Discrete Dynamics in Nature and Society

Volume 2011, Article ID 534165, 10 pages

http://dx.doi.org/10.1155/2011/534165

## Homotopy Perturbation Method for Solving Wave-Like Nonlinear Equations with Initial-Boundary Conditions

Mathematics and Computing Department, Beykent University, 34396 Istanbul, Turkey

Received 4 May 2011; Revised 27 June 2011; Accepted 20 July 2011

Academic Editor: Pham Huu Anh Ngoc

Copyright © 2011 Afgan Aslanov. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The homotopy perturbation method is employed to obtain approximate analytical solutions of the wave-like nonlinear equations with initial-boundary conditions. An efficient way of choosing the auxiliary operator is presented. The results demonstrate reliability and efficiency of the method.

#### 1. Introduction

In this paper, we consider the equation with initial conditions and boundary condition where , , , , and are known functions.

Problems like (1.1)-(1.2)-(1.3) model many problems in classical and quantum mechanics, solitons, and matter physics [1, 2]. If is a function of only, we obtain a Klein-Gordon or sine-Gordon-type equations.

In the last decade, some various approximate methods have been developed, such as the homotopy perturbation method (HPM) [3–13] and Adomian’s decomposition method (ADM) [14–20] to solve linear and nonlinear differential equations.

Unlike the various approximation techniques for solving nonlinear wave type problems, which are usually valid for initial value problems (without boundary conditions) or some special type of problems (homogenous, etc.), our technique is applicable for all initial-boundary problems of type (1.1)-(1.2)-(1.3). Chowdhury and Hashim [9] applied the HPM for solving Klein-Gordon and sine-Gordon equations, with initial conditions (1.2). El-Sayed [19] and Wazwaz and Gorguis [20] used ADM for solving wave-like and heat-like problems. Their approaches cannot be applied for all wave-like equations with initial-boundary conditions since the operator cannot control the boundary condition (1.3)—see Example 2.1 below.

The central idea here is that the problem has a unique solution (see, e.g., [21]) and therefore there exists an inverse of the operator .

The main idea of HPM is to introduce a homotopy parameter, say , which takes values from 0 to 1. When the equation usually reduces to a sufficiently simplified form (linear or very easy nonlinear). As increases to 1, the equation goes through a sequence of “deformations” (homotopics) and at takes the original form of the equation.

We rewrite (1.1) as and construct the following homotopy: Usually we take as a solution of the problem (1.1)-(1.2)-(1.3) with or simply . Assume that the solution of (1.6) is in the form and substituting (1.7) into (1.6) and equating terms of the same powers of we obtain a system of equations for . Solving these system of equations we obtain a solution in the form

#### 2. Applications

The HPM and ADM offer excellent choices for obtaining the closed-form analytical solutions of wave-like equations. Chowdhury and Hashim [9], El-Sayed [19], and Wazwaz and Gorguis [20] recently showed how the HPM and ADM can be applied to find an analytic approximate solution of wave-like equations with initial conditions (1.2). They mainly used the operator for solving the wave-like problems. But in case of inhomogeneous nonlinear or even linear equations with initial-boundary conditions, these approaches have some difficulties. If we construct the standard homotopy with , for solving wave-like equations with initial-boundary conditions, usually in the second or even in the first stage of HPM, we obtain an “overdetermined” or very difficult problem. To explain these difficulties we consider the following example.

*Example 2.1. *Consider the linear problem
The exact solution is
Using our HPM technique we can easily find this solution. Indeed, let us take , and construct the homotopy
Now substituting (1.7) into and equating the coefficients of like powers of , we get a system of linear equations
Solving correspondingly we obtain
. Thus we obtain an exact solution .

Now let us show that this problem can not be solved when is taken as .

Indeed if we choose or (which seems most natural and appropriate) and and construct the homotopy

we obtain an overdetermined problem for in the form

which has no solution.

If in the first stage we consider the boundary conditions , , , we obtain and in the second stage we need to solve the problem

which is overdetermined again and has no solution.

*Example 2.2. *Consider the problem
The exact solution is . We take again , , and construct the homotopy
Substituting (1.7) into (2.10) we obtain
and equating terms of the coefficients of like powers of gives

Solving these equations we obtain (see [21, Chapter 3.4])for and for . In a similar manner we have and therefore

for . In a similar manner we obtain for , where and so The absolute errors between the exact and three term approximation of the series solution for some values of are shown in Table 1.

*Example 2.3. *Now we consider the problem

The exact solution is . We take , and construct the homotopy Now substituting (1.7) into (2.20) and equating terms of the coefficients of like powers of , we obtain

Solving equations yields In a similar manner we get

for . Taking only two-term approximation for we have for . The absolute errors between the exact and two-term approximation of the series solution for some values of are shown in Table 2. A higher accuracy level can be attained by evaluating some more terms.

*Example 2.4. *Now we consider the problem
It is easy to show that the HPM can not be applied if is taken as . The exact solution is
We construct the homotopy
with . Substituting (1.7) into and equating the coefficients of like powers of , we get a system of linear equations
Solving we obtain
Continuing we obtain
Thus the three-term approximation is
and the error is less than for .

#### 3. Conclusion

Homotopy perturbation method has been successful for solving many linear and nonlinear wave-type problems. However, it has difficulties in dealing with initial boundary problems, namely, in including all initial and boundary conditions together into the process of homotopy perturbation and computation. Our main goal is to construct the homotopy perturbation scheme containing all initial and boundary conditions together. The goal is achieved by involving an auxiliary operator which includes both variables and .

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