is a Banach space and is a nonlinear mapping, and suppose that for some constant  < 1.Then  has a unique fixed point.Furthermore, the sequence with an arbitrary choice of  0 ∈  converges to the fixed point of .
According to Theorem 1, for the nonlinear mapping a sufficient condition for the convergence of the variational iteration method is strict contraction of .Furthermore, the sequence (2) converges to the fixed point of  which is also the solution of problem (1).Some modifications to prove the convergence speed and to lengthen the interval of convergence for VIM series solution are suggested in [17,[26][27][28][29][30].
Variational iteration method for KdV-Burgers and Lax's seventh-order KdV equations has been studied by Soliman [32].In this paper, we consider the generalized Degasperis-Procesi equation that was proposed in [33]. is the generalized perturbation item.We suppose  is a sufficiently smooth function of the variable.
Step 2. From [34], we find the special solution, when  is identical to 0: Here, ũ , ũ  , ũ  , and ũ  are considered as a restricted variation [35].That is, Step 4.Under the above condition, make the correct functional stationary with respect to   ; noticing that   (0) = 0, we have For arbitrary  +1 , from the above relation, we obtain the Euler-Language equation: Solve ( 19), we derive Substituted ( 20) into ( 16), we have the integration form: From the above solution procedure, we can see that the approximate solutions converge to its exact solution.That is,  (, ) = lim  → ∞   (, ),   (, ) is the approximate solution with arbitrary degree of accurate solitary wave of Degasperis-Procesi equation.
Step 5. Calculation of the approximate solution.
According to the integration form (21),we can calculate the approximate solution.Firstly, let (15) be the zero-order approximate solution: Substitute ( 15) into (21).We obtain the one-order approximate solution  1 ()  Then, substitute (23) into (21).We can obtain the secondorder approximate solution  2 (): Using the same method, we can get the higher order approximate solution.

Conclusion
By the analysis of structure on the left side of ( 8) and the properties of  about the variable and the analytical variational iteration formula, we can prove that the sequence of functions of {  ()} decided by ( 21) is uniform convergence.So the limit function of {  ()} is the solution of the equation.Moreover, the zero-order approximate solution  0 () is the soliton of (8) in which  = 0; it should be specially pointed out that the more accurate the identification of the multiplier is, the faster the approximations converge to their exact solution.