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Research Article | Open Access
Application of Optimal Homotopy Asymptotic Method to Some Well-Known Linear and Nonlinear Two-Point Boundary Value Problems
The objective of this paper is to obtain an approximate solution for some well-known linear and nonlinear two-point boundary value problems. For this purpose, a semianalytical method known as optimal homotopy asymptotic method (OHAM) is used. Moreover, optimal homotopy asymptotic method does not involve any discretization, linearization, or small perturbations and that is why it reduces the computations a lot. OHAM results show the effectiveness and reliability of OHAM for application to two-point boundary value problems. The obtained results are compared to the exact solutions and homotopy perturbation method (HPM).
Two-point boundary value problems (TPBVP) have many applications in the field of science and engineering [1, 2]. These problems arise in many physical situations like modeling of chemical reactions, heat transfer, viscous fluids, diffusions, deflection of beams, the solution of optimal control problems, etc. Due to the wide applications and importance of boundary value problems (BVP) in science and engineering we need solutions to these problems.
There are many techniques available for the solution of-of BVP like Adomian Decomposition Method (ADM) [3–7], Extended Adomian Decomposition Method (EADM), Differential Transformation Method (DTM) , Variational Iteration Method (VIM) , Perturbation methods(PMs) [1, 11–13], and so on. Perturbation methods are easy to solve but they require small parameters which are sometimes not an easy task. Recently V. Marinca et al. presented optimal homotopy asymptotic method (OHAM)  for the solution of BVP, which did not require small parameters. The method can also be applied to solve the stationary solution of some partial differential equations, e.g., gKdv equation, nonlinear parabolic problems, and so on [15–20]. In OHAM, the concept of homotopy is used together with the perturbation techniques. Here, OHAM is applied to TPBVP to check the applicability of OHAM for TPBVP.
2. Basics of OHAM
Let us take the BVP whose general form is the following:where is a linear operator, is independent variable, is the nonlinear operator, is a known function, and is a boundary operator.
Homotopy on OHAM can be constructed as where is an embedding parameter, is an unknown function, is a nonzero auxiliary function for , and is of the formClearly when then . And obviously, when then When then . So as increases from to , the solution varies from to the exact solution , where is obtained from (2) for The proposed solution of (1) will be of the formSubstituting this value of into (1), after some calculations, we can obtain the governing equations of by using (4) and , that is,where is the coefficient of in the series expansion of with respect to the embedding parameter . Andwhere is given by (5). The convergence of series (5) depends on the convergence of the constants , if these constants are convergent at , then the solution becomesGenerally, the order solution of the problem can be obtained in the formPutting this solution in (1) we get the following residual:If , then the solution is going to be exact, but generally, such a situation does not arise in nonlinear problems but the functional defined below can be minimizedwhere and are two constants depending on the given problem. The values of can be optimally found by the conditionAfter knowing these constants, the solution (10) is well determined.
To check the applicability of OHAM for TPBVP, in this section four examples of TPBVP are presented in which one example is linear and the remaining are nonlinear.
3.1. Example 1
The zeroth-order problem isThe solution of (15) isThe first-order problem isThe solution of (17) isThe second-order problem isThe solution of (19) is And the third-order approximate solution of the bvp (14) is as follows:Table 1 shows the comparison between the exact solution and the approximate solution obtained by OHAM. Figure 1 of the solution also shows well agreement with the exact solution.
3.2. Example 2
Consider the nonlinear two-point boundary value problem  of the typeAccording to OHAM and , while . The exact solution of (23) is . Now proceeding with the same lines as above we have the following zeroth-order problem:The solution of (24) isNow the first-order problem isThe solution of (26) isThe second-order problem isThe solution of (29) isThe third-order problem is The solution of the third-order problem results a large output, therefore not included here.
Now the third-order approximate solution is has the following values and then substituting in the above solution we will get the approximate solution. is given in Appendix (A.1).The solution at the points given in Table 2 and the graph of the solution is shown in Figure 2. Here it is third-order OHAM solution while the HPM  gives the accuracy up to 9 decimal places in 7th order.
3.3. Example 3
Now we consider higher order TPBVP of order four. The problem iswith the boundary conditions , , , and .
Where , , and , the exact solution of problem (34) is . After solving this by the method described in Section 2, we have the following zeroth-order problem:The solution to (35) isThe first-order problem is The second-order problem is The solutions of problem (38) and (40) are very large; therefore we did not write it here. The constants and have the values and , respectively. Table 3 and Figure 3 show a good agreement with the exact values. The approximate solution is given in Appendix (A.2).
3.4. Example 4
At last, consider the second-order nonlinear TPBVPThe exact solution of (42) is . Solving (42) by the method depicted in Section 2, we have the following zeroth order problem:The solution of (44) is given byThe first-, second-, and third-order problems are given in (46), (47), and (48) respectively. The solutions of problem (46), (47), and (48) are very large and therefore cannot be written here but the table of values and the graph are shown in Table 4 and Figure 4, respectively. The approximate solution is written in Appendix (A.3). The values of the constants can be found by (13) which are given as follows:
This paper reveals that OHAM is a very strong method for solving TPBVP and gives us a more accurate solution as compared to other methods. In these examples only second- and third-order solution gives us the accuracy up to 8 or 10 decimal places; therefore it is concluded that this method converges very fast to the exact solution and in some problems like example 1 it gives us the exact solution. The plots and tables show well agreement with the exact solution.