New Nonlinear Four-Step Method for

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1 Matematika, 003, Jilid 19, bil. 1, hlm c Jabatan Matematik, UTM. New Nonlinear Four-Step Method for y = ft, y) Nazeeruddin Yaacob & Phang Chang Department of Mathematics University Teknologi Malaysia Skudai, Johor, Malaysia Abstract In this paper, a study is made on the possibility of developing a nonlinear four-step method based on contraharmonic mean. The study is done since the four-step methods always give higher order than popular methods like Numerov and classical Runge-Kutta methods. A detailed study of consistency, stability, convergence and interval of periodicity has been done to convince ourselves of using this new method. The numerical results shows that the method is more accurate than the existing one. Keywords Contraharmonic mean, interval of periodicity, second order initial value problem. Abstrak Dalam kertas kerja ini, satu kajian telah dijalankan tentang kemungkinan untuk membangunkan satu kaedah empat-langkah taklinear berdasarkan min kontraharmonik. Kajian ini telah dijalankan kerana kaedah empat-langkah selalunya memberikan peringkat yang lebih tinggi daripada kaedah terkenal seperti kaedah Numerov dan kaedah Runge-Kutta klasik. Kajian yang terperinci tentang kekonsistenan, kestabilan, penumpuan dan selang berkala telah meyakinkan kita penggunaan kaedah baru ini. Keputusan berangka menunjukkan keputusan yang lebih jitu daripada kaedah yang sedia ada. Katakunci Min kontraharmonik, selang berkala, masalah nilai awal peringkat kedua. 1 Introduction For years, much work has been devoted to developing a better formulae to solve second order initial value problems of the type y = ft, y) with solution which is periodic or oscillating with known frequency. In recent years, multistep methods for this problem have

2 48 Nazeeruddin Yaacob & Phang Chang been particularly been of concern. It is so because single step methods are inefficient since they do not make full use of the available information. It seems plausible that more accuracy can be obtained if the value of y n1 is made to depend not only on y n, but also on y n 1 and y n.the same is true for the more recent method proposed by Simos [4] and Ixaru and Rizea [3]. The new nonlinear method is developed here based on some important hypotheses by Lambert and Watson [] and Sommeijer, Van der Houwen and Neta [1]. Development of Nonlinear Four-Step Method based on Contraharmonic Mean To develop a nonlinear four-step method, we use the formula of a linear multistep method as 1) and make a slightly modification to suit nonlinear four-step method as ). k a j y nj = h k c j ψ j f), k, 1) k 15 a j y nj = h c j ψ j f), k =4, ) where a j and c j are constants and are the function formed by taking nonlinear combination of f at some fixed discrete points of special second order IVP y = ft, y) which is characterized by the polynomials ρ and σ, where k ρζ) = a j ξ j, 3) σζ) = k c j ξ j. 4) Throughout this paper, we refer to the work of Lambert and Watson [] for linear multistep methods as a guide. We shall assume that of nonlinear multistep methods satisfy the following hypotheses: i) a k =1, a 0 a 5 =0, 15 j=1 c j =0. ii) ρ and σ have no common factors. iii) ρ1) = ρ 1) = 0, ρ 1) = σ1) ; this is necessary and sufficient for the method to be consistent, that is, to have order at least one. iv) The method ρ, σ) is zero-stable; that is, all the roots of ρ lie in or on the unit circle, and those on the unit circle having multiplicity not greater than two.

3 New Nonlinear Four-Step Method for y = ft, y) 49 If we intend to have the four-step formula to be based on contraharmonic mean, then we have to form the nonlinear combination as in 5) a j y nj = h c j ψ j 5) where j=1 ψ 1 = f n4,ψ = f n3, ψ 3 = f n,ψ 4 = f n1, ψ 5 = f n, ψ 6 = f n4 f n3 f n4 f n3,ψ 7 = f n3 f n f n3 f n,ψ 8 = f n f n1 f n f n1, ψ 9 = f n1 f n f n1 f n, ψ 10 = f n4 f n f n4 f n, ψ 11 = f n3 f n1 f n3 f n1, ψ 1 = f n4 f n1 f n4 f n1,ψ 13 = f n4 f n f n4 f n,ψ 14 = f n3 f n f n3 f n, ψ 15 = f n f n f n f n. To fulfill the hypothesis i) and properties of symmetric methods by Sommeijer, Van der Houwen and Neta [1], we shall let the left hand side of equation 5) as We may then write 5) as y n4 q 1 y n3 q y n1 y n. y n4 q 1 y n3 q y n1 y n = h c n4 c f n3 c 3 f n c 4 f n1 c 5 f n fn4 c f n3 6 f n4 fn3 c fn3 f n 7 f n3 fn c fn f n1 8 f n fn1 fn1 fn c 9 f n1 fn c fn4 fn 10 f n4 fn c fn3 fn1 11 f n3 fn1 fn4 fn1 c 1 f n4 fn1 c fn4 fn 13 f n4 fn c fn3 fn 14 f n3 fn c 15 f n f n f n fn 6) Using Taylor s series we expand both sides of equation 5), and compare each coefficient. We obtain the system of equations as follows:

4 50 Nazeeruddin Yaacob & Phang Chang Using MATHEMATICA, we get the solution for system of equations in free parameter q as follows: c 1 = q 480, c = q 0, c 3 = q 40 c 4 = q 0, c 5 = q 480, c 6 = 1 c 9 = ,c 10 = ,c 11 = 50000, c 7 = ,c 8 = , ,c 1 = ,c 13 = c 14 = ,c 15 = , and q 1 = 1 q , Thus, we obtain a new method for solving y = ft, y). We call it a nonlinear 4-step method based on contraharmonic mean. We abreviate it as 4-step CoM method ). y n4 1 q )y n3 q y n 1 q )y n1 y n = h q 480 )f n q 0 )f n q 40 )f n q 0 )f n q 480 )f n ) n4 fn3 ) f n4 f n ) n3 fn ) 4 f n3 f n ) n fn1 ) f n f n ) n1 fn ) 19 f f n1 f n ) n4 fn ) f n4 f n ) n3 fn1 ) f n3 f n1 7 f ) n4 fn1 ) f n4 f n ) n4 fn ) f n4 f n ) n3 fn ) f n3 f n ) n fn ) ) 7) f n f n

5 New Nonlinear Four-Step Method for y = ft, y) 51 3 Determination of Interval of Periodicity for 4-step CoM Method The first and the second characteristic polynomial for 4-step CoM method may be written as follows ρζ) =ζ 4 1 q ) ζ 3 q ζ 1 q ) ζ 1 8) σζ) = q ) ζ q ) ζ q ) ζ q ) ζ q ) ) 1 ζ 8 ζ 6 ) ζ 4 ζ 3 ) 1 ζ 6 ζ 4 ) ) 41 ζ 4 ζ ) ) 1 ζ ) ζ 3 ζ ζ ζ 6500 ζ 1 ) 19 ζ 8 ) ) 1 1 ζ 6 ζ ) ) 7 ζ 8 ζ ) ζ ζ 3 ζ ζ 4 ζ ) 1 ζ 8 ζ 4 ) ) 1 ζ 6 ) ) 1 1 ζ 4 ) ζ 4 ζ ζ ζ 9) 1 Next, we consider the equation φζ; H )=ρζ)h σζ). 10) The objective is to find H0 such that ζ j = 1 where ζ j are zeroes of the equation φζ; H )= 0, and H 0,H0 ). This is done by fixing some values of H which will give values of φ = 0 in the equation 10). We do this until the required H0 is found. We present some of the values obtained in Table 1 using q = where the method is consistent). From the Table 1, it is observed that the value of H0 is approximately This is true for H 0, 0.55), we have ζ j = 1. Hence, the interval of periodicity for 4-step CoM method is 0,0.55). 4 Numerical Results We shall test the new method using two different values of free parameter for two different problems. We compare our results to those of Method V developed by Lambert and Watson [] given as follows: y n4 q)y n3 q)y n q)y n1 y n [ ) ) ) 18 q 6 3q 14 97q = h f n4 f n3 f n ) ) ] 6 3q 18 q f n1 f n )

6 5 Nazeeruddin Yaacob & Phang Chang Table 1: Determination of H 0 H ζ j ζ j ± i ± i ± i ± i ± i ±0.4794i ± i ± i ± i ± i ± i ± i ± i ± i ± i ± i ± i ± i 1.

7 New Nonlinear Four-Step Method for y = ft, y) 53 Problem 1 Consider the problem of solving y = 4y, y0) = 0, y 0) = where the exact solution of this problem is yt) = sint). Using the 4-step CoM formula as in 7), we solve Problem 1 using q =1/1000 with step size h =0.1. The absolute errors for the numerical solution near the zeroes of yt) are given in Table. Table : Absolute errors in Method V and 4-step CoM method using step size h =0.or Problem 1 t Exact solution Absolute errors Absolute errors yt) near zero Method V) 4-step CoM method) π e e e-06 4π e e e-07 16π e e e-06 5π e e e-06 3π e e e-07 36π e e e-06 40π e e e-06 50π e e e-06 Problem : Undamped Duffing problem We use 4-step CoM method with q = and Method V to find the numerical solution of the following initial value problem, which is the nonlinear undamped Duffing equation y y y 3 = B cos Ωt forced by a harmonic function where B = 0.00 and Ω = The exact solution computed by the Galerkin method with a precision 10 1 of the coefficients is given by where yt) =A 1 cos Ωt A 3 cos 3Ωt A 5 cos 5Ωt A 7 cos 7Ωt A 9 cos 9Ωt A 1 = , A 3 = , A 5 = , A 7 = , A 9 = This problem are also solved by U.Anantha Krishnaiah [6]). Table 3. We present the results in

8 54 Nazeeruddin Yaacob & Phang Chang Table 3: Absolute error in Method V and 4-step CoM method using step size h = π/10 for Problem t Exact solution Absolute errors Absolute errors near zero Method V) 4-step CoM method) 4π e e e-04 4.π e e e π e e e π e e e π e e e π e e e π e e e π e e e π e e e-05 5 Consistency The 4-step CoM method is said to be consistent if and only if ρ 1) = ρ1) = 0 and ρ 1) = σ1). Proof: ρζ) =ζ 4 1 q )ζ3 q ζ 1 q )ζ 1 = ρ1) = 1) 4 1 q )1)3 q 1) 1 q )1) 1 = 0 and ρ ζ) =4ζ 3 31 q )ζ q ζ 1 q ) ρ 1) = 41) 3 31 q )1) q 1) 1 q )=0. So ρ 1) = ρ1) = 0. Next, we have ρ ζ) =1ζ 61 q )ζ q = ρ 1) = 11) 61 q )1) q =4 q

9 New Nonlinear Four-Step Method for y = ft, y) 55 and σ1) = q ) ) q ) ) q ) 1) q ) ) q ) ) 1 1) 8 1) 6 ) ) 4 1) 3 ) 1 1) 6 1) 4 ) ) 41 1) 4 1) ) ) 1 1) ) ) 3 1) ) 1) ) 1 ) 19 1) 8 ) ) 1 1 1) 6 1) ) ) 7 1) 8 1) ) ) ) ) 4 1 ) 1 1) 8 1) 4 ) ) 1 1) 6 ) ) 1 1 1) 4 ) ) 4 1) ) ) ) 1 = q 1) Since ρ 1) = σ1), the 4-step CoM method is consistent. 6 Zero-stability The multistep method is said to be zero-stable if no root of the first characteristic polynomial ρζ) has modulus greater than one, and every root of modulus one has multiplicity not greater than two. Proof: Since ρζ) =ζ 4 ζ 3 ζ ζ 1=0, we get ζ = i, i, 1 and 1. Thus ζ =1. So i) No modulus of root > 1, and ii) Every root of modulus one has multiplicity exactly. Hence the 4-step CoM method is zero-stable. 7 Convergence The necessary and sufficient conditions for a multistep method to be convergent are that it be consistent and zero-stable. Proof: The nonlinear multistep method based on contraharmonic mean is shown to be i) consistent, and ii) zero-stable. Thus, the 4-step CoM method is convergent.

10 56 Nazeeruddin Yaacob & Phang Chang 8 Conclusion In this paper, we have pioneered in using nonlinear mean to develop the nonlinear 4-step method for y = ft, y). Finally, we shall determine the agreement of the hypotheses mentioned in Section 1. i) a k =1, a 0 c 5 =0, 15 j=1 c j =0 ii) ρ and σ have no common factors. iii) ρ1) = ρ 1) = 0,ρ 1) = σ1); This is a necessary and sufficient conditions for the method ρ, σ) to be consistent, that is, to have order at least one. iv) The method ρ, σ) is zero-stable; That is all the roots of ρ lie in or on the unit circle, and those on the unit circle having multiplicity not greater than two. Proof: i) a 4 =1, a 0 =1,c 5 = q 480. So a 0 c 5 = 0 and σ1) = ) ) ) 40 ) ) ) = 0 ii), iii) and iv) are verified in Sections 5 to 7. Furthermore, from the Table and 3, we notice that this new nonlinear four-step method is generally more accurate than the Method V. References [1] B. P. Sommeijer, P. J. Van der Houwen and B. Neta, Symmetric Linear Multistep Methods for Second Order Differential Equations with Periodic Solutions, Journal on Applied Numerical Mathematics, 1986), [] J. D. Lambert and I.A. Watson, Symmetric Multistep Methods for Periodic Initial Value Problems, J. Inst. Math Applics., ),

11 New Nonlinear Four-Step Method for y = ft, y) 57 [3] L.G. Ixaru and M. Rizea, Four Step Methods for y = fx, y), Journal of Computational and Applied Mathematics, ), [4] T.E. Simos, An Explicit Four-Step Phase-Fitted Method for the Numerical Integration of Second-Order Periodic Initial Value Problems, J. Comput. Appl. Math., ), [5] N.Yaacob, New Nonlinear Runge-Kutta Methods for Solving Initial Value Problems, PhD. Thesis, UTM, 1996). [6] U.Anantha Krishnaiah, Adaptive Methods for Periodic Initial Value Problem of Second Order Differential Equations, Journal of Computational and Applied Mathematics, 8 198),