A Comparison between Adomian ’ s Polynomials and He ’ s Polynomials for Nonlinear Functional Equations

We will compare the standard Adomian decomposition method and the homotopy perturbation method applied to obtain the solution of nonlinear functional equations. We prove analytically that the two methods are equivalent for solving nonlinear functional equations. In Ghorbani (2009), Ghorbani presented a new definition which he called as He’s polynomials. In this paper, we also show that He’s polynomials are only the Adomian polynomials.


Introduction
The Adomian decomposition method (ADM) and the homotopy perturbation method (HPM) are two powerful methods which consider the approximate solution of a nonlinear equation as an infinite series usually converging to the accurate solution.These methods have been used in obtaining analytic and approximate solutions to a wide class of linear and nonlinear, differential, and integral equations.
Özis ¸and Yıldırım compared Adomian's method and He's homotopy perturbation method [1] for solving certain nonlinear problems.Li also has shown that the ADM and HPM for solving nonlinear equations are equivalent [2].In [3], Ghorbani has presented a definition which he called it as He's polynomials.
Consider the following nonlinear functional equation: where  is a nonlinear operator from Hilbert space  to ,  is an unknown function, and  is a known function in .We are looking for a solution  of (1) belonging to .We will suppose that (1) admits a unique solution.If (1) does not possess a unique solution, the ADM and HPM will give a solution among many (possible) other solutions.However, relatively few papers deal with the comparison of these methods with other existing techniques.In [4], a useful comparison between the decomposition method and the perturbation method showed the efficiency of the decomposition method compared to the tedious work required by the perturbation techniques.In [5], the advantage of the decomposition method over the Picard's method has been emphasized.Sadat has shown that the Adomian decomposition method and perturbation method are closely related and lead to the same solution in many heat conduction problems [6].In [7,8] the HPM has compared with Liao's homotopy analysis method and showed the HPM is special case of HAM, and the advantage of the HAM over the HPM has been emphasized.In this paper, we want to prove that He's polynomials are only Adomian's polynomials.We will also show that the standard Adomian decomposition method and the standard HPM are equivalent when applied for solving nonlinear functional equations.

Adomian's Decomposition Method (ADM)
Let us consider the nonlinear equation (1) which can be written in the following canonical form: The standard ADM consists of representing the solution of (1) as a series and the nonlinear function as the decomposed form: where   ,  = 0, 1, 2, . . .are the Adomian polynomials of  0 ,  1 , . . .,   given by [9, 10] Substituting ( 3) and ( 4) into (1) yields The convergence of the series in (6) gives the desired relation It should be pointed out that  0 depends only on  0 ,  1 depends only on  0 and  1 ;  2 depends only on  0 ,  1 , and  2 , and so on.The Adomian technique is very simple in its principles.The difficulties consist in proving the convergence of the introduced series.

Homotopy Perturbation Method (HPM)
This is a basic idea of homotopy method which is to continuously deform a simple problem easy to solve into the difficult problem under study.
In this section, we apply the homotopy perturbation method [11][12][13] to the discussed problem.To illustrate the homotopy perturbation method (HPM), we consider (1) as with solution ().The basic idea of the HPM is to construct a homotopy (V; ) :  × [0, 1] →  which satisfies where (V) is a proper function with known solution which can be obtained easily.The embedding parameter  monotonically increases from 0 to 1 as the trivial problem (V) = 0 is continuously transformed to the original problem V −  − (V) = 0. From H(V; ) = 0, we have (V; 0) = (V) = 0 and It is better to take (V) as a deformation of (V).For example, in (9), (V) = V − ().By selecting (V) = V − () we can define another convex homotopy H(V; ) by The embedding parameter  ∈ (0, 1] can be considered as an expanding parameter [14,15].The HPM uses the embedding parameter  as a "small parameter, " and writes the solution of (10) as a power series of , that is, Setting  = 1 results in the approximate solution of (10): Substituting ( 11) into (10) and equating the terms with identical powers of , we can obtain a series of equations of the following form: where It is obvious that the system of nonlinear equations in ( 13) is easy to solve, and the components V  ,  ≥ 0 of the homotopy perturbation method can be completely determined, and the series solutions are thus entirely determined.For the convergence of the previous method we refer the reader to the work of He [12,17,18].

Equivalence between ADM and HPM
In this section, we prove that the HPM and the ADM give same solution for solving nonlinear functional equations.We also show that the He polynomials are like the Adomian polynomials.In [3], Ghorbani has presented the following definition.
Note 1.Comparison between ( 5) and ( 15) has shown that the He polynomials are only Adomian's polynomials, and it is calculated like Adomian's polynomials.
Theorem 2. Suppose that nonlinear function () and the parameterized representation of V are V() = ∑ ∞ =0 V    , where  is a parameter, then we have =0 . ( Proof (see [3,19]).In Theorem 3, we prove that the He polynomials are the Adomian polynomials.
Theorem 3. The He polynomials which are given by (15) are the Adomian polynomials.
Proof.From Taylor's expansion of (V), we have substituting ( 11) in ( 17) and expanding it in terms of  leads to where   ,  = 0, 1, 2, . .., depends only on V 0 , V 1 , . . ., V  .In order to obtain   , we give -order derivative of both sides of (18) with respect to  and let  = 0, that is, According to Theorem 2 We know that   just depends on 20) in ( 19) leads us to find   in the following form: which is called for the first time by Ghorbani as the He polynomials [3]! Theorem 4. The homotopy perturbation method for solving nonlinear functional equations is the Adomian decomposition method with the homotopy H(V; ) given by Proof.Substituting ( 11) and ( 18) into (10) and equating the terms with the identical powers of , we have From (24) we have According to Theorem 3 we have   =   .Substituting (25) in (11) leads us to Therefore, by letting we observe that the power series corresponds to the solution of the equation H(V; ) = V − () − (V) = 0 and becomes the approximate solution of (1) if  → 1.This shows that the homotopy perturbation method is the Adomian decomposition method with the homotopy H(V; ) given by (28).The proof of Theorem 4 is completed.
These two approaches give the same equations for highorder approximations.This is mainly because the Taylor series of a given function is unique, which is a basic theory in calculus.Thus, nothing is new in Ghorbani's definition, except the new name "He's polynomial." He just employed the early ideas of ADM.

Mathematical Problems in Engineering
Example 5 (see [20]).Consider the following nonlinear Volterra integral equation: with the exact solution () = tan .
We apply standard ADM and HPM.For applying standard HPM, we construct following homotopy: In view of ( 13), we have In view of (7), we have following recursive formula:  (34)

Conclusion
It has been shown that the standard HPM provides exactly the same solutions as the standard Adomian decomposition method for solving functional equations.It has been proved that He's polynomials are only Adomian's polynomials with different name.