Herişanu et al. [1] proposed a new technique called the optimal homotopy asymptotic method (OHAM). The main advantage of OHAM is that it is reliable and straight forward. Also, the OHAM does not need to worry about curves as homotopy asymptotic method (HAM). Moreover, the OHAM provides controls the convergence of the series solution and its solution agrees with the exact one at large domains, for more infor- mation see [2] - [6] .

On the other hand, the standard Bratu problem is used in a large variety of applications, such as the fuel ignition model of the theory of thermal combustion, the thermal reaction process model, the Chandrasekhar model of the expansion of the universe, radiative heat transfer, nanotechnology and theory of chemical reaction, for more information see [7] [8] and references therein.

The Bratu initial value problems have been studied extensively because of its mathematical and physical properties. In [9] , Batiha studied a numerical solution of Bratu-type equations by the variational iteration method; Feng et al. [10] considered Bratu’s problems by means of modified homotopy perturbation method; Rashidinia et al. [11] applied Sinc-Galerkin method for numerical solution of the Bratu’s problems; Syam and Hamdan [12] used variational iteration method for numerical solutions of the Bratu-type problems; Wazwaz [13] applied Adomian decomposition method to study the Bratu-type equations.

The main goal of this paper is to extend OHAM method to solve the initial value problems of second order differential equations of Bratu-type. The OHAM is very useful to get an approximate solution of the initial value problems of second order differential equations of Bratu-type. Our numerical examples of OHAM are compared with exact ones.

In this section we start by describing the basic formulation of OHAM, see for example [1] [3] - [5] . Consider the boundary value problem

where is a given function and is an unknown function. Here, , and represent a linear operator, a nonlinear operator and a boundary operator, respectively.

By means of OHAM one constructs a homotopy, which satisfies the following fa- mily of equations

where is an embedding parameter, is a non-zero auxiliary function for and. It is easy to see that when and we have and, respectively, where is obtained from (2.2) for

Therefore, the unknown function goes from to as changes from to.

In the sequel, we choose auxiliary function in the form

where, , are constants to be determined.

In order to obtain an approximate solution, we expand, , in the form of Taylor’s series about as

Now, substituting by Equation (2.5) into Equation (2.2) and equating the coefficients of like powers of in the resulting equation, we obtain the governing problem of, given by Equation (2.3). In addition, the governing problems of and are given in the forms

and

respectively. Also, the general governing problems of are given by

where is the coefficient of in the expansion of about the embedding parameter:

where, , is given by Equation (2.5).

Observe that the convergence of the series (2.5) depends upon the auxiliary constants,. If the series (2.5) converges when, one has

The m-th order approximations are given by

By substituting Equation (2.11) into Equation (2.1), we get the following expression for residual

If, then will be the exact solution and this, in general, does not happen especially in nonlinear problems. In order to find the optimal values of, , we apply the method of least squares as under

where and are numbers properly chosen in the domain of the problem. Next, minimizing with

After knowing those constants, the approximate solution of order is well determined.

Example 1 Consider the second order initial value problem of Bratu type

The initial value problem (3.1) has as the exact solution.

Next, we apply the OHAM method to the initial value problem (3.1). We have

, and. Therefore, according to the OHAM method,

we have

Problem of zero order:

which has a solution.

Problem of first order:

Problem (3.3) has a solution

The problem of second order

The solution of Problem (3.5) is given by

Third order problem is

and its solution is given in the form

Finally, fourth order problem is

which has a solution in the form

Now, by using equations (3.4), (3.6), (3.8) and (3.10), the fourth order approximate solution, using OHAM with, is given by

Next, we follow the procedure presented in Section 2, we obtain the following values of’s:

, , and (table 1).

Example 2 In this example, let us consider the Bratu initial value problem

which has

exact solution.

Now, we apply the OHAM method presented in previous section. In this example, we have

, and. Now,

Problem of zero order:

Problem (3.13) has a solution.

Problem of first order:

The solution of Problem (3.14) is given by

The problem of second order

and its solution is given by

Third order problem is

The solution of Problem (3.18) is given by

In the end, the fourth order problem is given by

which has a solution in the form

Now, by using equations (3.4), (3.6), (3.8) and (3.10), the fourth order approximate solution, using OHAM with, is given by

Next, we follow the procedure presented in Section 0.2, we obtain the following values of’s:

, , and (table 2).

Throughout this paper, an technique for obtaining a numerical solution for second order initial value problems of Bratu-type, is optimal homotopy asymptotic method (OHAM). The main advantage of the used technique is achieving high accurate approximate solutions. In the numerical tables and graphics, our numerical results are compared with the exact ones.

Mohamed AbdallaDarwish,Bothayna S.Kashkari, (2014) Numerical solutions of second order initial value problems of Bratu-type via optimal homotopy asymptotic method. American Journal of Computational Mathematics,04,47-54. doi: 10.4236/ajcm.2014.42005