International Journal of Engineering Mathematics

Volume 2015, Article ID 107978, 35 pages

http://dx.doi.org/10.1155/2015/107978

## New Improvement of the Expansion Methods for Solving the Generalized Fitzhugh-Nagumo Equation with Time-Dependent Coefficients

Department of Applied Mathematics, Faculty of Mathematics Science, University of Tabriz, Tabriz 51569 84687, Iran

Received 30 May 2015; Revised 10 September 2015; Accepted 10 September 2015

Academic Editor: Josè A. Tenereiro Machado

Copyright © 2015 Jalil Manafian and Mehrdad Lakestani. 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

An improvement of the expansion methods, namely, the improved -expansion method, for solving nonlinear second-order partial differential equation, is proposed. The implementation of the new approach is demonstrated by solving the generalized Fitzhugh-Nagumo equation with time-dependent coefficients. As a result, many new and more general exact travelling wave solutions are obtained including periodic function solutions, soliton-like solutions, and trigonometric function solutions. The exact particular solutions contain four types: hyperbolic function solution, trigonometric function solution, exponential solution, and rational solution. We obtained further solutions comparing this method with other methods. The results demonstrate that the new -expansion method is more efficient than the Ansatz method and Tanh method applied by Triki and Wazwaz (2013). Recently, this method is developed for searching exact travelling wave solutions of nonlinear partial differential equations. Abundant exact travelling wave solutions including solitons, kink, and periodic and rational solutions have been found. These solutions might play an important role in engineering fields. It is shown that this method, with the help of symbolic computation, provides a straightforward and powerful mathematical tool for solving the nonlinear physics.

#### 1. Introduction

Nonlinear evolution equations (NLEEs) are very important model equations in mathematical physics, engineering, and applied mathematics for describing diverse types of physical mechanisms of natural phenomena in the field of applied sciences, biochemistry, and engineering. Nonlinear wave equations play a major role in various fields such as plasma physics, fluid mechanics, optical fibers, solid state physics, chemical kinetics, geochemistry, and nonlinear optics [1, 2]. Much work has been done over the years on the subject of obtaining the analytical solutions to the nonlinear partial differential equations (NPDEs). One of the most exciting advances of nonlinear science and theoretical physics has been the development of methods to look for exact solutions for NPDEs. The advent of symbolic computation also enables performing some complicated and tedious algebraic processes coupled with differential calculations. With the rapid development of nonlinear sciences based on computer algebraic system, many effective methods have been presented, such as the homotopy analysis method [3, 4], the variational iteration method [5–7], the homotopy perturbation method [8, 9], the sine-cosine method [10], the tanh-coth method [11–13], the modified extended tanh-function method [14, 15], the Exp-function method [16, 17], the exp()-expansion method [18, 19], the -expansion method [20, 21], the modified simple equation method [22], the novel -expansion method [23], the new approach of the generalized -expansion method [21], the Jacobi elliptic function method [24], and the homogeneous balance method [25]. Recently, Naher and Abdullah [21] presented an effective and straightforward method, called the new approach of generalized -expansion method, to obtain exact travelling wave solutions of nonlinear evolution equations. Here, we use an effective method, namely, the improved -expansion method, for constructing a range of exact solutions for the following ordinary partial differential equations that in this paper we developed solutions as well. In this paper, we put forth the new approach of improved -expansion method to construct exact travelling wave solutions including solitons, kink, and periodic and rational solutions to the generalized Fitzhugh-Nagumo equation with time-dependent coefficients. In [22, 26], the standard form of the Fitzhugh-Nagumo equation is given:where and is of the unknown function depending on the temporal variable and the spatial variable . When , (1) reduces to the real Newell-Whitehead equation which describes the dynamical behavior near the bifurcation point for the Rayleigh-Bénard convection of binary fluid mixtures [27]. The FN equation has various applications in the fields of flame propagation, logistic population growth, neurophysiology, branching Brownian motion process, autocatalytic chemical reaction, and nuclear reactor theory [28]. This equation combines diffusion and nonlinearity which is controlled by the term . Many physicists and mathematicians have paid much attention to the Fitzhugh-Nagumo equation in recent years due to its importance in mathematical physics. Remarkably, this nonlinear evolution equation is an important nonlinear reaction-diffusion equation and is usually used to model the transmission of nerve impulses [29–31]. Shih et al. utilized the approximate conditional symmetry method to determine approximate solutions admitted by a perturbation of (1). Equation (1) has been presented by Hariharan and Kannan via Haar wavelet method [32]. Kawahara and Tanaka [33] have found new exact solutions of (1), by applying the nonclassical symmetry reductions approach by using Hirota method. In [34], Nucci and Clarkson have obtained some new exact solutions with Jacobi elliptic function. Li and Guo [35] have obtained the new exact solutions of the Fitzhugh-Nagumo equation by using first integral method. In this paper, we consider that the generalized Fitzhugh-Nagumo equation with time-dependent coefficients is given aswhere , , and are arbitrary functions of . The nonlinear models with variable coefficients are needed to describe the propagation of pulses. Triki and Wazwaz [36] obtained a new variety of soliton solutions by means of specific solitary wave Ansatz and the tanh method for (2). Also, Jiwari et al. [37] applied polynomial differential quadrature method to find the numerical solution of the generalized Fitzhugh-Nagumo equation with time-dependent coefficients in one-dimensional space. In [38], the effect of diffusion on pattern formation in FitzHugh-Nagumo model has been surveyed. Van Gorder applied the method of homotopy analysis to study the Fitzhugh-Nagumo equation [39]. In [40], a FitzHugh-Nagumo monodomain model has been used to describe the propagation of the electrical potential in heterogeneous cardiac tissue. Abbasian et al. have searched symmetric bursting behaviors in the generalized FitzHugh-Nagumo model [41]. Ray and Sahooz used the fractional subequation method for solving the space-time fractional Zakharov-Kuznetsov and modified Zakharov-Kuznetsov equations [42]. Also, the authors of [43] applied the fractional subequation method to the time fractional KdV-Zakharov-Kuznets and space-time fractional modified KdV-Zakharov-Kuznetsov equations. The purpose of this paper is to obtain exact solutions of the generalized Fitzhugh-Nagumo equation and to determine the accuracy of the improved -expansion method in solving these kinds of problems. The paper is organized as follows. In Section 2, we describe the improved -expansion method. In Section 3, we examine the generalized Fitzhugh-Nagumo equation with method introduced in Section 2. Also, conclusion and advantages are given in Section 4. Finally, some references are given at the end of this paper.

#### 2. Description of Improved -Expansion Technique

*Step 1. *We suppose the given nonlinear fractional partial differential equation for to be in the formwhich can be converted to an ODE:the transformation, , is wave variable. Also, is constant to be determined later.

*Step 2. *Suppose the traveling wave solution of (4) can be expressed as follows:where and are constants to be determined, such that , and satisfies the following ordinary differential equation:We will consider the following special solutions of (6): *Family 1*:* When ** and *, then *Family 2*:* When ** and *, then *Family 3: When ** and *, then *Family 4: When ** and *, then *Family 5: When ** and *, then *Family 6: When ** and *, then *Family 7: When ** and *, then *Family 8*:* When *, then *Family 9*:* When *, then *Family 10: When ** and *, then *Family 11: When *, then *Family 12: When *, then *Family 13: When *, then *Family 14: When *, then *Family 15: When ** and *, then *Family 16: When ** and *, then *Family 17: When ** and *, then ,
where , and are constants to be determined later. But the positive integer can be determined by considering the homogeneous balance between the highest order derivatives and nonlinear terms appearing in (6). If is not an integer, then a transformation formula should be used to overcome this difficulty.

*Step 3. *Substituting (5) into (4) with the value of obtained in Step 2, collecting the coefficients of , and then setting each coefficient to zero, we can get a set of overdetermined partial differential equations for , and with the aid of symbolic computation Maple.

*Step 4. *Solve the algebraic equations in Step 3, and then substitute in (5).

#### 3. The Generalized Fitzhugh-Nagumo Equation

We consider the generalized Fitzhugh-Nagumo equation with time-dependent coefficients as follows:where , and are arbitrary functions of . By using the wave variable , reduce it to an ODE as follows:where and . Balancing the the linear term of the highest order with the highest order nonlinear term by using homogenous principal, we haveThen, the trail solutions areSubstituting (10) and (6) into (8) and collecting all terms with the same order of together, the left-hand side of (10) is converted into polynomial in . Setting each coefficient of each polynomial to zero, we derive a set of algebraic equations for , and as follows:Solving the set of algebraic equations using Maple, we get the following results.

*Set 1. *We have the followingwhere , and are arbitrary constants. By using (13), Families 7, 9, and 13 can be written, respectively, aswhere .

*Set 2. *We have the following: where , and are arbitrary constants. By using (17), Families 7, 9, and 13 can be written, respectively, aswhere .

*Set 3. *We have the following: where , and are arbitrary constants. By using (20), Families 7, 9, and 13 can be written, respectively, aswhere .

*Set 4. *We have the following: where , and are arbitrary constants. By using (23), Families 7, 9, and 13 can be written, respectively, aswhere .

*Set 5. *We have the following: where , and are arbitrary constants. By using (26), Families 7, 9, and 13 can be written, respectively, aswhere .

*Set 6. *We have the following: where , and are arbitrary constants. By using (29), Families 7, 9, and 13 can be written, respectively, aswhere .

*Set 7. *We have the following: where , and are arbitrary constants. By using (32), Family 2 can be written aswhere .

*Set 8. *We have the following: where , and are arbitrary constants. By using (35), Family 2 can be written aswhere .

*Set 9. *We have the following: where , and are arbitrary constants. By using (38), Family 2 can be written aswhere + + .

*Set 10. *We have the following: where , and are arbitrary constants. By using (41), Family 2 can be written aswhere .

*Set 11. *We have the following: where , and are arbitrary constants. By using (44), Family 2 can be written aswhere .

*Set 12. *We have the following: