J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 Journal of the Nigerian Society of Physical Sciences Original Research Comparison of the Solution of the Van der Pol Equation Using the Modified Adomian Decomposition Method and Truncated Taylor Series Method J. N. Ndama,∗, O. Adedireb aDepartment of Mathematics, University of Jos, Nigeria bFederal College of Forestry, Jos, Nigeria Abstract In this paper, we compare the solution of the van der Pol equation obtained by using truncated Taylor series method and modified Adomian decomposition method with the solution obtained by the Poincare-Lindstedt (P-L) method. The approximating 4-component modified Adomian decomposition method behaves more like approximate P-L analytic method than tenth-order Taylor series. Also, with addition of just one term, the approximating 5-component modified Adomian decomposition method produces more convergent solution to that of P-L analytic method than the twenty second-order Taylor series approximation as the independent variable t representing time progressively increases. A general comparison of the two solutions revealed that the absolute errors generated by the approximating polynomial from Taylor series are greater than the ones generated from modified Adomian decomposition method. It was further revealed that very few components of the modified Adomian decomposition could yield a series of about 3 times the order of the one obtained by using the Taylor series method. Hence, we recommend inclusion of the modified Adomian Decomposition Method in modern mathematical tools. Keywords: van der Pol oscillator, modified Adomian decomposition method, Taylor series method Article History : Received: 01 January 2020 Received in revised form: 04 March 2020 Accepted for publication: 09 March 2020 Published: 14 May 2020 c©2020 Journal of the Nigerian Society of Physical Sciences. All rights reserved. Communicated by: T. Latunde 1. Introduction The van der Pol equation, otherwise called the van der Pol oscillator is a model which describes the behaviour of electri- cal circuits [1-19]. It was formulated by an electrical engineer and physicist, Balthasar van der Pol, and no exact solution has been obtained for the second order differential equation since then [12]. However, asymptotic and numerical solutions have been obtained. One important method of obtaining approximate analytic solutions to differential equations is the Taylor series ∗Corresponding author tel. no: 07031950138 Email address: ndamj@unijos.edu.ng (J. N. Ndam) method. However, arguments have been advanced against it because the method has the problem of producing tedious com- putational work in determining the coefficient an which can be observed in deriving recurrence relation, particularly when the product of two or more infinite series are involved [17]. Recently, another form of series solution procedure for solv- ing both linear and nonlinear differential equations has been developed. This procedure, which generates series solutions to differential equations that converge very rapidly, is called Adomian decomposition method, named after the author [18]. The Adomian decomposition method has been used to obtain approximate analytic solutions to a wide range of differential equations, including linear and nonlinear equations, and has 106 J. N. Ndam & O. Adedire / J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 107 been shown to converge more rapidly with proofs of conver- gence than other forms of series solutions ([1, 4, 13, 17, 19]). There have been various modifications to this procedure which have significantly improved the convergence of the method ([1- 3, 5, 8, 9, 14, 18]). Wazwaz [17] compared the solutions of a linear and nonlinear differential equations using the Taylor se- ries and the Adomian decomposition methods and concluded that the solutions obtained by the decomposition method con- verged much more rapidly than the Taylor series procedure. However, consideration of Wazwaz [17] of the two series meth- ods was in the sense of ease of getting Adomian decomposition series compared with that of Taylor series but did not consider error analysis of the series methods as his focus was on infinite series solution. In this work, we attempt approximate solutions of the van der Pol equation using few orders of Taylor series and few com- ponents of the modified Adomian decomposition procedures with emphasis on finite series solution. The aim is to com- pare the solution of the Van der Pol equation using the Modified Adomian Decomposition Method (MADM) and the Truncated Taylor Series Method (TTSM) with approximate Poincare-Lindstedt (P-L) technique of pertubation method. The specific objec- tives are to investigate the behaviour of the errors generated from both methods for parameter values of van der Pol equa- tion and to graphically display them. These objectives of the study will be achieved by comparing the solution of the van der Pol equation from finite orders of TTSM and finite components of MADM with that of approximate P-L analytic method. The choice of P-L analytic method for comparison in this study is based on its suitability and accuracy for the solution of nonlin- ear differential equations ([6], [11], [16]). The remaining parts of this paper are organised as follows: the second section considers the Taylor series solution, while section 3 is dedicated to the modified Adomian decomposition procedure. Results and discussion comes up in section 4. Fi- nally, conclusion will be the subject of section 5. 2. Taylor series method The van der Pol oscillator is given by u′′(t) + µ(u2(t) − 1)u′(t) + u(t) = 0 (1) with the initial conditions u(0) = 1, u′(0) = 0, where µ � 1 or µ � 1. The Taylor series solution about t = 0 is obtained as u(t) = u(0)+tu′(0)+ t2 2! u′′(0)+ t3 3! u′′′(0)+· · ·+ tn n! u(n)(0)(2) Equation (1) can be written as u′′(t) = µ(1 − u2(t))u′(t) − u(t) u′′′(t) = −2µu(t)u′2(t) + µ(1 − u2(t))u′′(t) − u′(t) uiv(t) = −2µu′3 − 6µu(t)u′(t)u′′(t) − 8µu(t)u′(t)u′′′(t) +µ(1 − u(t)2)uiv(t) − u′′′(t) ... Computing the derivatives as far as possible and evaluating at t = 0, we obtain u(0) = 1 u′(0) = 0 u(2)(0) = −1 u(3)(0) = 0 u(4)(0) = 1 u(5)(0) = −6µ u(6)(0) = −1 u(7)(0) = 66µ u(8)(0) = −252µ2 + 1 u(9)(0) = −612µ u(10)(0) = 11052µ2 − 1 u(11)(0) = 5532µ− 31752µ3 u(12)(0) = −341316µ2 + 1 u(13)(0) = −49818µ + 3714552µ3 u(14)(0) = 6259644µ2 − 8845200µ4 − 1 u(15)(0) = 448398µ− 274369248µ3 u(16)(0) = 2211091344µ4 − 241794792µ2 + 1 u(17)(0) = −4604457312µ5 + 15983461728µ3 − 4035384µ ... Using the above values, tenth-order Taylor series yields the fol- lowing approximating polynomial: u(t) ≈ 1 − t2 2! + t4 4! − 6µ t5 5! + t6 6! + 66µ t7 7! −(252µ2 + 1) t8 8! − 612µ t9 9! + (11052µ2 − 1) t10 10! (3) Analogously, twenty second-order Taylor series yields the fol- lowing approximating polynomial: u(t) ≈ 1 − t2 2! + t4 4! − 6µ t5 5! + t6 6! + 66µ t7 7! −(252µ2 + 1) t8 8! − 612µ t9 9! + (11052µ2 − 1) t10 10! +(5532µ− 31752µ3) t11 11! − (341316µ2 − 1) t12 12! −(49818µ− 3714552µ3) t13 13! + ( 6259644µ2 − 8845200µ4 −1) t14 14! + (448398µ− 274369248µ3) t15 15! 107 J. N. Ndam & O. Adedire / J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 108 +(2211091344µ4 − 241794792µ2 + 1) t16 16! +(−4604457312µ5 + 15983461728µ3 − 4035384µ) t17 17! +(−325537770960µ4 + −325537770960µ2 − 1) t18 18! +(2136491544096µ5 − 924215233680µ3 +36320664µ) t19 19! + (−4018487578560µ6 +37614999137616µ4 − 154525796280µ2 + 1) t20 20! +(−557011268077248µ5 + 48491799847920µ3 −326886030µ) t21 21! + (3140584492618944µ6 −3803681636636400µ4 + 3873523538520µ2 − 1) t22 22! (4) 3. Adomian decomposition method In this section, we follow the approach used by El-Kalla [4] in which he considered kth order nonlinear ordinary differential equation whose nonlinear term has Adomian polynomial repre- sentation. This approach is similar to that of Wazwaz [17]. The Adomian decomposition procedure seeks a solution of the form u(t) = ∞∑ n=0 un(t) (5) Equation (1) can be expressed in operator form as Ltu = µ(1 − u 2)u′ − u (6) where Lt = d2 dt2 , and hence the solution of (1) can be expressed as u(t) = u(0) + L−1t { µ(u′ − u2u′) − u } (7) where L−1t = ∫ ∫ (.)dtdt is the two-fold integration operator. Initial condition of eq. (1) given by u(0) = 1 is denoted by u0 = 1 so that from eq. (5), the components that make up the series becomes u0 = 1 (8) un+1(t) = ∫ t 0 ∫ t 0 { µ((un(s)) ′ − An(s)) − un(s) } d sd s (9) where An = 1 n! dn dλn N  ∞∑ i=0 uiλ i   λ=0 = u2n(un) ′ n = 0, 1, 2, · · · and λ is a parameter, are called the Adomian polynomials. However, we shall use the modified procedure for generating the Adomian polynomials as discussed below: Suppose the nonlinear term in (1) is denoted by N(u), then we decompose it as A0 = N(u0) A1 = N(u0 + u1) − A0 A2 = N(u0 + u1 + u2) − (A0 + A1) A3 = N(u0 + u1 + u2 + u3) − (A0 + A1 + A2) ... Thus we obtain the relation for generating the Adomian poly- nomials as An = N  n∑ i=0 ui  − n−1∑ i=0 Ai (10) From the initial value problem (1), N(u) = u2u′ and hence by using (10), the Adomian polynomials are obtained as A0 = u 2 0u ′ 0 A1 = (u0 + u1) 2(u′0 + u ′ 1) − u 2 0u ′ 0 = u20u ′ 1 + 2u0u1u ′ 0 + 2u0u1u ′ 1 + u 2 1u ′ 0 + u 2 1u ′ 1 A2 = (u0 + u1 + u2) 2(u′0 + u ′ 1 + u ′ 2) − (A0 + A1) = u20u ′ 2 + 2u0u1u ′ 2 + 2u0u2u ′ 0 + 2u0u2u ′ 1 +2u0u2u ′ 2 + 2u1u2u ′ 0 + 2u1u2u ′ 1 + 2u1u2u ′ 2 +u22u ′ 1 + u 2 2u ′ 2 + u 2 1u ′ 2 + u 2 2u ′ 0 Accordingly, we obtain u0 = 1 u1 = − t2 2 u2 = µ 168 t7 − µ 20 t5 + t4 4! u3 = − µ4 312947712 t22+ µ4 11289600 t20− µ4 1299600 t18− µ3 12870144 t19 + µ3 399840 t17− µ 240 ( 5µ 32256 − µ3 1600 ) t16− 617µ3 21168000 t15+ 3µ2 125440 t14 − µ 156 ( − 31µ2 1400 + 1 3456 ) t13− 41µ2 120960 t12− µ 110 ( − 5 576 + µ2 40 ) t11 + 19µ2 8400 t10 − 13µ 9072 t9 − µ2 160 t8 + µ 140 t7 − t6 6! ... The series solution for 4-components Adomian decomposition then becomes u(t) = u0 + u1 + u2 + u3 (11) which gives u(t) = 1 − t2 2 + µ 168 t7 − µ 20 t5 + t4 4! − µ4 312947712 t22 + µ4 11289600 t20 108 J. N. Ndam & O. Adedire / J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 109 − µ4 1299600 t18 − µ3 12870144 t19 + µ3 399840 t17 − µ 240 ( 5µ 32256 − µ3 1600 ) t16 − 617µ3 21168000 t15 + 3µ2 125440 t14 − µ 156 ( − 31µ2 1400 + 1 3456 ) t13 − 41µ2 120960 t12 − µ 110 ( − 5 576 + µ2 40 ) t11 + 19µ2 8400 t10 − 13µ 9072 t9 − µ2 160 t8 + µ 140 t7 − t6 6! (12) and the series solution for 5-component Adomian decomposi- tion then gives u(t) = u0 + u1 + u2 + u3 + u4 (13) We simplify (13) and choose µ to be arbitrarily small because approximate P-L method to be used for comparison can work effectively on differential equations of the form u′′ + w20u = µF(t, u, u ′), 0 < µ << 1 whose leading order is oscillatory with frequency w0 [10]. So eq.(13) using µ = 2 x 10−2 gives u(t) = 1 − 1/2 t2 + 0.0002619047619 t7 −0.001000000000 t5 − t6 720 + 1/24 t4 +1.329776164 × 10−50 t67 − 1.139867761 × 10−48 t65 +5.077535944 × 10−47 t64 + 4.357672234 × 10−47 t63 −4.592882729 × 10−45 t62 + 8.593423967 × 10−44 t61 −8.269818866 × 10−42 t59 − 1.845062816 × 10−13 t23 +1.866090683 × 10−43 t60 + 8.309132396 × 10−41 t58 +3.568516858 × 10−40 t57 − 8.714821000 × 10−39 t56 +4.811105327 × 10−38 t55 + 4.002845402 × 10−37 t54 −5.886109478 × 10−36 t53 + 1.424302351 × 10−35 t52 +2.902430667 × 10−34 t51 − 2.595201219 × 10−33 t50 −1.043753849 × 10−33 t49 + 1.407479522 × 10−31 t48 −7.032084376 × 10−31 t47 − 3.022579830 × 10−30 t46 +4.519343938 × 10−29 t45 − 8.093847241 × 10−29 t44 −1.418636597 × 10−27 t43 + 8.881156719 × 10−27 t42 +1.617446587 × 10−26 t41 − 3.583414553 × 10−25 t40 +7.270414170 × 10−25 t39 + 8.689959175 × 10−24 t38 −4.931206414 × 10−23 t37 − 1.020281668 × 10−22 t36 +1.652954579 × 10−21 t35 − 1.931801068 × 10−21 t34 −3.707212443 × 10−20 t33 + 1.482428707 × 10−19 t32 +5.109844495 × 10−19 t31 − 4.724699345 × 10−18 t30 +2.377613320 × 10−19 t29 + 1.016506228 × 10−16 t28 −2.556365546 × 10−16 t27 − 1.543943729 × 10−15 t26 +8.696083800 × 10−15 t25 + 1.257108256 × 10−14 t24 +1.611259824 × 10−13 t22 − 9.420685884 × 10−12 t20 +0.00002230158730 t8 + 2.799612144 × 10−12 t21 −3.001809900 × 10−11 t19 + 0.0000000002238474420 t18 +0.0000000001499617141 t17 − 0.000000003494172630 t16 +0.000000003368584091 t15 + 0.00000003773049937 t14 −0.0000001349519000 t13 − 0.0000002741969964 t12 +0.000002674211961 t11 + 0.000001218253968 t10 −0.00003373015873 t9 (14) 4. Results and Discussion Here, comparison of results from the approximating poly- nomials (3), (4), (12) and (14) with approximate P-L method for the solution of (1) and its initial conditions are shown in Tables 1, 2, 3 and 4. The choice of approximate P-L method is because exact solution has not been obtained for the Van der Pol equation. Details of the approximate method used here can be obtained from ([10], [15]) and references contained therein. It should be noted that the initial conditions used in this research suggest that the periodic behaviour exhibits the amplitude 1 as against the amplitude 2 used in [15]. Absolute errors (ABS Er- rors) are also obtained and are tabulated as follows: Figure 1: Graphical solutions using P-L method and approximating polynomi- als (3) and (12) of TTSM and 4-component MADM. While the graphs of solutions of equation (1) and its initial conditions using approximate P-L analytic method and approx- imating polynomials (3), (4), (12) and (14) are shown in Figures 109 J. N. Ndam & O. Adedire / J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 110 Table 1: Solutions and errors from (3) and (12) of TTSM series and 4-component MADM t Poincare-Lindstedt TTSM MADM ABS Error TTSM ABS Error MADM 0 1.000000000000 1.000000000000 1.000000000000 0.000000000000 0.000000000000 0.3 0.955854836508 0.955334115584 0.955334114058 0.000520720924 0.000520722450 0.6 0.828944246584 0.825264812016 0.825264451699 0.003679434568 0.003679794885 0.9 0.631240082296 0.621131026775 0.621122751091 0.010109055521 0.010117331205 1.2 0.378578337374 0.360630621478 0.360560141843 0.017947715896 0.018018195531 1.5 0.0906245246636 0.066327550293 0.065998964725 0.024296974371 0.024625559939 1.8 -0.20868615269 -0.23659702897 -0.23752492072 0.027910876280 0.028838768030 2.1 -0.491935529476 -0.52423546969 -0.52542008925 0.032299940214 0.033484559774 2.4 -0.731188366817 -0.78110829991 -0.77773900461 0.049919933093 0.046550637793 2.7 -0.902481373797 -1.01309133371 -0.98448885633 0.110609959913 0.082007482533 3.0 -0.989925613354 -1.26963992857 -1.15176986217 0.279714315216 0.161844248816 Table 2: Solutions and errors from (4) and (12) of TTSM and 4-component MADM t Poincare-Lindstedt TTSM MADM ABS Error TTSM ABS Error MADM 0 1.000000000000 1.000000000000 1.000000000000 0.000000000000 0.000000000000 0.3 0.955854836508 0.955334115588 0.955334114058 0.000520720920 0.000520722450 0.6 0.828944246584 0.825264821264 0.825264451699 0.003679425320 0.003679794885 0.9 0.631240082296 0.621131785279 0.621122751091 0.010108297017 0.010117331205 1.2 0.378578337374 0.360647539325 0.360560141843 0.017930798049 0.018018195531 1.5 0.0906245246636 0.066512092157 0.065998964725 0.024112432507 0.024625559939 1.8 -0.20868615269 -0.23531787096 -0.23752492072 0.026631718270 0.028838768030 2.1 -0.491935529476 -0.51774914669 -0.52542008925 0.025813617214 0.033484559774 2.4 -0.731188366817 -0.75481924354 -0.77773900461 0.023630876723 0.046550637793 2.7 -0.902481373797 -0.9216069535 -0.98448885633 0.019125579703 0.082007482533 3.0 -0.989925613354 -0.97208752941 -1.15176986217 0.017838083944 0.161844248816 Table 3: Solutions and errors from (3) and (14) of TTSM and 5-component MADM t Poincare-Lindstedt TTSM MADM ABS Error TTSM ABS Error MADM 0 1.000000000000 1.000000000000 1.000000000000 0.000000000000 0.000000000000 0.3 0.955854836508 0.955334115584 0.95533411559 0.000520720924 0.000520720918 0.6 0.828944246584 0.825264812016 0.825264822641 0.003679434568 0.003679423943 0.9 0.631240082296 0.621131026775 0.621131859045 0.010109055521 0.010108223251 1.2 0.378578337374 0.360630621478 0.360648788531 0.017947715896 0.017929548843 1.5 0.0906245246636 0.066327550293 0.066523502482 0.024296974371 0.024101022182 1.8 -0.20868615269 -0.23659702897 -0.23524709918 0.027910876280 0.026560946490 2.1 -0.491935529476 -0.52423546969 -0.51742153967 0.032299940214 0.025486010194 2.4 -0.731188366817 -0.78110829991 -0.75375297048 0.049919933093 0.022564603663 2.7 -0.902481373797 -1.01309133371 -0.92099274209 0.110609959913 0.018511368293 3.0 -0.989925613354 -1.26963992857 -0.99998289494 0.279714315216 0.010057281586 110 J. N. Ndam & O. Adedire / J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 111 Table 4: Solutions and errors from (4) and (14) of TTSM and 5-component MADM t Poincare-Lindstedt TTSM MADM ABS Error TTSM ABS Error MADM 0 1.000000000000 1.000000000000 1.000000000000 0.000000000000 0.000000000000 0.3 0.955854836508 0.955334115588 0.95533411559 0.000520720920 0.000520720918 0.6 0.828944246584 0.825264821264 0.825264822641 0.003679425320 0.003679423943 0.9 0.631240082296 0.621131785279 0.621131859045 0.010108297017 0.010108223251 1.2 0.378578337374 0.360647539325 0.360648788531 0.017930798049 0.017929548843 1.5 0.0906245246636 0.066512092157 0.066523502482 0.024112432507 0.024101022182 1.8 -0.20868615269 -0.23531787096 -0.23524709918 0.026631718270 0.026560946490 2.1 -0.491935529476 -0.51774914669 -0.51742153967 0.025813617214 0.025486010194 2.4 -0.731188366817 -0.75481924354 -0.75375297048 0.023630876723 0.022564603663 2.7 -0.902481373797 -0.9216069535 -0.92099274209 0.019125579703 0.018511368293 3.0 -0.989925613354 -0.97208752941 -0.99998289494 0.017838083944 0.010057281586 1, 2, 3 and 4 for µ = 2 x 10−2, the graphs of absolute errors are also indicated in Figures 5, 6, 7 and 8. Figure 2: Graphical solutions using P-L method and approximating polynomi- als (4) and (12) of TTSM and 4-component MADM. Figure 3: Graphical solutions using P-L method and approximating polynomi- als (3) and (14) of TTSM and 5-component MADM. 111 J. N. Ndam & O. Adedire / J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 112 Figure 4: Graphical solutions using P-L method and approximating polynomi- als (4) and (14) of TTSM and 5-component MADM. Figure 5: Plot of absolute errors from approximating polynomials (3) and (12) of TTSM and 4-component MADM. Figure 6: Plot of absolute errors from approximating polynomials (4) and (12) of TTSM and 4-component MADM. Figure 7: Plot of absolute errors from approximating polynomials (3) and (14) of TTSM and 5-component MADM. 112 J. N. Ndam & O. Adedire / J. Nig. Soc. Phys. Sci. 2 (2020) 106–114 113 Figure 8: Plot of absolute errors from approximating polynomials (4) and (14) of TTSM and 5-component MADM. Equation (12) of approximating 4-component MADM be- have more like approximate P-L method than approximating polynomial (3) of TTSM as shown in Figure 1 and Table 1. On the other hand, many more terms are added to approximating TTSM (3) to obtain TTSM (4). Solutions from approximating polynomial (4) of TTSM with twenty-two order behave more closely as that of approximate P-L method than equation (12) of approximating 4-component MADM series and are indicated in Figure 2 and Table 2. However, results from 5-component MADM (14) shown in Tables 3 and 4 are similar to the results obtained from approximate P-L method than those got from TTSM of equations (3) and (4) as shown in Figures 3 and 4. The absolute errors generated by the approximating polynomial (3) are greater than the ones generated by (12) as shown in Fig- ure 5. Absolute errors obtained from TTSM (4) are generally lower than the ones generated from (12) of MADM due to many numbers of terms added to TTSM (3) as shown in Figure 6. Fi- nally, Figures 7 and 8 indicate that (14) shows smaller absolute errors compared with (3) and (4). At t=3 from Table 1, equation (3) gives u(3) ≈ −1.27 and (12) gives u(3) ≈ −1.15. Observe that with several terms added to (3) to obtain (4), the solution obtained in Table 4 at t = 3 for (4) is not as close to that of approximate P-L method as (14) which is obtained from (12) by addition of just one component. The theoretical implication of this is that few components from MADM - obtained with ease - can produce solution that is more convergent to that of P- L method than for tediuos computational work of many terms added to TTSM. 5. Conclusion In this paper, approximate series solutions of the van der Pol equation have been obtained using the truncated Taylor series method and the modified Adomian decomposition method. The two series solutions for Van der Pol equation obtained as ap- proximating polynomials (3), (4), (12) and (14) agree. From the approximating polynomials of the two series solutions, MADM produces more convergent results to the P-L methods than the TTSM, as only very few components of the MADM yielded a series much longer than order twenty of Taylor series. Hence, we recommend inclusion of the MADM in modern mathemati- cal tools. References [1] E. A. Az-Zobi, K. Al-Khaled & A. Darweesh, “Numeric-Analytic so- lutions for nonlinear oscillators via the modified multi-stage decom- position method.”, MDPI Journal of Mathematics 7 (2019) 1. doi: 10.3390/math7060550 [2] J. Biazar and Y. Shafiof, “A simple algorithm for calculating Adomian polynomials. Int. J. Contemp. Math. Sciences, 975 - 982. [3] Y. 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