American Journal of Computational Mathematics
Vol.4 No.3(2014), Article ID:47197,13 pages
DOI:10.4236/ajcm.2014.43021
An Iterative Method for Solving Two Special Cases of Lane-Emden Type Equation
Pedro Pablo Cárdenas Alzate
Department of Mathematics, Universidad Tecnológica de Pereira, Pereira R, Colombia
Email: ppablo@utp.edu.co
Copyright © 2014 by author and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/
Received 18 March 2014; revised 22 April 2014; accepted 8 May 2014
Abstract
In this work we apply the differential transformation method or DTM for solving
some classes of Lane-Emden type equations as a model for the dimensionless density
distribution in an isothermal gas sphere
and as a study of the gravitational potential of (white-dwarf) stars
, which are nonlinear
ordinary differential equations on the semi-infinite domain
[1] [2] . The efficiency of the DTM is
illustrated by investigating the convergence results for this type of the Lane-Emden
equations. The numerical results show the reliability and accuracy of this method.
Keywords: Differential Transformation, Lane-Emden, Isothermal Gas Sphere, White-Dwarfs, Iterative Method
1. Introduction
Other classical nonlinear equation, which has been the object of much study, is Lane-Emden’s equation. This equation has the form
(1)
with
and the subject to initial conditions
(2)
where
and
are constants and
is a real-valued continuous function where
and
are constants and
is a real-valued continuous function. The Equation (1) was used to model various
problems, including the isothermal gas spheres, theory of thermionic currents and
the gravitational potential of stars [1] among
others.
Let us consider a spherical cloud of gas (see Figure 1)
and denote its hydrostatic pressure at a distance
from the centre by
. Let
be the mass of the spheres of radius
the gravitational potential of the gas and
the acceleration of gravity.
Then, we have the following equation
(3)
where
is the gravitational constant.
Thus, three conditions are assumed for the determination of
and
(4)
where
is the density of the gas.
(5)
and
(6)
where
and
are arbitrary constants.
Now, solving (4) and (6) with
when
we have
(7)
or
(8)
where
and
. If this value of
is replaced into Equation (5), we obtain
(9)
where.
Figure 1. Spherical cloud of gas.
Now, since, by integration
, that is,
. If
is the central densitythen
must be zero, a change from the condition in the previous case where
was zero only at the boundary of the sphere.
Poisson’s equation is now replaced by
(10)
where, equation which is known as Liouville’s equation.
If we assume symmetry as before, Equation (1) in polar coordinates reduces to the
following
(11)
which replaces Equation (9).
If we let
and
, then (11) becomes
(12)
which is to solved subject to the boundary conditions
and
. The counterpart [2] of the Equation (12) in which
is replaced by
appears in Richardson’s theory of thermionic currents when one seeks to determine
the density and electric force of an electron gas in the neighborhood of a hot body
in thermal equilibrium.
Finally, now consider, then Equation (1) is
turned to the white-dwarf equation, which introduced by
[2] in his study of gravitational potential
of the degenerate stars. This Equation is defined in the form
With
and subject to initial conditions
and
. For instance if
, we have Lane-Emden
equation of index
[3] .
The Differential Transformation Method is a semi-numerical-analytic method for solving
ordinary and partial differential equations. Zhou first introduced the concept of
DTM in 1986 [4] . This technique constructs an
analytical solution in the form of a polynomial. DTM is an alternative procedure
for obtaining analytical Taylor series solution of the differential equations. The
series often coincides with the Taylor expansion of the true solution at point, in the value case, although the series can be
rapidly convergent in a very small region.
Many numerical methods were developed for this type of nonlinear ordinary differential equations, specifically on Lane-Emden type equations such as the Adomian Decomposition Method (ADM) [5] , the Homotopy Perturbation Method (HPM) [6] [7] , the Homotopy Analysis Method (HAM) [8] and Berstein Operational Matrix of Integration [9] . In this paper, we show superiority of DTM by applying them on the some type LaneEmden type equations. The power series solution of the reduced equation transforms into an approximate implicit solution of the original equation. A spectral method (Legendre-Spectral method) was proposed to solve white-dwarf equation; this spectral method provides the most convenient computer implementation [10] .
2. Description of DTM
Differential transformation method of the function
is defined as follows
(13)
In (13),
is the original function and
is the transformed function and the inverse differential transformation is defined
by
(14)
In real applications, function
is expressed by a finite series and Equation (14) can be written as
(15)
Equation (15) implies that
The following theorems can be deduced from Equations (13) and (15).
Theorem 1.
Theorem 2.
Theorem 3.
Theorem 4.
Theorem 5
Theorem 6 (Cárdenas)
The proofs of Theorems are available in [11] .
3. Test Problems
To illustrate the ability of DTM for the Lane-Emden type equation, three examples are provided. The results reveal that this method is very effective.
Example 1 Consider the nonlinear initial-value problem
subject to
. Multiplying both sides
by
we obtain
(16)
Applying theorems 1-6 to Equation (16)
(17)
where
(18)
(19)
(20)
for all.
Now, from the initial conditions
we can obtain
(21)
Substituting Equation (21) into Equation (17) and by recursive method, the results are listed as follows.
.
and then,. For
we have:
Now, as
and
, then
For
we have:
In this case as
and
, then
.
The lector can see that
For
we have:
Now, we can see:
and then
For
we have:
Here,
and
and so,. Consequently,
.
For
we have:
Here
and
Consequently,. Finally,
Therefore using (15), the closed form of the solution can be easily written as:
A series solution obtained by Wazwaz [5] and series expansion respectively is
(22)
Table 1 shows the comparison of
obtained by the DTM (method proposed in this work) and those obtained by Wazwaz.
The resulting graph of the isothermal gas spheres equation in comparison to the
present method and those obtained by Wazwaz is shown in
Figure 2.
Example 2 Consider the following problem
subject to
. Multiplying both sides
by
(23)
As before, using theorems 1-6 we obtain
(24)
where
and
are as (18), (19) and (20) respectively for all
. Now, from the initial conditions
we have
(25)
Figure 2. Comparison between DTM and Wazwaz’s algorithm.
Table 1. Comparison between DTM and Wazwaz’s algorithm.
Substituting Equation (25) into Equation (24) and by recursive method, the results are listed as follows.
For, we have respectively
So on, we can use (15) and the closed form of the solution can be easily written as
A solution obtained by Yahya [12] by using the power series method is
We can see Figure 3 and compare with [13] , the results are very good.
As final example and to illustrate the ability of DTM for white-dwarf equation,
the next problem is provided for.
Figure 3. Numerical results by using DTM.
Example 3 Consider the problem
subject to
. Multiplying both sides
by
we obtain
(27)
Here, is easy to verify that the function
has a series expansion
(28)
where. Therefore, Equation (27) takes the form
(29)
Using in (29) the above theorems we have the following
(30)
or
(31)
where
and successively. Also,
(32)
(33)
(34)
for all. Now, from the initial conditions
we have
(35)
Substituting (35) into Equation (31) and by recursive method, the results are listed as follows.
For. or
and then
and then
. For
and so
. For
therefore
. Using (15), the closed
form of the soluyion can be easily written as
A series solution obtained by Chandrasekhar [2] using series expansion was
(36)
Table 2 shows the comparison of
obtained by the DTM and those obtained by Parand [14]
. The resulting graph of the white-dwarfs equation in comparison to the present
method and the obtained by [14] is show in
Figure 4.
4. On Convergence of DTM
We can write the DTM as
Table 2 . Comparison between DTM and Legendre-Spectral method.
Figure 4. Comparison between DTM and a Legendre-Spectral Method.
(37)
where
increase function depends on its arguments through the function
. The method (37) means
steps, needed for early values
to calculate
. It is therefore necessary
to have bootstrap values
.
The method (37) is said to be convergent if for all IVP has to
Remark. The condition
on the bootstrap values is equivalent to asking that
for
. Here, we are asking that bootstrap values
approximate well and the initial data
; if this is not, then
no reason to expect that numerical solution closely matches the theoretical.
Now let us consider the following form of the Equation (1)
(38)
Here
is a nonlinear differential operator, which encloses the linear and nonlinear term
of the Lane-Emden type equation. Now, the linear term
is always invertible and the nonlinear term is
Therefore (38) may be written as
(39)
or
(40)
Applying DTM in (40) we can obtain
(41)
Remember that differential transformation of
and
are computed by using theorems 1 - 6.
Let us consider the Equation (38) in the following form
(42)
Here,
is a nonlinear operator. It is noted that Equation
(15) is equivalent to the sequence
(43)
This sequence is determined using the iterative scheme
(44)
and associated with.
The following theorem guarantees that the scheme of DTM converges to the solution
of Lane-Emden Equation (1).
Theorem 7 Let
be a nonlinear operator from a Banach space
and
be the solution (exact) of Equation (42). The series solution (14) converges to
, if there exists a constant
such that
for
.
Proof. We prove that the sequence
is a Cauchy sequence in
. Therefore,
Thus, for any
so
implying that he sequence
is Cauchy, i.e. since
then
, therefore there exists
such that
, i.e.
converges.
Now, we can say too that Equation (42) is similar to solve, therefore this implies
that if
is continuous then
i.e. T is a solution of
and this completes the proof.
Figure 5 shows the maximum point-wise error between the numerical solution obtained by using DTM and the Chandrasekhar solution. It is observed that both schemes are almost the same accuracy.
Figure 5. Example 1.
error obtained by using DTM.
5. Conclusion
In this work, we presented the definition and handling of one-dimensional differential transformation method. Using DTM, the Lane-Emden equations were transformed into algebraic equations (iterative equations). The new scheme obtained by using DTM yields an analytical solution in the form of a rapidly convergent series. This method makes the solution procedure much more attractive. The figures and tables clearly show the high efficiency of DTM and the convergence of the method for three examples in investigated.
Acknowledgements
Foremost, I would like to express my sincere gratitude to Jean-Christophe Nave (Department of Mathematics and Statics McGill University) for the support of my research and the support of the Department of Mathematics of the Universidad Tecnológica de Pereira (Colombia) and the group GEDNOL.
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