# Computation of the General Relativistic Perihelion Precession and of Light Deflection via the Laplace-Adomian Decomposition Method

Authors: Man Kwong Mak; Chun Sing Leung; Tiberiu Harko
Journal: Advances in High Energy Physics
 (2018)

---

## Abstract

We study the equations of motion of the massive and massless particles in the Schwarzschild geometry of general relativity by using the Laplace-Adomian Decomposition Method, which proved to be extremely successful in obtaining series solutions to a wide range of strongly nonlinear differential and integral equations. After introducing a general formalism for the derivation of the equations of motion in arbitrary spherically symmetric static geometries and of the general mathematical formalism of the Laplace-Adomian Decomposition Method, we obtain the series solution of the geodesics equation in the Schwarzschild geometry. The truncated series solution, containing only five terms, can reproduce the exact numerical solution with a high precision. In the first order of approximation we reobtain the standard expression for the perihelion precession. We study in detail the bending angle of light by compact objects in several orders of approximation. The extension of this approach to more general geometries than the Schwarzschild one is also briefly discussed.


---

## Body

## 1. Introduction

General relativity is a very successful theory of the gravitational field, whose predictions are in excellent agreement with a large number of astronomical observations and experiments performed at the scale of the Solar System. In particular, three fundamental tests of general relativity, the perihelion precession of planet Mercury [1, 2], the bending of light by the Sun [3, 4], and the radar echo delay experiment [5, 6] have all fully confirmed, within the range of observational/experimental errors, the predictions of Einstein’s theory of gravity. But the importance of these effects goes far beyond the limits of the Solar System. A fast full general relativistic method to simultaneously constrain the mass of massive black holes, their spin, and the spin direction by considering both the motion of a star and the propagation of photons from the star to a distant observer was developed in [7]. The spin-induced effects on the projected trajectory and redshift curve of a star depend on both the value and the direction of the spin. The maximum effects over a full orbit can differ by a factor up to more than one order of magnitude for cases with significantly different spin directions. In [8] it was shown that the spin of the massive black hole at the Galactic Center can be constrained with 1σ error <~0.1 or even greater than 0.02 by monitoring the orbital motion of a star with semi major axis <~300 AU and eccentricity >~0.95 over a period shorter than a decade through future facilities. An improvement in astrometric precision would be more effective at improving the quality of constraining the spin than an improvement in velocity precision. Short-period stars orbiting around the supermassive black hole in our Galactic Center can successfully be used to probe the gravitational theory in a strong regime [9]. By using 19 years of observations of the two best measured short-period stars orbiting our Galactic Center constraints on a hypothetical fifth force that arises in some extended theories of gravity or in some models of dark matter and dark energy were obtained in [9]. No deviations from general relativity were found, and the fifth force strength was restricted to an upper 95% confidence limit of |α|<0.016 at a length scale of λ=150 astronomical units. Moreover, 95% confidence upper limit on a linear drift of the argument of periastron of the short-period star S0-2 was obtained, a result that opens the possibility of testing gravitational theories using orbital dynamic in the strong gravitational regime of a supermassive black hole. The S-star cluster in the Galactic Center allows the study of physics close to a supermassive black hole, including distinctive dynamical tests of general relativity [10], where a new and practical method for the investigation of the relativistic orbits of stars in the gravitational field near Sgr A∗ was developed, by using a first-order post-Newtonian approximation to calculate the stellar orbits with a broad range of periapse distance rp. For in depth discussions of the experimental and Solar System tests of general relativity, see [11] and [12], respectively.Due to its importance in many applications, the study of the motion of massive or massless particles in different geometries obtained as solutions of Einstein’s gravitational field equations and of their extensions is a fundamental field of general relativity. The first exact solution of the vacuum field equations was the Schwarzschild solution [13], which can be used efficiently to explain all astronomical observations at the scale of the Solar System. The exact equation of motion in Schwarzschild geometry is highly nonlinear, and therefore to obtain the observable physical parameters approximate methods must be used. The first-order approximation of the equation of motion already gives the correct approximation of the perihelion precession of Mercury and of the deflection of light by the Sun [11]. However, due to the importance of the problem many mathematical techniques for the study of the astrometric properties of the planetary motions and of the light have been developed. A standard approach is based on the solution of the Hamilton-Jacobi equation [13](1)gμν∂S∂xμ∂S∂xν-m2c2=0,where gμν are the components of the metric tensor and m is the mass of the particle. By representing S as S=-Et+Mφ+Sr(r), where E and M are the constants of the energy and angular momentum, one can obtain the full solution of the equation of motion in Schwarzschild geometry in an integral form as [13](2)ct=Emc2∫dr1-rg/rE/mc22-1+M2/m2c2r21-rg/r1/2,(3)φ=∫Mdrr2E2/c2-m2c2+M2/r21-rg/r.The parameters of the orbits can be obtained by solving the integrals by using some approximate methods. The geodesic equations obtained from the Schwarzschild gravitational metric in the presence of a cosmological constant were solved exactly and expressed in a closed form in [14] as(4)u=1r=4αS℘φ+ϵ+13αS,where αS=2GM/c2, ϵ is an arbitrary integration constant, ℘ is the Weierstrass function that gives the inversion of the elliptic integral ∫U(dU/4U3-g2U-g3)=ϕ, by the Weierstrass function, U=℘(ϕ+ϵ). In this approach the exact expression of the perihelion precession is given by Δω=2ω-π, where ω=∫e1∞(dt/4t3-g2t-g3) and e1 is a root of the cubic equation in the integral. The perihelion precession and deflection of light have been investigated in the four-dimensional general spherically symmetric spacetime in [15], where the master equation has also been obtained. As an application of this master equation, the Reissner-Nordstorm solution and Clifton-Barrow solution in f(R) gravity have been investigated. The homotopy perturbation method, which was introduced in [16], was applied for calculating the perihelion precession angle of planetary orbits in general relativity in [17, 18]. The basic ideas behind the homotopy perturbation method are as follows [16]. We start from the nonlinear differential equation A(u)=g(r),r∈Ω, where A is a general differential operator and g(r) is a known analytic function, with the boundary conditions B(u,∂u/∂n)=0;r∈Γ, where B is a boundary operator and Γ is the boundary of the domain Ω. We assume that the operator A can be divided into two parts M and N, and we reformulate our initial equation as M(u)+N(u)=g(r). Then the homotopy v(r,p):Ω×[0,1]→R is constructed in the following way: H(v,p)=(1-p)[M(v)-M(y0)]+p[A(v)-g(r)]=0, where r∈Ω and p∈[0,1] is an imbedding parameter, and y0 is an initial approximation of the equation. Since 0≤p≤1, it can be considered as a small parameter, and one can assume that the solution of the equation can be expressed as a power series in p as v=∑i=0∞vipi. When p→1, then this series becomes the approximate solution of the equation, that is, u(x)=limp→1⁡v=∑i=0∞vi. This series is generally convergent.The study and the applications of Adomian’s Decomposition Method (ADM) [19–22], which allows investigating the solutions of many kinds of ordinary, partial, stochastic differential, and integral equations that describe numerous physical and/or mathematical problems, have attracted a lot of attention in recent years. Historically, the ADM was first proposed and applied in the 1980s [23–25]. An essential advantage of the ADM is that with its use one can obtain analytical approximations to the solutions of a rather wide class of nonlinear (and stochastic) differential and integral equations, without the need of linearization, perturbation, closure approximations, or discretization methods. Usually the application of these methods could lead to the necessity of intensive numerical computation. Moreover, to make solvable and to obtain closed-form analytical solutions of a nonlinear problem imply the necessity of introducing some simplifying and restrictive assumptions.It is important to mention that ADM can generate the solution of a given equation in the form of a power series. The terms of the series are obtained by recursive relations using the Adomian polynomials. Another important advantage of the ADM is that usually the series solution of the differential equation converges fast, and therefore the use of this method saves a lot of computational time. Moreover, in the ADM there is no need to linearize or discretize the differential equation. Reviews of ADM and its applications in applied mathematics and physics can be found in [19, 20], respectively. Many studies have been devoted to the modification and improvement of the ADM in an attempt to increase its accuracy and/or to extend the applicability of the initial method [21, 22, 26–40]. An important improvement of the ADM is represented by the Laplace-Adomian Decomposition Method [41], in which the Adomian Decomposition Method is applied not to initial equation, but to the Laplace transformed one.Even that the ADM has been extensively used in the study of many problems in different fields of physics and engineering, it has been applied very little in astronomy, astrophysics, or general relativity, the only exception from this “rule” known to authors being the papers [42, 43]. It is the purpose of the present paper to investigate the equations describing the perihelion precession and light bending in general relativity for static gravitational fields by using the Adomian Decomposition Method, representing a very powerful mathematical method for the investigation of the solutions of nonlinear differential equations. For the geometry outside a compact, stellar type object (the Sun) we adopt some specific static and spherically symmetric vacuum solutions of general relativity. In particular the power series solution of the equation describing the motion of massive and massless particles in Schwarzschild geometry is investigated in detail. As a first step in our analysis we derive the equations of motion for particles in arbitrary spherically symmetric spacetimes, and we develop a general formalism for obtaining the equations of motion that can be used for any given metric. As the next step in our study we adopt the Schwarzschild form of the metric, and we apply the Laplace-Adomian Decomposition Method to obtain its approximate analytical power series solution for both massive and massless particles. We compare our solutions with the exact numerical solutions of the equations of motion, and it turns out that by truncating our series to five terms only we obtain a very good description of the solution of the equation of motion. Moreover, in the first approximation we can reobtain easily the standard expressions of the perihelion precession and the bending angle of light.The present paper is organized as follows. We derive the equations of motion of massive and massless particles in arbitrary static spherically symmetric spacetimes in Section2. The application of the Laplace-Adomian Decomposition Method to the case of second-order nonlinear differential equations is presented in Section 3. The power series solution of the equation of motion of massive particles by using the Laplace-Adomian Decomposition Method is obtained in Section 4, where the comparison with the exact numerical solution is also performed. The motion of photons in Schwarzschild geometry is investigated in Section 5. Finally, in Section 6, we discuss and conclude our results.


## 2. Particle Motion in Arbitrary Spherically Symmetric Static Spacetimes

In the following we will restrict our analysis to the case of static and spherically symmetric metrics, given by(5)ds2=-eνrdt2+eλrdr2+r2dΩ2,where we have denoted dΩ2=dθ2+sin2θdφ2 and θ and φ are the standard coordinates on the three-sphere. The time variable takes real values only, t∈R while the radial coordinate r ranges over a finite open interval r∈rmin,rmax, so that -∞≤rmin≤rmax≤∞. Moreover, we also require that the functions ν(r) and λ(r) are strictly positive and that on the interval rmin,rmax they are (at least piecewise) differentiable. This form of the metric is relevant for the study of the dynamics of particles (both massive and massless) in the Solar System.Important observational evidence for the correctness of the theory of general relativity is provided, at the level of the Solar System, by three fundamental tests, which also allow the testing of its extensions and generalization, as well as of alternative theories of gravitation. These three essential tests are the perihelion precession of Mercury, the deflection of photons by the Sun, and the radar echo delay observations. These three effects have been successfully used to test the Schwarzschild solution of general relativity, as well as other predictions of the theory. However, it is also important to study these physical phenomena in arbitrary static spherically symmetric spacetimes for any given metric. In the present Section, we develop a formalism that can be used for obtaining the equations of motion and compute the perihelion precession and light bending angle in any static spherically symmetric metric. This formalism was first introduced to study the Solar System tests for some modified gravity vacuum solutions in [44–47].


### 2.1. The Equation of Motion of Massive Test Particles

The geodesic equations of motion of a massive test particle in the gravitational field of the metric given by (5) can be derived with the use of the variational principle(6)δ∫eνc2t˙2-eλr˙2-r2θ˙2+sin2θφ˙2ds=0,where by a dot we have denoted d/ds. It can be easily checked that the orbit is planar, and therefore without any loss of generality we can take θ=π/2. Hence φ is the only the angular coordinate in this problem. Since t and φ do not appear explicitly in (6), their conjugate momenta give two constants of motion, denoted by E and L, so that(7)eνc2t˙=E=constant,r2φ˙=L=constant.The constant E gives the energy of the particle, while the constant L is related to its angular momentum.From the line element (5) we obtain the following equation of motion for r:(8)r˙2+e-λr2φ˙2=e-λeνc2t˙2-1.Substitutingt˙ and φ˙ from (7) gives the following relation:(9)r˙2+e-λL2r2=e-λE2c2e-ν-1.We introduce now a new variableu, defined as r=1/u, as well as the transformation d/ds=Lu2d/dφ. Then (9) takes the form(10)dudφ2+e-λu2=1L2e-λE2c2e-ν-1.We represent formallye-λ as e-λ=1-f(u), thus obtaining(11)dudφ2+u2=fuu2+E2c2L2e-ν-λ-1L2e-λ≡Gu.We take the derivative of the above equation with respect toφ, which gives(12)d2udφ2+u=Fu,where(13)Fu=12dGudu.Equation (12) gives the equation of motion of a particle in an arbitrary spherically symmetric geometry.


#### 2.1.1. The Precession of the Perihelion

The root of the equationu0=Fu0 gives a circular orbit with u=u0. Any deviation δ=u-u0 from it can be obtained from the substitution of u=u0+δ into (12), which gives the equation(14)d2δdφ2+u0+δ=Fu0+δ.In the first order of approximation Fu0+δ≈Fu0+dF/duu=u0δ+Oδ2 and hence(15)d2δdφ2+1-dFduu=u0δ=Oδ2.Therefore, to first order inδ, the trajectory of the massive particle can be obtained as(16)δ=δ0cos⁡1-dFduu=u0φ+β,where δ0 and β are two arbitrary constants of integration. The angles for which r is minimum are the angles of the perihelia of the orbit. Therefore they are determined from the condition that u or δ is maximum. Hence, from one perihelion to the next the orbital angle varies by a quantity Δφ, given by(17)Δφ=2π1-dF/duu=u0=2π1-σ.The parameterσ introduced in the previous equation is called the perihelion advance. From a physical point of view it represents the rate of advance of the perihelion after one rotation. As the test particle advances through φ radians in its orbit, its perihelion precesses by σΔφ radians. From (17), σ can be expressed as(18)σ=1-1-dFduu=u0,or for small dF/duu=u0, as(19)σ=12dFduu=u0.For a complete rotation of the planet we obtainφ≈2π(1+σ). Hence the advance of the perihelion is(20)δφ=φ-2π≈2πσ.To be able to obtain effective estimations of the perihelion precession we must know the expression of the angular momentum of the particle L as a function of the geometric parameters of the orbit. We will obtain now the expression of L in the Newtonian limit [44–47].Let us assume that the planet moves on a Keplerian ellipse, with semiaxesa¯ and b¯, respectively, where b¯=a¯1-e2 and by e we have denoted the eccentricity of the orbit. The ellipse has a surface area πa¯b¯. The oriented surface area of the ellipse is dA→=r→×dr→/2, and consequently the areolar velocity of the planet is given by dA→/dt=r→×dr→/2=r2dφ/dt/2≈πa¯21-e2/T, where T is the period of the planetary motion. On the other hand T can be obtained from Kepler’s third law as T2=4π2a¯3/GM [48]. In the Newtonian limit of small velocities ds≈cdt, and the conservation equation of the angular momentum reduces to r2dφ/dt=cL. Hence we obtain first L=2πa¯21-e2/cT, and hence(21)1L2=c2GMa¯1-e2.


#### 2.1.2. The Equation of Motion of Massive Particles in Schwarzschild Geometry

As a first astronomical application of the formalism introduced in the previous section we obtain the precession of the perihelion of a planet in the Schwarzschild geometry, with(22)eν=e-λ=1-2GMc2r=1-2GMc2u.Then we immediately obtain f(u)=2GM/c2u. On the other hand since for this geometry ν+λ=0, we easily find(23)Gu=2GMc2u3+1L2E2c2-1+2GMc2L2u,and(24)Fu=3GMc2u2+GMc2L2,respectively. Therefore the equation of motion of a massive test particle in Schwarzschild geometry is given by(25)d2udφ2+u=3GMc2u2+GMc2L2.The radiusu=u0 of the circular orbit is found as the solution of the algebraic equation(26)u0=3GMc2u02+GMc2L2,with the only physically acceptable solution given by(27)u0=1±1-12G2M2/c4L26GM/c2≈GMc2L2.Therefore(28)δφ=πdFduu=u0=6πGMc2a¯1-e2,which is the standard general relativistic result [13].


### 2.2. Equation of Motions of Photons and the Deflection of Light

In a gravitational field a photon follows a null geodesic, given byds2=0. In this case the affine parameter along the trajectory of the photon can be taken as an arbitrary quantity λ~. In the following we denote again by a dot the derivatives with respect to it. Similar to the case of the motion of massive particles we have two constants of motion, the energy E and the angular momentum L, which can be obtained from (7).The equation of motion of the photon is given by(29)r˙2+e-λr2φ˙2=eν-λc2t˙2.By using the constants of motion the above equation can be transformed into(30)r˙2+e-λL2r2=E2c2e-ν-λ.We change now the independent variabler to u=1/r. With the use of the conservation equations we eliminate the derivative with respect to the affine parameter, thus obtaining(31)dudφ2+u2=fuu2+1c2E2L2e-ν-λ≡Pu.We take the derivative of (31) with respect to φ, and thus we find the basic equation of the photon in an arbitrary static spherically symmetric geometry, as given by(32)d2udφ2+u=Vu,where we have denoted(33)Vu=12dPudu.In the particular case of the Schwarzschild geometry we haveν+λ=0 and f(u)=2GM/c2u, giving P(u)=2GM/c2u3 and V(u)=3GM/c2u2, respectively. Hence the equation of motion of photons in the Schwarzschild metric is obtained as(34)d2udφ2+u=3GMc2u2.


#### 2.2.1. The Deflection Angle of Light

In the lowest order of approximation we can neglect the term on the right-hand side of (32). Then the solution is given by a straight line,(35)u=cos⁡φR,whereby R we have denoted the distance of the closest approach to the central massive gravitating object. In the next order of approximation (35) is substituted into the right-hand side of (32). Hence the equation of the trajectory is given by a second-order linear inhomogeneous differential equation,(36)d2udφ2+u=Vcos⁡φR,which has a general solution given by u=uφ. The photons travel towards the star from infinity at the asymptotic angle φ=-π/2+ε and are deflected to infinity at the asymptotic angle φ=π/2+ε. The angle ε can be computed by solving the algebraic equation uπ/2+ε=0. For the total deflection angle of the photon beam we find δ=2ε.(a) The Deflection of Light in Schwarzschild Geometry. We consider now the case of the Schwarzschild geometry. In the lowest order of approximation from (35) and (36) we obtain for the photon trajectory the second-order linear differential equation(37)d2udφ2+u=3GMc2R2cos2⁡φ=3GM2c2R21+cos⁡2φ,having the general solution given by(38)u=cos⁡φR+3GM2c2R21-13cos⁡2φ.By substitutingφ=π/2+ε, u=0 into (38) we easily find(39)ε=2GMc2R,where we have used the simple trigonometric relations cosπ/2+ε=-sin⁡ε, cos⁡π+2ε=-cos⁡2ε, and the approximations sin⁡ε≈ε and cos⁡2ε≈1, respectively. The total deflection angle of light in the Schwarzschild geometry is thus δ=2ε=4GM/c2R, a well-known result in general relativity [13].


## 2.1. The Equation of Motion of Massive Test Particles

The geodesic equations of motion of a massive test particle in the gravitational field of the metric given by (5) can be derived with the use of the variational principle(6)δ∫eνc2t˙2-eλr˙2-r2θ˙2+sin2θφ˙2ds=0,where by a dot we have denoted d/ds. It can be easily checked that the orbit is planar, and therefore without any loss of generality we can take θ=π/2. Hence φ is the only the angular coordinate in this problem. Since t and φ do not appear explicitly in (6), their conjugate momenta give two constants of motion, denoted by E and L, so that(7)eνc2t˙=E=constant,r2φ˙=L=constant.The constant E gives the energy of the particle, while the constant L is related to its angular momentum.From the line element (5) we obtain the following equation of motion for r:(8)r˙2+e-λr2φ˙2=e-λeνc2t˙2-1.Substitutingt˙ and φ˙ from (7) gives the following relation:(9)r˙2+e-λL2r2=e-λE2c2e-ν-1.We introduce now a new variableu, defined as r=1/u, as well as the transformation d/ds=Lu2d/dφ. Then (9) takes the form(10)dudφ2+e-λu2=1L2e-λE2c2e-ν-1.We represent formallye-λ as e-λ=1-f(u), thus obtaining(11)dudφ2+u2=fuu2+E2c2L2e-ν-λ-1L2e-λ≡Gu.We take the derivative of the above equation with respect toφ, which gives(12)d2udφ2+u=Fu,where(13)Fu=12dGudu.Equation (12) gives the equation of motion of a particle in an arbitrary spherically symmetric geometry.


### 2.1.1. The Precession of the Perihelion

The root of the equationu0=Fu0 gives a circular orbit with u=u0. Any deviation δ=u-u0 from it can be obtained from the substitution of u=u0+δ into (12), which gives the equation(14)d2δdφ2+u0+δ=Fu0+δ.In the first order of approximation Fu0+δ≈Fu0+dF/duu=u0δ+Oδ2 and hence(15)d2δdφ2+1-dFduu=u0δ=Oδ2.Therefore, to first order inδ, the trajectory of the massive particle can be obtained as(16)δ=δ0cos⁡1-dFduu=u0φ+β,where δ0 and β are two arbitrary constants of integration. The angles for which r is minimum are the angles of the perihelia of the orbit. Therefore they are determined from the condition that u or δ is maximum. Hence, from one perihelion to the next the orbital angle varies by a quantity Δφ, given by(17)Δφ=2π1-dF/duu=u0=2π1-σ.The parameterσ introduced in the previous equation is called the perihelion advance. From a physical point of view it represents the rate of advance of the perihelion after one rotation. As the test particle advances through φ radians in its orbit, its perihelion precesses by σΔφ radians. From (17), σ can be expressed as(18)σ=1-1-dFduu=u0,or for small dF/duu=u0, as(19)σ=12dFduu=u0.For a complete rotation of the planet we obtainφ≈2π(1+σ). Hence the advance of the perihelion is(20)δφ=φ-2π≈2πσ.To be able to obtain effective estimations of the perihelion precession we must know the expression of the angular momentum of the particle L as a function of the geometric parameters of the orbit. We will obtain now the expression of L in the Newtonian limit [44–47].Let us assume that the planet moves on a Keplerian ellipse, with semiaxesa¯ and b¯, respectively, where b¯=a¯1-e2 and by e we have denoted the eccentricity of the orbit. The ellipse has a surface area πa¯b¯. The oriented surface area of the ellipse is dA→=r→×dr→/2, and consequently the areolar velocity of the planet is given by dA→/dt=r→×dr→/2=r2dφ/dt/2≈πa¯21-e2/T, where T is the period of the planetary motion. On the other hand T can be obtained from Kepler’s third law as T2=4π2a¯3/GM [48]. In the Newtonian limit of small velocities ds≈cdt, and the conservation equation of the angular momentum reduces to r2dφ/dt=cL. Hence we obtain first L=2πa¯21-e2/cT, and hence(21)1L2=c2GMa¯1-e2.


### 2.1.2. The Equation of Motion of Massive Particles in Schwarzschild Geometry

As a first astronomical application of the formalism introduced in the previous section we obtain the precession of the perihelion of a planet in the Schwarzschild geometry, with(22)eν=e-λ=1-2GMc2r=1-2GMc2u.Then we immediately obtain f(u)=2GM/c2u. On the other hand since for this geometry ν+λ=0, we easily find(23)Gu=2GMc2u3+1L2E2c2-1+2GMc2L2u,and(24)Fu=3GMc2u2+GMc2L2,respectively. Therefore the equation of motion of a massive test particle in Schwarzschild geometry is given by(25)d2udφ2+u=3GMc2u2+GMc2L2.The radiusu=u0 of the circular orbit is found as the solution of the algebraic equation(26)u0=3GMc2u02+GMc2L2,with the only physically acceptable solution given by(27)u0=1±1-12G2M2/c4L26GM/c2≈GMc2L2.Therefore(28)δφ=πdFduu=u0=6πGMc2a¯1-e2,which is the standard general relativistic result [13].


## 2.1.1. The Precession of the Perihelion

The root of the equationu0=Fu0 gives a circular orbit with u=u0. Any deviation δ=u-u0 from it can be obtained from the substitution of u=u0+δ into (12), which gives the equation(14)d2δdφ2+u0+δ=Fu0+δ.In the first order of approximation Fu0+δ≈Fu0+dF/duu=u0δ+Oδ2 and hence(15)d2δdφ2+1-dFduu=u0δ=Oδ2.Therefore, to first order inδ, the trajectory of the massive particle can be obtained as(16)δ=δ0cos⁡1-dFduu=u0φ+β,where δ0 and β are two arbitrary constants of integration. The angles for which r is minimum are the angles of the perihelia of the orbit. Therefore they are determined from the condition that u or δ is maximum. Hence, from one perihelion to the next the orbital angle varies by a quantity Δφ, given by(17)Δφ=2π1-dF/duu=u0=2π1-σ.The parameterσ introduced in the previous equation is called the perihelion advance. From a physical point of view it represents the rate of advance of the perihelion after one rotation. As the test particle advances through φ radians in its orbit, its perihelion precesses by σΔφ radians. From (17), σ can be expressed as(18)σ=1-1-dFduu=u0,or for small dF/duu=u0, as(19)σ=12dFduu=u0.For a complete rotation of the planet we obtainφ≈2π(1+σ). Hence the advance of the perihelion is(20)δφ=φ-2π≈2πσ.To be able to obtain effective estimations of the perihelion precession we must know the expression of the angular momentum of the particle L as a function of the geometric parameters of the orbit. We will obtain now the expression of L in the Newtonian limit [44–47].Let us assume that the planet moves on a Keplerian ellipse, with semiaxesa¯ and b¯, respectively, where b¯=a¯1-e2 and by e we have denoted the eccentricity of the orbit. The ellipse has a surface area πa¯b¯. The oriented surface area of the ellipse is dA→=r→×dr→/2, and consequently the areolar velocity of the planet is given by dA→/dt=r→×dr→/2=r2dφ/dt/2≈πa¯21-e2/T, where T is the period of the planetary motion. On the other hand T can be obtained from Kepler’s third law as T2=4π2a¯3/GM [48]. In the Newtonian limit of small velocities ds≈cdt, and the conservation equation of the angular momentum reduces to r2dφ/dt=cL. Hence we obtain first L=2πa¯21-e2/cT, and hence(21)1L2=c2GMa¯1-e2.


## 2.1.2. The Equation of Motion of Massive Particles in Schwarzschild Geometry

As a first astronomical application of the formalism introduced in the previous section we obtain the precession of the perihelion of a planet in the Schwarzschild geometry, with(22)eν=e-λ=1-2GMc2r=1-2GMc2u.Then we immediately obtain f(u)=2GM/c2u. On the other hand since for this geometry ν+λ=0, we easily find(23)Gu=2GMc2u3+1L2E2c2-1+2GMc2L2u,and(24)Fu=3GMc2u2+GMc2L2,respectively. Therefore the equation of motion of a massive test particle in Schwarzschild geometry is given by(25)d2udφ2+u=3GMc2u2+GMc2L2.The radiusu=u0 of the circular orbit is found as the solution of the algebraic equation(26)u0=3GMc2u02+GMc2L2,with the only physically acceptable solution given by(27)u0=1±1-12G2M2/c4L26GM/c2≈GMc2L2.Therefore(28)δφ=πdFduu=u0=6πGMc2a¯1-e2,which is the standard general relativistic result [13].


## 2.2. Equation of Motions of Photons and the Deflection of Light

In a gravitational field a photon follows a null geodesic, given byds2=0. In this case the affine parameter along the trajectory of the photon can be taken as an arbitrary quantity λ~. In the following we denote again by a dot the derivatives with respect to it. Similar to the case of the motion of massive particles we have two constants of motion, the energy E and the angular momentum L, which can be obtained from (7).The equation of motion of the photon is given by(29)r˙2+e-λr2φ˙2=eν-λc2t˙2.By using the constants of motion the above equation can be transformed into(30)r˙2+e-λL2r2=E2c2e-ν-λ.We change now the independent variabler to u=1/r. With the use of the conservation equations we eliminate the derivative with respect to the affine parameter, thus obtaining(31)dudφ2+u2=fuu2+1c2E2L2e-ν-λ≡Pu.We take the derivative of (31) with respect to φ, and thus we find the basic equation of the photon in an arbitrary static spherically symmetric geometry, as given by(32)d2udφ2+u=Vu,where we have denoted(33)Vu=12dPudu.In the particular case of the Schwarzschild geometry we haveν+λ=0 and f(u)=2GM/c2u, giving P(u)=2GM/c2u3 and V(u)=3GM/c2u2, respectively. Hence the equation of motion of photons in the Schwarzschild metric is obtained as(34)d2udφ2+u=3GMc2u2.


### 2.2.1. The Deflection Angle of Light

In the lowest order of approximation we can neglect the term on the right-hand side of (32). Then the solution is given by a straight line,(35)u=cos⁡φR,whereby R we have denoted the distance of the closest approach to the central massive gravitating object. In the next order of approximation (35) is substituted into the right-hand side of (32). Hence the equation of the trajectory is given by a second-order linear inhomogeneous differential equation,(36)d2udφ2+u=Vcos⁡φR,which has a general solution given by u=uφ. The photons travel towards the star from infinity at the asymptotic angle φ=-π/2+ε and are deflected to infinity at the asymptotic angle φ=π/2+ε. The angle ε can be computed by solving the algebraic equation uπ/2+ε=0. For the total deflection angle of the photon beam we find δ=2ε.(a) The Deflection of Light in Schwarzschild Geometry. We consider now the case of the Schwarzschild geometry. In the lowest order of approximation from (35) and (36) we obtain for the photon trajectory the second-order linear differential equation(37)d2udφ2+u=3GMc2R2cos2⁡φ=3GM2c2R21+cos⁡2φ,having the general solution given by(38)u=cos⁡φR+3GM2c2R21-13cos⁡2φ.By substitutingφ=π/2+ε, u=0 into (38) we easily find(39)ε=2GMc2R,where we have used the simple trigonometric relations cosπ/2+ε=-sin⁡ε, cos⁡π+2ε=-cos⁡2ε, and the approximations sin⁡ε≈ε and cos⁡2ε≈1, respectively. The total deflection angle of light in the Schwarzschild geometry is thus δ=2ε=4GM/c2R, a well-known result in general relativity [13].


## 2.2.1. The Deflection Angle of Light

In the lowest order of approximation we can neglect the term on the right-hand side of (32). Then the solution is given by a straight line,(35)u=cos⁡φR,whereby R we have denoted the distance of the closest approach to the central massive gravitating object. In the next order of approximation (35) is substituted into the right-hand side of (32). Hence the equation of the trajectory is given by a second-order linear inhomogeneous differential equation,(36)d2udφ2+u=Vcos⁡φR,which has a general solution given by u=uφ. The photons travel towards the star from infinity at the asymptotic angle φ=-π/2+ε and are deflected to infinity at the asymptotic angle φ=π/2+ε. The angle ε can be computed by solving the algebraic equation uπ/2+ε=0. For the total deflection angle of the photon beam we find δ=2ε.(a) The Deflection of Light in Schwarzschild Geometry. We consider now the case of the Schwarzschild geometry. In the lowest order of approximation from (35) and (36) we obtain for the photon trajectory the second-order linear differential equation(37)d2udφ2+u=3GMc2R2cos2⁡φ=3GM2c2R21+cos⁡2φ,having the general solution given by(38)u=cos⁡φR+3GM2c2R21-13cos⁡2φ.By substitutingφ=π/2+ε, u=0 into (38) we easily find(39)ε=2GMc2R,where we have used the simple trigonometric relations cosπ/2+ε=-sin⁡ε, cos⁡π+2ε=-cos⁡2ε, and the approximations sin⁡ε≈ε and cos⁡2ε≈1, respectively. The total deflection angle of light in the Schwarzschild geometry is thus δ=2ε=4GM/c2R, a well-known result in general relativity [13].


## 3. The Laplace-Adomian Method for Nonlinear Second-Order Ordinary Differential Equations

### 3.1. The General Formalism

Let us consider a nonlinear differential equation of the form(40)y′′+ω2y+b2+gy=0,where ω and b are constants and g is an arbitrary nonlinear function of dependent variable y. Equation (40) must be integrated with the initial conditions y(0)=y0=a and y′(0)=0, respectively.In the Laplace-Adomian method we first apply the Laplace transformation operatorL to (40), thus obtaining(41)Ly′′+ω2Ly+Lb2+Lgy=0.With the use of the properties of the Laplace transformation we easily find(42)s2+ω2Ly-sy0-y′0+b2s+Lgy=0.With the use of the initial conditions for our problem we obtain(43)Ly=ass2+ω2-b2ss2+ω2-1s2+ω2Lgy.As a next step in our analysis we assume that the solution can be represented in the form of an infinite series,(44)yx=∑n=0∞ynx,where the terms yn(x) are computed recursively. As for the nonlinear operator g(y), it is decomposed as(45)gy=∑n=0∞An,where An’s are the so-called Adomian polynomials, defined generally as [20](46)An=1n!dndϵnf∑i=0∞ϵiyiϵ=0.The first five Adomian polynomials can be obtained in the following form:(47)A0=fy0,(48)A1=y1f′y0,(49)A2=y2f′y0+12y12f′′y0,(50)A3=y3f′y0+y1y2f′′y0+16y13f′′′y0,(51)A4=y4f′y0+12!y22+y1y3f′′y0+12!y12y2f′′′y0+14!y14fivy0.Substituting (44) and (45) into (43) we obtain(52)L∑n=0∞ynx=ass2+ω2-b2ss2+ω2-1s2+ω2L∑n=0∞An.Matching both sides of (52) yields the following iterative algorithm for the power series solution of (40),(53)Ly0=ass2+ω2-b2ss2+ω2,(54)Ly1=-1s2+ω2LA0,(55)Ly2=-1s2+ω2LA1,(56)⋯(57)Lyk+1=-1s2+ω2LAk.By applying the inverse Laplace transformation to (53), we obtain the value of y0. Substituting y0 into (47) to find the first Adomian polynomial A0, then we substitute A0 into (54), and we evaluate the Laplace transform of the quantities on the right-hand side of it. The application of the inverse Laplace transformation yields then the value of y1. The other terms y2, y3,…,yk+1 can be computed recursively in a similar step by step approach.


### 3.2. The Particular Caseg(y)=∑l=0mal+2yl+2

Let us consider a nonlinear differential equation of the form(58)d2ydx2+ω2y+b2+∑l=0mal+2yl+2=0,where ω, b, and al+2, l=0,…,m, are constants. Equation (58) must be integrated with the initial conditions y(0)=y0=a and y′(0)=0, respectively. By applying the Laplace-Adomian method we first take the Laplace transform to (58), thus obtaining(59)Ld2ydx2+ω2Ly+b2L1+∑l=0mal+2Lyl+2=0.Now we easily find(60)Lys2+ω2=sy0+y′0-b2s-∑l=0mal+2Lyl+2=0,thus obtaining(61)Ly=sy0+y′0s2+ω2-b2ss2+ω2-1s2+ω2∑l=0mal+2Lyl+2.From (61) we obtain y(x) in the form(62)yx=L-1sy0+y′0s2+ω2-b2ss2+ω2-L-11s2+ω2∑l=0mal+2Lyl+2.We assume a power series solution fory(x) as y(x)=∑n=0∞yn(x), and we write the nonlinear terms as(63)yl+2=∑n=0∞An,l+2x,where An,l+2 are the Adomian polynomial corresponding to yl+2. Then we obtain(64)∑n=0∞ynx=L-1sy0+y′0s2+ω2-b2ss2+ω2-L-11s2+ω2∑l=0mal+2L∑n=0∞An,l+2x.We rewrite (64) in the form(65)y0x+∑n=0∞yn+1x=L-1sy0+y′0s2+ω2-b2ss2+ω2-∑n=0∞L-11s2+ω2∑l=0mal+2LAn,l+2x.Equation (65) can be written as the recursive relations(66)y0x=L-1sy0+y′0s2+ω2-b2ss2+ω2,(67)…,(68)yk+1x=-L-11s2+ω2∑l=0mal+2LAk,l+2x.For the functionyl+2 a few Adomian polynomials are(69)A0,l+2=y0l+2,(70)A1,l+2=l+2y1y0l+1,(71)A2,l+2=l+2y2y0l+1+l+1l+2y122!y0l,(72)A3,l+2=l+2y3y0l+1+l+1l+2y1y2y0l+ll+1l+2y133!y0l-1.Fork=0 we obtain the first-order approximation to the solution as(73)y1x=-L-1La2A0,2x+a3A0,3x+a4A0,4x+⋯s2+ω2=-L-1La2y02+a3y03+a4y04+⋯s2+ω2.For k=1 we find(74)y2x=-L-1La2A1,2x+a3A1,3x+a4A1,4x+⋯s2+ω2=-L-1L2a2y1y0+3a3y1y02+4a4y1y03+⋯s2+ω2.k=2 gives(75)y3x=-L-1La2A2,2x+a3A2,3x+a4A2,4x+⋯s2+ω2=-L-1La22y2y0+y12+3a3y2y02+y12y0+a44y2y03+6y12y02+⋯s2+ω2.Finally, for k=3 we obtain(76)y4x=-L-1La2A3,2x+a3A3,3x+a4A3,4x+⋯s2+ω2=-L-11s2+ω2L2a2y3y0+y1y2+a33y3y02+6y1y2y0+y13+a44y3y03+12y1y2y02+4y13y0+⋯.Hence we have obtained the truncated power series solution of (58) as given by(77)yx=∑n=0∞ynx=y0x+y1x+y2x+y3x+y4x+⋯.


## 3.1. The General Formalism

Let us consider a nonlinear differential equation of the form(40)y′′+ω2y+b2+gy=0,where ω and b are constants and g is an arbitrary nonlinear function of dependent variable y. Equation (40) must be integrated with the initial conditions y(0)=y0=a and y′(0)=0, respectively.In the Laplace-Adomian method we first apply the Laplace transformation operatorL to (40), thus obtaining(41)Ly′′+ω2Ly+Lb2+Lgy=0.With the use of the properties of the Laplace transformation we easily find(42)s2+ω2Ly-sy0-y′0+b2s+Lgy=0.With the use of the initial conditions for our problem we obtain(43)Ly=ass2+ω2-b2ss2+ω2-1s2+ω2Lgy.As a next step in our analysis we assume that the solution can be represented in the form of an infinite series,(44)yx=∑n=0∞ynx,where the terms yn(x) are computed recursively. As for the nonlinear operator g(y), it is decomposed as(45)gy=∑n=0∞An,where An’s are the so-called Adomian polynomials, defined generally as [20](46)An=1n!dndϵnf∑i=0∞ϵiyiϵ=0.The first five Adomian polynomials can be obtained in the following form:(47)A0=fy0,(48)A1=y1f′y0,(49)A2=y2f′y0+12y12f′′y0,(50)A3=y3f′y0+y1y2f′′y0+16y13f′′′y0,(51)A4=y4f′y0+12!y22+y1y3f′′y0+12!y12y2f′′′y0+14!y14fivy0.Substituting (44) and (45) into (43) we obtain(52)L∑n=0∞ynx=ass2+ω2-b2ss2+ω2-1s2+ω2L∑n=0∞An.Matching both sides of (52) yields the following iterative algorithm for the power series solution of (40),(53)Ly0=ass2+ω2-b2ss2+ω2,(54)Ly1=-1s2+ω2LA0,(55)Ly2=-1s2+ω2LA1,(56)⋯(57)Lyk+1=-1s2+ω2LAk.By applying the inverse Laplace transformation to (53), we obtain the value of y0. Substituting y0 into (47) to find the first Adomian polynomial A0, then we substitute A0 into (54), and we evaluate the Laplace transform of the quantities on the right-hand side of it. The application of the inverse Laplace transformation yields then the value of y1. The other terms y2, y3,…,yk+1 can be computed recursively in a similar step by step approach.


## 3.2. The Particular Caseg(y)=∑l=0mal+2yl+2

Let us consider a nonlinear differential equation of the form(58)d2ydx2+ω2y+b2+∑l=0mal+2yl+2=0,where ω, b, and al+2, l=0,…,m, are constants. Equation (58) must be integrated with the initial conditions y(0)=y0=a and y′(0)=0, respectively. By applying the Laplace-Adomian method we first take the Laplace transform to (58), thus obtaining(59)Ld2ydx2+ω2Ly+b2L1+∑l=0mal+2Lyl+2=0.Now we easily find(60)Lys2+ω2=sy0+y′0-b2s-∑l=0mal+2Lyl+2=0,thus obtaining(61)Ly=sy0+y′0s2+ω2-b2ss2+ω2-1s2+ω2∑l=0mal+2Lyl+2.From (61) we obtain y(x) in the form(62)yx=L-1sy0+y′0s2+ω2-b2ss2+ω2-L-11s2+ω2∑l=0mal+2Lyl+2.We assume a power series solution fory(x) as y(x)=∑n=0∞yn(x), and we write the nonlinear terms as(63)yl+2=∑n=0∞An,l+2x,where An,l+2 are the Adomian polynomial corresponding to yl+2. Then we obtain(64)∑n=0∞ynx=L-1sy0+y′0s2+ω2-b2ss2+ω2-L-11s2+ω2∑l=0mal+2L∑n=0∞An,l+2x.We rewrite (64) in the form(65)y0x+∑n=0∞yn+1x=L-1sy0+y′0s2+ω2-b2ss2+ω2-∑n=0∞L-11s2+ω2∑l=0mal+2LAn,l+2x.Equation (65) can be written as the recursive relations(66)y0x=L-1sy0+y′0s2+ω2-b2ss2+ω2,(67)…,(68)yk+1x=-L-11s2+ω2∑l=0mal+2LAk,l+2x.For the functionyl+2 a few Adomian polynomials are(69)A0,l+2=y0l+2,(70)A1,l+2=l+2y1y0l+1,(71)A2,l+2=l+2y2y0l+1+l+1l+2y122!y0l,(72)A3,l+2=l+2y3y0l+1+l+1l+2y1y2y0l+ll+1l+2y133!y0l-1.Fork=0 we obtain the first-order approximation to the solution as(73)y1x=-L-1La2A0,2x+a3A0,3x+a4A0,4x+⋯s2+ω2=-L-1La2y02+a3y03+a4y04+⋯s2+ω2.For k=1 we find(74)y2x=-L-1La2A1,2x+a3A1,3x+a4A1,4x+⋯s2+ω2=-L-1L2a2y1y0+3a3y1y02+4a4y1y03+⋯s2+ω2.k=2 gives(75)y3x=-L-1La2A2,2x+a3A2,3x+a4A2,4x+⋯s2+ω2=-L-1La22y2y0+y12+3a3y2y02+y12y0+a44y2y03+6y12y02+⋯s2+ω2.Finally, for k=3 we obtain(76)y4x=-L-1La2A3,2x+a3A3,3x+a4A3,4x+⋯s2+ω2=-L-11s2+ω2L2a2y3y0+y1y2+a33y3y02+6y1y2y0+y13+a44y3y03+12y1y2y02+4y13y0+⋯.Hence we have obtained the truncated power series solution of (58) as given by(77)yx=∑n=0∞ynx=y0x+y1x+y2x+y3x+y4x+⋯.


## 4. The Solution of the Equation of the Motion of the Massive Test Particles in Schwarzschild Geometry by the Laplace-Adomian Decomposition Method

In the following we will use a system of units withG=c=1. Then (25), describing the motion of a massive test particle in the Schwarzschild geometry, takes the form(78)d2udφ2+u=ML2+3Mu2.In order to simplify the mathematical formalism we rescale the function u so that(79)u=13MU.Thus (78) becomes(80)d2Udφ2+U=b2+U2,where we have denoted b2=3M2/L2. Equation (80) must be solved with the initial conditions U(0)=3Mu(0)=a and U′(0)=0, respectively.


### 4.1. Power Series Solution of the Equation of Motion

Assume that the solution of (80) can be obtained in power series form,(81)Uφ=∑n=0∞Unφ.Now taking Laplace transform to (80) yields(82)Ld2Udφ2+LU=b2L1+LU2.Hence we obtain(83)s2LU-sU0-U′0+LU=b2s+LU2,(84)LU=sU0+U′0s2+1+b2ss2+1+1s2+1LU2.We write down a few Adomian polynomials forU2,(85)A0=U02,(86)A1=2U1U0,(87)A2=2U2U0+U12.(88)A3=2U3U0+2U1U2,Substituting (81) and U2=∑n=0∞Anφ into (84) gives the relation(89)L∑n=0∞Unφ=sU0+U′0s2+1+b2ss2+1+1s2+1L∑n=0∞Anφ,or equivalently,(90)U0φ+∑n=1∞Unφ=U0φ+∑n=0∞Un+1φ=L-1sU0+U′0s2+1+b2ss2+1+∑n=0∞L-1LAnφs2+1.Next we rewrite (90) in the recursive forms(91)U0φ=L-1sU0+U′0s2+1+b2ss2+1,(92)…,(93)Uk+1φ=L-1LAkφs2+1.With the help of the explicit expressions of the Adomian polynomials, we obtain(94)U0φ=a-b2cos⁡φ+b2,(95)A0=U02=a-b2cos⁡φ+b22,(96)U1φ=L-1LA0φs2+1=L-1LU02s2+1,k=0,(97)U1φ=16-2a2-2ab2+4b4cos⁡φ+3a2-2ab2+3b4+a-b2b2-acos⁡2φ+6b2φsin⁡φ,(98)A1=2U1U0=13a-b2cos⁡φ+b2-2a2-2ab2+4b4cos⁡φ+3a2-2ab2+3b4+a-b2b2-acos⁡2φ+6b2φsin⁡φ,(99)U2φ=L-1LA1φs2+1=2L-1LU1U0s2+1,k=1,(100)U2φ=114416a2-5ab2+7b4a-b2cos⁡2φ+cos⁡φ29a3-183a2b2+3ab4125-24φ2+b672φ2-509+12φ5a3-19a2b2+41ab4-39b6×sin⁡φ-48a3-6a2b2+12ab4-13b6-48φb3-ab2sin⁡2φ+3a-b23cos⁡3φ,(101)U3φ=L-1LA2φs2+1=L-1L2U2U0+U12s2+1,k=2,(102)U3φ=1432-12φ5a3-23a2b2+73ab4-79b6a-b2sin⁡2φ-3φsin⁡φ60a4-449a3b2+1335a2b4+3ab68φ2-721+b81721-24φ2+cos⁡φ×-119a4+4a3b2206-45φ2+3a2b4204φ2-961+2ab62461-666φ2+4b8279φ2-1262-24cos⁡2φ4a4-24a3b2+72a2b4-112ab6-6b4φ2a-b22+63b8+925a4-164a3b2+534a2b4-868ab6+737b8+27b2φa-b23sin⁡3φ+a-b24cos⁡4φ-9a-4b2a-2b2a-b22cos⁡3φ,(103)U4φ=L-1LA3φs2+1=2L-1LU3U0+U1U2s2+1,k=3,(104)U4φ=120736322a2-13ab2+17b4a-b23cos⁡4φ+108φ5a3-27a2b2+105ab4-119b6a-b22sin⁡3φ+36a-b2cos⁡3φ31a4-196a3b2+642a2b4-1036ab6-54b4φ2a-b22+595b8+24φsin⁡φ580a5-4938a4b2+a3b420129-180φ2+7a2b684φ2-6583+3ab820397-428φ2+5b10204φ2-8221+192φsin⁡2φ20a5-223a4b2+884a3b4-2036a2b6+2608ab8+24b6φ2a-b22-1289b10+384cos⁡2φ16a5+a4b215φ2-152+8a3b474-9φ2+8a2b627φ2-164+104ab816-3φ2+b10153φ2-862-3cos⁡φa5600φ2-2357+a4b223209-5880φ2+2a3b415588φ2-49577+2a2b6126457-39324φ2+ab896φ21235-3φ2-362713+b10288φ4-83088φ2+298693-28850a5-475a4b2+1934a3b4-4604a2b6+6208ab8-4337b10-192b2φa-b24sin⁡4φ+5a-b25cos⁡5φ.The power series solution of the equation of motion of the massive test particles in Schwarzschild geometry is thus given by(105)Uφ=U0φ+U1φ+U2φ+U3φ+U4φ+⋯.


### 4.2. Comparison with the Exact Numerical Solution

In order to estimate the results obtained by the Laplace-Adomian Decomposition Method, in Figure1 we present the comparison between the exact numerical solution of (80) and the analytical, power series representation, given by (105), for b=0.15, b2=0.0225, and a=0.001, respectively.Figure 1
Comparison of the numerical solutionU(φ) of (80) (solid curve) and of the truncated power series solution obtained by the Laplace-Adomian Decomposition Method, given by (105) (dashed curve), for b2=0.0225 and a=0.001.The absolute differenceΔ(φ), defined as(106)Δφ=Uφnum-UφLADM,between the numerical solution U(φ)num and the truncated Laplace-Adomian power series solution, is represented in Figure 2.Figure 2
The absolute differenceΔ(φ) between the numerical solution U(φ)num and the truncated Laplace-Adomian power series solution U(φ)LADM of (80) for b2=0.0225 and a=0.001.


### 4.3. Application: The Motion of Planet Mercury

In order to integrate the equation of motion of the massive test particles in Schwarzschild geometry we need to know the initial conditions of the motion. For the initial value ofu and U we adopt the values(107)u0=ML21+e,a=U0=3M2L21+e=b21+e,where e is the eccentricity of the orbit. The angular momentum L2 can be expressed in terms of the geometric parameters of the orbit by using (21). Thus in physical units we obtain(108)b2=3GMc2a¯1-e2,a=3GMc2a¯1-e.Therefore for this choice of parameters and initial conditions the successive terms in the power series solution of the equation of motion can be obtained by the Laplace-Adomian Decomposition Method as follows:(109)u0φ=1+ecos⁡φa¯1-e2,(110)u1φ=-GM-3e2+2eφsin⁡φ+2+e2cos⁡2φ+2e2+3cos⁡φ2c2a¯21-e22,(111)u2φ=G2M216c4a¯3e2-13e-3e2cos⁡3φ-16e-3e+3cos⁡2φ+12φsin⁡φ8ecos⁡φ+4-5ee-18+e96-29ee+72φ2-96+288×cos⁡φ+48ee-3e+3+3φsin⁡φ-6,(112)u3φ=G3M316c6a¯41-e24e4-cos⁡4φ+27e3φsin⁡3φ-24-6e2φ2+4eee-2e+6-6+3cos⁡2φ-9e-3e-1e2cos⁡3φ+-119e4+348e3-1125e2-36ee5e-2+18-6φ2+1152e-2304cos⁡φ-3φeee60e-209+348+24φ2-25+504sin⁡φ+12e8-5ee-42+24eφsin⁡2φ+9eee25e-64+192-192+264,(113)u4φ=G4M4256c8a¯5e2-15-5e5cos⁡5φ+192e4φsin⁡4φ-32e2e-9+6×e3cos⁡4φ-108ee5e-12+66-36e2φsin⁡3φ+24φ-580e5+2038e4-6177e3+9522e2+12ee15e-4+54-12φ2-12564e+10224sinφ+3-2357e5+11424e4-29888e3+71136e2-288eφ4+24eee5e5e-24+569-600+1428-720φ2-73296e+110592cos⁡φ-192φeeee20e-123+192+64φ2-87+396-36sin⁡2φ-36eeee31e-72-54φ2+240-216+36cos⁡3φ-3843eee5e-4+30-12φ2+8eeee2e-9+18-36+36-54cos⁡2φ+288eee25e2e-9+534-1152+1152-1224.In the case of the planet Mercury its orbital parameters area¯=57.91×1011 cm and e=0.205615, respectively. M is the mass of the Sun, given by M=M⊙=1.989×1033 g. Therefore the successive approximations of the solution of the equation of motion of planet Mercury in the Schwarzschild metric created by the gravitational field of the Sun can be obtained as(114)u0φ=1.80305×10-131+0.205615cos⁡φ,(115)u1φ=-2.39927×10-21-30.41123φsin⁡φ+2.04228+6.08456cos⁡φ+0.0422775cos⁡2φ,(116)u2φ=-7.98169×10-300.20561572φ2-77.487+288.00cos⁡φ+483φsin⁡φ-5.50129+0.205615-38.8069cos⁡2φ-0.126833cos⁡3φ+12φsin⁡φ1.64492cos⁡φ-17.3889,(117)u3φ=2.12421×10-37-3φ0.20561524φ2-25+63.2396+504sin⁡φ+84.2407φ2-2111.88cos⁡φ-24-0.253665φ2-0.982493cos⁡2φ+38.6365φsin⁡2φ+0.234708φsin⁡3φ-0.844636cos⁡3φ-0.00178739cos⁡4φ+2089.15,(118)u4φ=-3.53332×10-4624φ7992.95-11.2261φ2sin⁡φ-7.402140.2056150.205615226.506-54φ2-216+36cos⁡3φ-192φ0.2056150.20561564φ2-87+34.4518+396-36sin⁡2φ-384-3.67467φ2-5.82984cos⁡2φ+3-59.2171φ4-10728.2φ2+98288.4cos⁡φ+104.53φsin⁡3φ+0.343179φsin⁡4φ-1.17779cos⁡4φ-0.00183757cos⁡5φ-297094..(119)uφ=6.88343×10-44φ3+8.20724×10-36φsin⁡2φ+sin⁡φ-3.14475×10-36φ3+2.95996×10-21φ-3.23948×10-29φcos⁡φ+1.29321×10-36φ2+5.00886×10-36×cos⁡2φ+-5.97095×10-45φ2-1.79419×10-37cos⁡3φ+6.27699×10-44φ4-1.18163×10-28φ2+3.70733×10-14cos⁡φ+4.98570×10-38φsin⁡3φ-1.21256×10-46φsin⁡4φ-1.01435×10-22cos⁡2φ+2.08152×10-31cos⁡3φ-3.79680×10-40cos⁡4φ+6.49272×10-49cos⁡5φ+1.80304×10-13.


#### 4.3.1. The Perihelion Precession

In the first-order approximation the solution of the equation of motion for massive particles in Schwarzschild geometry is obtained as(120)uφ≈u0φ+u1φ=1+ecos⁡φa¯1-e2+GM3e2+2eϕsin⁡φ+2-e2cos⁡2φ-2e2+3cos⁡φ2a¯2c21-e22.By neglecting the constant terms and those who oscillate through two cycles on each orbit we obtain(121)uφ≈1+ecos⁡φ+eαφsin⁡φl=1+ecos⁡1-αφl,where we have denoted l=a¯1-e2 and α=3GM/c2l<<1. At perihelion we now have 1-αφ=2nπ, where n is an integer or φ=2nπ+6nπGM/lc2. This relation shows that the perihelion advances by Δφ=6nπGM/lc2 per rotation and the rate of precession is(122)ΔφΔt=6πGMa¯c21-e2Δt,n=1.We have thus recovered from the first-order Laplace-Adomian solution of the equation of motion the standard general relativistic result for the perihelion precession [13].


## 4.1. Power Series Solution of the Equation of Motion

Assume that the solution of (80) can be obtained in power series form,(81)Uφ=∑n=0∞Unφ.Now taking Laplace transform to (80) yields(82)Ld2Udφ2+LU=b2L1+LU2.Hence we obtain(83)s2LU-sU0-U′0+LU=b2s+LU2,(84)LU=sU0+U′0s2+1+b2ss2+1+1s2+1LU2.We write down a few Adomian polynomials forU2,(85)A0=U02,(86)A1=2U1U0,(87)A2=2U2U0+U12.(88)A3=2U3U0+2U1U2,Substituting (81) and U2=∑n=0∞Anφ into (84) gives the relation(89)L∑n=0∞Unφ=sU0+U′0s2+1+b2ss2+1+1s2+1L∑n=0∞Anφ,or equivalently,(90)U0φ+∑n=1∞Unφ=U0φ+∑n=0∞Un+1φ=L-1sU0+U′0s2+1+b2ss2+1+∑n=0∞L-1LAnφs2+1.Next we rewrite (90) in the recursive forms(91)U0φ=L-1sU0+U′0s2+1+b2ss2+1,(92)…,(93)Uk+1φ=L-1LAkφs2+1.With the help of the explicit expressions of the Adomian polynomials, we obtain(94)U0φ=a-b2cos⁡φ+b2,(95)A0=U02=a-b2cos⁡φ+b22,(96)U1φ=L-1LA0φs2+1=L-1LU02s2+1,k=0,(97)U1φ=16-2a2-2ab2+4b4cos⁡φ+3a2-2ab2+3b4+a-b2b2-acos⁡2φ+6b2φsin⁡φ,(98)A1=2U1U0=13a-b2cos⁡φ+b2-2a2-2ab2+4b4cos⁡φ+3a2-2ab2+3b4+a-b2b2-acos⁡2φ+6b2φsin⁡φ,(99)U2φ=L-1LA1φs2+1=2L-1LU1U0s2+1,k=1,(100)U2φ=114416a2-5ab2+7b4a-b2cos⁡2φ+cos⁡φ29a3-183a2b2+3ab4125-24φ2+b672φ2-509+12φ5a3-19a2b2+41ab4-39b6×sin⁡φ-48a3-6a2b2+12ab4-13b6-48φb3-ab2sin⁡2φ+3a-b23cos⁡3φ,(101)U3φ=L-1LA2φs2+1=L-1L2U2U0+U12s2+1,k=2,(102)U3φ=1432-12φ5a3-23a2b2+73ab4-79b6a-b2sin⁡2φ-3φsin⁡φ60a4-449a3b2+1335a2b4+3ab68φ2-721+b81721-24φ2+cos⁡φ×-119a4+4a3b2206-45φ2+3a2b4204φ2-961+2ab62461-666φ2+4b8279φ2-1262-24cos⁡2φ4a4-24a3b2+72a2b4-112ab6-6b4φ2a-b22+63b8+925a4-164a3b2+534a2b4-868ab6+737b8+27b2φa-b23sin⁡3φ+a-b24cos⁡4φ-9a-4b2a-2b2a-b22cos⁡3φ,(103)U4φ=L-1LA3φs2+1=2L-1LU3U0+U1U2s2+1,k=3,(104)U4φ=120736322a2-13ab2+17b4a-b23cos⁡4φ+108φ5a3-27a2b2+105ab4-119b6a-b22sin⁡3φ+36a-b2cos⁡3φ31a4-196a3b2+642a2b4-1036ab6-54b4φ2a-b22+595b8+24φsin⁡φ580a5-4938a4b2+a3b420129-180φ2+7a2b684φ2-6583+3ab820397-428φ2+5b10204φ2-8221+192φsin⁡2φ20a5-223a4b2+884a3b4-2036a2b6+2608ab8+24b6φ2a-b22-1289b10+384cos⁡2φ16a5+a4b215φ2-152+8a3b474-9φ2+8a2b627φ2-164+104ab816-3φ2+b10153φ2-862-3cos⁡φa5600φ2-2357+a4b223209-5880φ2+2a3b415588φ2-49577+2a2b6126457-39324φ2+ab896φ21235-3φ2-362713+b10288φ4-83088φ2+298693-28850a5-475a4b2+1934a3b4-4604a2b6+6208ab8-4337b10-192b2φa-b24sin⁡4φ+5a-b25cos⁡5φ.The power series solution of the equation of motion of the massive test particles in Schwarzschild geometry is thus given by(105)Uφ=U0φ+U1φ+U2φ+U3φ+U4φ+⋯.


## 4.2. Comparison with the Exact Numerical Solution

In order to estimate the results obtained by the Laplace-Adomian Decomposition Method, in Figure1 we present the comparison between the exact numerical solution of (80) and the analytical, power series representation, given by (105), for b=0.15, b2=0.0225, and a=0.001, respectively.Figure 1
Comparison of the numerical solutionU(φ) of (80) (solid curve) and of the truncated power series solution obtained by the Laplace-Adomian Decomposition Method, given by (105) (dashed curve), for b2=0.0225 and a=0.001.The absolute differenceΔ(φ), defined as(106)Δφ=Uφnum-UφLADM,between the numerical solution U(φ)num and the truncated Laplace-Adomian power series solution, is represented in Figure 2.Figure 2
The absolute differenceΔ(φ) between the numerical solution U(φ)num and the truncated Laplace-Adomian power series solution U(φ)LADM of (80) for b2=0.0225 and a=0.001.


## 4.3. Application: The Motion of Planet Mercury

In order to integrate the equation of motion of the massive test particles in Schwarzschild geometry we need to know the initial conditions of the motion. For the initial value ofu and U we adopt the values(107)u0=ML21+e,a=U0=3M2L21+e=b21+e,where e is the eccentricity of the orbit. The angular momentum L2 can be expressed in terms of the geometric parameters of the orbit by using (21). Thus in physical units we obtain(108)b2=3GMc2a¯1-e2,a=3GMc2a¯1-e.Therefore for this choice of parameters and initial conditions the successive terms in the power series solution of the equation of motion can be obtained by the Laplace-Adomian Decomposition Method as follows:(109)u0φ=1+ecos⁡φa¯1-e2,(110)u1φ=-GM-3e2+2eφsin⁡φ+2+e2cos⁡2φ+2e2+3cos⁡φ2c2a¯21-e22,(111)u2φ=G2M216c4a¯3e2-13e-3e2cos⁡3φ-16e-3e+3cos⁡2φ+12φsin⁡φ8ecos⁡φ+4-5ee-18+e96-29ee+72φ2-96+288×cos⁡φ+48ee-3e+3+3φsin⁡φ-6,(112)u3φ=G3M316c6a¯41-e24e4-cos⁡4φ+27e3φsin⁡3φ-24-6e2φ2+4eee-2e+6-6+3cos⁡2φ-9e-3e-1e2cos⁡3φ+-119e4+348e3-1125e2-36ee5e-2+18-6φ2+1152e-2304cos⁡φ-3φeee60e-209+348+24φ2-25+504sin⁡φ+12e8-5ee-42+24eφsin⁡2φ+9eee25e-64+192-192+264,(113)u4φ=G4M4256c8a¯5e2-15-5e5cos⁡5φ+192e4φsin⁡4φ-32e2e-9+6×e3cos⁡4φ-108ee5e-12+66-36e2φsin⁡3φ+24φ-580e5+2038e4-6177e3+9522e2+12ee15e-4+54-12φ2-12564e+10224sinφ+3-2357e5+11424e4-29888e3+71136e2-288eφ4+24eee5e5e-24+569-600+1428-720φ2-73296e+110592cos⁡φ-192φeeee20e-123+192+64φ2-87+396-36sin⁡2φ-36eeee31e-72-54φ2+240-216+36cos⁡3φ-3843eee5e-4+30-12φ2+8eeee2e-9+18-36+36-54cos⁡2φ+288eee25e2e-9+534-1152+1152-1224.In the case of the planet Mercury its orbital parameters area¯=57.91×1011 cm and e=0.205615, respectively. M is the mass of the Sun, given by M=M⊙=1.989×1033 g. Therefore the successive approximations of the solution of the equation of motion of planet Mercury in the Schwarzschild metric created by the gravitational field of the Sun can be obtained as(114)u0φ=1.80305×10-131+0.205615cos⁡φ,(115)u1φ=-2.39927×10-21-30.41123φsin⁡φ+2.04228+6.08456cos⁡φ+0.0422775cos⁡2φ,(116)u2φ=-7.98169×10-300.20561572φ2-77.487+288.00cos⁡φ+483φsin⁡φ-5.50129+0.205615-38.8069cos⁡2φ-0.126833cos⁡3φ+12φsin⁡φ1.64492cos⁡φ-17.3889,(117)u3φ=2.12421×10-37-3φ0.20561524φ2-25+63.2396+504sin⁡φ+84.2407φ2-2111.88cos⁡φ-24-0.253665φ2-0.982493cos⁡2φ+38.6365φsin⁡2φ+0.234708φsin⁡3φ-0.844636cos⁡3φ-0.00178739cos⁡4φ+2089.15,(118)u4φ=-3.53332×10-4624φ7992.95-11.2261φ2sin⁡φ-7.402140.2056150.205615226.506-54φ2-216+36cos⁡3φ-192φ0.2056150.20561564φ2-87+34.4518+396-36sin⁡2φ-384-3.67467φ2-5.82984cos⁡2φ+3-59.2171φ4-10728.2φ2+98288.4cos⁡φ+104.53φsin⁡3φ+0.343179φsin⁡4φ-1.17779cos⁡4φ-0.00183757cos⁡5φ-297094..(119)uφ=6.88343×10-44φ3+8.20724×10-36φsin⁡2φ+sin⁡φ-3.14475×10-36φ3+2.95996×10-21φ-3.23948×10-29φcos⁡φ+1.29321×10-36φ2+5.00886×10-36×cos⁡2φ+-5.97095×10-45φ2-1.79419×10-37cos⁡3φ+6.27699×10-44φ4-1.18163×10-28φ2+3.70733×10-14cos⁡φ+4.98570×10-38φsin⁡3φ-1.21256×10-46φsin⁡4φ-1.01435×10-22cos⁡2φ+2.08152×10-31cos⁡3φ-3.79680×10-40cos⁡4φ+6.49272×10-49cos⁡5φ+1.80304×10-13.


### 4.3.1. The Perihelion Precession

In the first-order approximation the solution of the equation of motion for massive particles in Schwarzschild geometry is obtained as(120)uφ≈u0φ+u1φ=1+ecos⁡φa¯1-e2+GM3e2+2eϕsin⁡φ+2-e2cos⁡2φ-2e2+3cos⁡φ2a¯2c21-e22.By neglecting the constant terms and those who oscillate through two cycles on each orbit we obtain(121)uφ≈1+ecos⁡φ+eαφsin⁡φl=1+ecos⁡1-αφl,where we have denoted l=a¯1-e2 and α=3GM/c2l<<1. At perihelion we now have 1-αφ=2nπ, where n is an integer or φ=2nπ+6nπGM/lc2. This relation shows that the perihelion advances by Δφ=6nπGM/lc2 per rotation and the rate of precession is(122)ΔφΔt=6πGMa¯c21-e2Δt,n=1.We have thus recovered from the first-order Laplace-Adomian solution of the equation of motion the standard general relativistic result for the perihelion precession [13].


## 4.3.1. The Perihelion Precession

In the first-order approximation the solution of the equation of motion for massive particles in Schwarzschild geometry is obtained as(120)uφ≈u0φ+u1φ=1+ecos⁡φa¯1-e2+GM3e2+2eϕsin⁡φ+2-e2cos⁡2φ-2e2+3cos⁡φ2a¯2c21-e22.By neglecting the constant terms and those who oscillate through two cycles on each orbit we obtain(121)uφ≈1+ecos⁡φ+eαφsin⁡φl=1+ecos⁡1-αφl,where we have denoted l=a¯1-e2 and α=3GM/c2l<<1. At perihelion we now have 1-αφ=2nπ, where n is an integer or φ=2nπ+6nπGM/lc2. This relation shows that the perihelion advances by Δφ=6nπGM/lc2 per rotation and the rate of precession is(122)ΔφΔt=6πGMa¯c21-e2Δt,n=1.We have thus recovered from the first-order Laplace-Adomian solution of the equation of motion the standard general relativistic result for the perihelion precession [13].


## 5. Solving the Equation of Motion for Light in Schwarzschild Geometry

The equation of motion of light in Schwarzschild geometry is a particular case of the general equation of motion (80) for b2=0, and it is given by(123)d2Udφ2+U=U2.


### 5.1. Power Series Solution

The truncated power series solution of (123) can be obtained immediately by using the Laplace-Adomian Decomposition Method as(124)U0φ=acos⁡φ,(125)U1φ=23a2sin2φ2cos⁡φ+2,(126)U2φ=1144a360φsin⁡φ+29cos⁡φ+16cos⁡2φ+3cos⁡3φ-48,(127)U3φ=-1432a4180φsin⁡φ+60φsin⁡2φ+119cos⁡φ+96cos⁡2φ+9cos⁡3φ+cos⁡4φ-225,(128)U4φ=a5207367071-1800φ2cos⁡φ+6144cos⁡2φ+1116cos⁡3φ+64cos⁡4φ+512232φsin⁡φ+64φsin⁡2φ+9φsin⁡3φ-240+cos⁡5φ,(129)Uφ=a20736a3842a8a2-6a+3-9cos⁡2φ+a36a31a-12+12×cos⁡3φ+a164a-3cos⁡4φ+5acos⁡5φ+60φsin⁡φ18acos⁡2φ+324a-3cos⁡φ+241a-144+8640φsin⁡φ-144a25a4a-3+48-72+3aaaa2357-600φ2-1904+1392-2304+6912cos⁡φ.The comparison of the numerical solution of (123) and its truncated power series solution (129), as well as their difference, is presented in Figures 3 and 4, respectively.Figure 3
Comparison of the numerical solutionU(φ) of (123), describing the motion of a photon in Schwarzschild geometry (solid curve), and of its truncated power series solution obtained by the Laplace-Adomian Decomposition Method, given by (129) (dashed curve), for a=0.015.Figure 4
The absolute differenceΔ(φ) between the numerical solution U(φ)num and the truncated Laplace-Adomian power series solution U(φ)LADM of Eq. (123) for a=0.015.


### 5.2. The Bending Angle of Light

In the Newtonian approximation the solution of the equation of motion for the light has the solutionu0=cos⁡φ/R, where R is the distance of the closest approach to the massive object. This fixes the value of the constant a as a=3GM/c2R. In the first order of approximation the equation of motion of the photon in the Schwarzschild geometry has the solution(130)uφ≈u0φ+u1φ=1Rcos⁡φ+2GMc2R2sin2φ22+cos⁡φ.By taking φ=π/2+ϵ, we have(131)uφ≈GM2c2R221-c2RGMsin⁡ϵ+cos⁡2ϵ+3.By performing a first-order series expansion with respect toϵ and taking u=0, it gives the light deflection angle as(132)δ=2ϵ≈4GMc2R1-GM/c2R,which in the limit GM/c2R<<1 reduces to the well-known general relativistic result δ≈4GM/c2R [13]. In the second order of approximation, with u≈u0+u1+u2, we obtain by using the same procedure(133)δ=4GMc2R1+15π/16-2GM/c2R1-GM/c2R-5G2M2/2c4R2.Finally, in the fourth order of approximation we obtain(134)δ≈4GMc2R1+15π-3216GMc2R-59π-3216G2M2c4R2+5669π-2048256G3M3c6R31-GMc2R-5G2M22c4R2-15π-224G3M3c6R3-4816-1920π+225π2128G4M4c8R4-1.


## 5.1. Power Series Solution

The truncated power series solution of (123) can be obtained immediately by using the Laplace-Adomian Decomposition Method as(124)U0φ=acos⁡φ,(125)U1φ=23a2sin2φ2cos⁡φ+2,(126)U2φ=1144a360φsin⁡φ+29cos⁡φ+16cos⁡2φ+3cos⁡3φ-48,(127)U3φ=-1432a4180φsin⁡φ+60φsin⁡2φ+119cos⁡φ+96cos⁡2φ+9cos⁡3φ+cos⁡4φ-225,(128)U4φ=a5207367071-1800φ2cos⁡φ+6144cos⁡2φ+1116cos⁡3φ+64cos⁡4φ+512232φsin⁡φ+64φsin⁡2φ+9φsin⁡3φ-240+cos⁡5φ,(129)Uφ=a20736a3842a8a2-6a+3-9cos⁡2φ+a36a31a-12+12×cos⁡3φ+a164a-3cos⁡4φ+5acos⁡5φ+60φsin⁡φ18acos⁡2φ+324a-3cos⁡φ+241a-144+8640φsin⁡φ-144a25a4a-3+48-72+3aaaa2357-600φ2-1904+1392-2304+6912cos⁡φ.The comparison of the numerical solution of (123) and its truncated power series solution (129), as well as their difference, is presented in Figures 3 and 4, respectively.Figure 3
Comparison of the numerical solutionU(φ) of (123), describing the motion of a photon in Schwarzschild geometry (solid curve), and of its truncated power series solution obtained by the Laplace-Adomian Decomposition Method, given by (129) (dashed curve), for a=0.015.Figure 4
The absolute differenceΔ(φ) between the numerical solution U(φ)num and the truncated Laplace-Adomian power series solution U(φ)LADM of Eq. (123) for a=0.015.


## 5.2. The Bending Angle of Light

In the Newtonian approximation the solution of the equation of motion for the light has the solutionu0=cos⁡φ/R, where R is the distance of the closest approach to the massive object. This fixes the value of the constant a as a=3GM/c2R. In the first order of approximation the equation of motion of the photon in the Schwarzschild geometry has the solution(130)uφ≈u0φ+u1φ=1Rcos⁡φ+2GMc2R2sin2φ22+cos⁡φ.By taking φ=π/2+ϵ, we have(131)uφ≈GM2c2R221-c2RGMsin⁡ϵ+cos⁡2ϵ+3.By performing a first-order series expansion with respect toϵ and taking u=0, it gives the light deflection angle as(132)δ=2ϵ≈4GMc2R1-GM/c2R,which in the limit GM/c2R<<1 reduces to the well-known general relativistic result δ≈4GM/c2R [13]. In the second order of approximation, with u≈u0+u1+u2, we obtain by using the same procedure(133)δ=4GMc2R1+15π/16-2GM/c2R1-GM/c2R-5G2M2/2c4R2.Finally, in the fourth order of approximation we obtain(134)δ≈4GMc2R1+15π-3216GMc2R-59π-3216G2M2c4R2+5669π-2048256G3M3c6R31-GMc2R-5G2M22c4R2-15π-224G3M3c6R3-4816-1920π+225π2128G4M4c8R4-1.


## 6. Discussions and Concluding Remarks

In the present paper we have considered the applications of a very powerful mathematical method, the Laplace-Adomian Decomposition Method, for the study of the general relativistic equations describing the motions of massive test particles and of photons in spherical symmetric geometries. The Laplace-Adomian Decomposition Method has been used extensively for obtaining power series solutions of many classes of nonlinear differential and integral equations. High precision solutions can be obtained by using only a few terms in a truncated series expansion. The main “ingredients” in this method are the Adomian polynomials, which are generated recursively in a step by step procedure.After introducing a general formalism for obtaining the equations of motion in arbitrary spherically symmetric spacetimes and after the presentation of the Laplace-Adomian Decomposition Method, we have investigated in detail the dynamics of massive particles and photons in Schwarzschild geometry. The equations of motion can be easily solved, and we have obtained their series solution. One of the advantages of the Laplace-Adomian Decomposition Method is that it gives directly the solution in terms of trigonometric functions, and not in terms of power series of the angular variableφ. A truncated series solution containing only five terms can reproduce the exact numerical solution with a high precision. Moreover, obtaining the terms in the Adomian series can be easily done with the use of some symbolic software. The basic known physical results can be easily reobtained in the first-order approximation, as well as for higher orders. For the perihelion precession we have investigated only the first-order effects, but in the case of the deflection of light we have obtained the bending angle up to the fourth order of approximation.Several important generalizations and extensions of the Schwarzschild metric are known. In the case of the vacuum spacetimes outside charged spherically symmetric objects carrying a chargeQ the geometry is given by the Reissner-Nordstrom metric,(135)eν=e-λ=1-2GMc2r+Q2r2.In the case of rotating star the first-order monopole correction to the Schwarzschild metric is given by [49](136)eν=e-λ=1-2GMc2r+J2r4,where J=IΩ, where I is the moment of inertia, Ω is the angular velocity, and J is the angular momentum of the star, respectively.The physics of the particle motion in these metrics can be easily investigated by using the methods developed in the present paper. For example, in the case of the Reissner-Nordstrom metric,f(u)=2GMu/c2-Qu2,(137)Gu=2GMc2u3-Qu4+E2c2L2-1L2+2GMuc2L2-Qu2L2,and(138)Fu=3GMc2u2-2Qu3+GMc2L2-QuL2,respectively. Therefore the equation of motion in the Reissner-Nordstrom metric is given by(139)d2udφ2+1+QL2u=3GMc2u2-2Qu3+GMc2L2,which can be solved easily by using the Laplace-Adomian Decomposition Method. The radius of the circular orbit u0 can be obtained as a solution of the algebraic equation(140)3GMc2u02-u0+GMc2L2=2Qu03+QL2u0.In the first order of approximation and with the assumption Q/L2≪1, u0 can be approximated as u0≈GM/c2L2. Thus, the perihelion precession Δϕ in the Reissner-Nordstrom geometry is given by(141)Δϕ=6πGMc2a1-e2-πc2QGMa1-e2.The first term in (141) gives the general relativistic correction term for the perihelion precession, while the second term gives the correction due to the presence of the charge. In the case of the motion of massless particles we have (142)Pu=2GMc2u3-Qu4+E2c2L2,and(143)Vu=3GMc2u2-2Qu3.Hence the equation of motion for photons is given by(144)d2udϕ2+u=3GMc2u2-2Qu3.This equation can also be investigated easily by using the Laplace-Adomian Decomposition Method, and the methods are developed in the present paper.



---

*Source: 7093592-2018-06-28.xml*