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

An improvement of the expansion methods, namely, the improved tan(Φ(ξ)/2)-expansion method, for solving nonlinear secondorder 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 tan(Φ(ξ)/2)-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.


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][6][7], the homotopy perturbation method [8,9], the sine-cosine method [10], the tanh-coth method [11][12][13], the modified extended tanhfunction method [14,15], the Exp-function method [16,17], the exp(−Φ())-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 tan(Φ()/2)-expansion 2 International Journal of Engineering Mathematics 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 tan(Φ()/2)-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 0 <  < 1 and (, ) is of the unknown function depending on the temporal variable  and the spatial variable .When  = −1, (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 (1 − )( − ).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][30][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 as where (), (), 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 timedependent 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 tan(Φ()/2)-expansion method in solving these kinds of problems.The paper is organized as follows.In Section 2, we describe the improved tan(Φ()/2)-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.

Description of Improved tan(Φ(𝜉)/2)-Expansion Technique
Step 1.We suppose the given nonlinear fractional partial differential equation for (, ) to be in the form which can be converted to an ODE: the transformation,  =  − , is wave variable.Also,  is constant to be determined later.

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   =  2 / 2 .Balancing the the linear term of the highest order   with the highest order nonlinear term  3 by using homogenous principal, we have Then, the trail solutions are Substituting ( 10) and ( 6) into (8) and collecting all terms with the same order of tan(Φ()/2) together, the left-hand side of ( 10) is converted into polynomial in tan(Φ()/2).Setting each coefficient of each polynomial to zero, we derive a set of algebraic equations for , , , , (), ,  0 ,  1 , and  1 as follows: International Journal of Engineering Mathematics )) ))

4
: Solving the set of algebraic equations using Maple, we get the following results.
Remark 1.In Figure 1, we plot two-and three-dimensional graphics of imaginary and real values of (14), which denote the dynamics of solutions with appropriate parametric selections.Also, in Figure 2, we draw two-and three-dimensional graphics of imaginary and real values of (15), which demonstrate the dynamics of solutions with convenient parametric choices.In Figure 3, we plot two-and three-dimensional graphics of (96), which represent the dynamics of solutions with proper parametric values.Moreover, in Figure 4, we