The Extended Fractional Subequation Method for Nonlinear Fractional Differential Equations

An extended fractional subequation method is proposed for solving fractional differential equations by introducing a new general ansätz and Bäcklund transformation of the fractional Riccati equation with known solutions. Being concise and straightforward, this method is applied to the space-time fractional coupled Burgers’ equations and coupled MKdV equations. As a result, many exact solutions are obtained. It is shown that the considered method provides a very effective, convenient, and powerful mathematical tool for solving fractional differential equations.


Introduction
In recent years, nonlinear fractional differential equations NFDEs have been attracted great interest.It is caused by both the development of the theory of fractional calculus itself and by the applications of such constructions in various sciences such as physics, engineering, and biology 1-7 .For better understanding the mechanisms of the complicated nonlinear physical phenomena as well as further applying them in practical life, the solution of fractional differential equation 8-15 is much involved.In the past, many analytical and numerical methods have been proposed to obtain solutions of NFDEs, such as finite difference method 16 , finite element method 17 , differential transform method 18, 19 , Adomian decomposition method 20-22 , variational iteration method 23-25 , and homotopy perturbation method 26-28 .Recently, He 29 introduced a new method called fractional sub-equation method to look for traveling wave solutions of NFDEs.The method is based on the homogeneous balance principle 30 and Jumarie's modified Riemann-Liouville derivative 31-33 .

Mathematical Problems in Engineering
By using fractional sub-equation method, Zhang et  The present paper is motivated by the desire to improve the work made in 30, 34, 35 by proposing a new and more general ansätz so that it can be used to construct more general exact solutions which contain not only the results obtained by using the method in 30, 34, 35 as special cases but also a series of new and more general exact solutions.To illustrate the validity and advantages of the method, we will apply it to the space-time fractional coupled Burgers' equations and coupled MKdV equations.
The rest of this paper is organized as follows.In Section 2, we will describe the Modified Riemann-Liouville derivative and give the main steps of the method here.In Section 3, we illustrate the method in detail with space-time fractional coupled Burgers' equations and coupled MKdV equations.In Section 4, some conclusions are given.

Description of Modified Riemann-Liouville Derivative and the Proposed Method
The Jumarie's modified Riemann-Liouville derivative is defined as

2.1
Some properties for the proposed modified Riemann-Liouville derivative are listed in 32, 33, 36 as follows: We present the main steps of the extended fractional sub-equation method as follows.
Step 1.For a given NFDEs with independent variables X x 1 , x 2 , x 3 , . . ., x m , t and dependent variable u, where D α t u, D α x 1 u, D α x 2 u, and D α x 3 u are the modified Riemann-Liouville derivatives of u with respect to t, x 1 , x 2 , and x 3 , and P is a polynomial in u u x 1 , x 2 , x 3 , . . ., x m , t and its various partial derivatives, in which the highest order derivatives and nonlinear terms are involved.
Step 2. By means of the traveling wave transformation, where k i and c are constants to be determined later; 2.6 becomes the following form: Step 3. We suppose that 2.6 has the following solution: where a i i −m, −m 1, . . ., m − 1, m are constants to be determined later, and Here B, D are arbitrary parameters, and φ ξ satisfies the following fractional Riccati equation: where σ is a constant.Recently, Zhang et al.'s 30 first obtained the following solutions of 2.11 : where the generalized hyperbolic and trigonometric functions are defined as

2.13
Here, E α z Step 4. Substituting 2.9 into 2.8 and collecting all terms with the same order of ψ ξ , then setting each coefficient of ψ k ξ to zero, yields a set of overdetermined nonlinear algebraic system for a i i −m, . . ., m , k j j 1, 2, . . ., m − 1 and c.
Step 5. Using the results obtained in the above steps and the solutions of 2.11 into 2.9 , we can finally obtain exact solutions of 2.6 .
Remark 2.2.As we know, the choice of an appropriate ansätz is very important when using the direct method to look for exact solutions.It can be easily found that the transformation 2.9 is more general than that introduced in 30, 34, 35 .To be more precise, if B 0, then 2.9 becomes 15 in 34 .If we set a i i −m, −m 1, . . ., −1 0, then 2.9 becomes 10 in 35 .It shows that taking full advantage of the transformation 2.9 , we may obtain new and more general exact solutions including not only all solutions obtained by the methods 30, 34, 35 but also other new solutions.It should be noted that the method can also be extended to other similar sub-equations 38 easily.

Application of the Proposed Method
Example 3.1.We first consider the space-time fractional coupled Burgers' equations 39, 40 :

3.1
Using the traveling wave transformations ξ x ct ξ 0 , 3.1 can be reduced to the following nonlinear fractional ODEs:

3.2
According to the method described in Section 2, we suppose that 3.1 has the following formal solution:

3.3
Substituting 3.3 into 3.2 , and setting each coefficients of ψ k ξ to zero, we get a system of underdetermined equations for a i , b i i −1, 0, 1 and c.To avoid tediousness, we omit the overdetermined nonlinear equations.Solving the system, we get the following solution sets.
From Cases 2 and 3, we can obtain many other exact solutions of 3.1 .Here, we omit them for simplicity.

3.12
Using the traveling wave transformations ξ x ct ξ 0 , 3.12 can be reduced to the following nonlinear fractional ODEs:

3.13
According to the method described in Section 2, we suppose that 3.12 has the following two formal solutions:

3.15
Substituting 3.14 into 3.12 , and setting each coefficients of ψ k ξ to zero, we get a system of underdetermined equations for a i , b i i −1, 0, 1 and c.To avoid tediousness, we omit the overdetermined, highly nonlinear equations.Solving the system, we get the following solution sets.
Case 1.Consider where a 0 is an arbitrary constant.
Case 2. Consider where a 0 is an arbitrary constant.
Case 3. Consider where a 0 is an arbitrary constant.
From Cases 2 and 3, we can obtain many other exact solutions of 3.12 .Here, we do not list them for simplicity.
It should be noted that when using ansätz 3.15 , we can also obtain more exact solutions of 3.12 by the same procedure.For the sake of simplicity, we omit them too.Remark 3.3.It seems that the Exp-function method 30 is more general than the extend subequation method.The adopted sub-equation method actually uses the same idea as the one using an FDE D α ξ v a bv 2 with constants a and b.You can choose either of these methods for analyzing a new equation or a previously unstudied or partially studied problem.

Conclusion
In this paper, based on a new general ansätz and Bäcklund transformation of the fractional Riccati equation with known solutions, we propose a new method called extended fractional sub-equation method to construct exact solutions of fractional differential equations.In order to illustrate the validity and advantages of the algorithm, we apply it to space-time fractional coupled Burgers' equations and coupled MKdV equations.As a result, many exact solutions are obtained.The results show that the extended fractional sub-equation method is direct, effective, and can be used for many other fractional differential equations in mathematical physics.
al. successfully obtained traveling wave solutions of nonlinear time fractional biological population model and 4 1 -dimensional space-time fractional Fokas equation.More recently, Guo et al. 34 and Lu 35 improved Zhang et al.'s work 30 and obtained exact solutions of some nonlinear fractional differential equations.