J. Nig. Soc. Phys. Sci. 2 (2020) 160–165 Journal of the Nigerian Society of Physical Sciences Original Research L-Stable Block Hybrid Numerical Algorithm for First-Order Ordinary Differential Equations B. I. Akinnukawea,∗, K. O. Mukab aDepartment of Mathematics, University of Lagos, Lagos, Nigeria bDepartment of Mathematics, University of Benin, Benin City, Nigeria Abstract In this work, a one-step L-stable Block Hybrid Multistep Method (BHMM) of order five was developed. The method is constructed for solving first order Ordinary Differential Equations with given initial conditions. Interpolation and collocation techniques, with power series as a basis function, are employed for the derivation of the continuous form of the hybrid methods. The discrete scheme and its second derivative are derived by evaluating at the specific grid and off-grid points to form the main and additional methods respectively. Both hybrid methods generated are composed in matrix form and implemented as a block method. The stability and convergence properties of BHMM are discussed and presented. The numerical results of BHMM have proven its efficiency when compared to some existing methods. Keywords: Second derivative, Stability, Hybrid, Block, Collocation techniques Article History : Received: 25 May 2020 Received in revised form: 13 July 2020 Accepted for publication: 14 July 2020 Published: 01 August 2020 c©2020 Journal of the Nigerian Society of Physical Sciences. All rights reserved. Communicated by: J. O. Kuboye 1. Introduction Most Mathematical models are formulated using ordinary differential equations (ODEs) of different orders. However, math- ematical models that are based on ODEs of order one occur in several engineering, applied sciences and economics problems. Some of the ODEs have been proven not to have closed-form solution hence the need to develop numerical methods to pro- vide approximate solutions to these problems. Consider the Ini- tial Value Problem of the form y′(x) = f (x, y(x)), y(x0) = y0 (1) Several numerical methods have been developed by different researchers to circumvent the Dahlquist’s order Barrier Theo- ∗Corresponding author tel. no: Email address: iakinnukawe@unilag.edu.ng (B. I. Akinnukawe ) rem [1 - 3] that restricted the use of Linear Multistep Methods of high order to solve (1). Some of such researchers are En- right [4], Akinnukawe and Okunuga [5], Gear [6], Okunuga [7], Cash [8], Akinfenwa et al. [9, 10], Adesanya et al. [11], Ijezie and Muka [12] to mention but a few. High-order A-stable and L-stable numerical methods are developed by incorporat- ing off-step points, additional stages and/or employing higher differentiation of the solution (Lambert [13]). Generating ap- proximate solutions for (1) in instances where they exist and are unique but are insoluble via analytical means is very impor- tant because they help in the analysis and validation of models in which they evolve from. One of the objectives of developing numerical schemes for solving (1) is to obtain methods with wider stability regions, better convergent rates and computa- tional efficiency. 160 Akinnukawe & Muka / J. Nig. Soc. Phys. Sci. 2 (2020) 160–165 161 In this article, Continuous Hybrid Methods (CHM) of order 5 are derived through Interpolation and Collocation techniques (Onumanyi et al. [14]). Both methods incorporate off-step points and the second derivative of the scheme to produce the discrete hybrid methods which are composed and implemented as a block method (BHMM) to simultaneously produce approx- imation at nodal points. The BHMM has the advantage of being self-starting, L-stable in nature and possesses high accuracy be- cause it implemented as a block method (see [15], [16], [17]). 2. Derivation of BHMM This section describes the derivation of the k-step hybrid multistep method of the form: k∑ j=0 α jyn+ j + αvyn+v = h k∑ j=0 β j fn+ j + hβv fn+v + h 2φkgn+k (2) where h, n and v = 12 are the step size, grid index and off- step point respectively while α j, β j, j = 0, 1, ..., k and φk are parameters to be determined uniquely. An approximate solution to (1) by the interpolating function y(x) = 4k+1∑ j=0 b jt j (3) where b j, j = 0, 1, ..., 4k + 1 are the unknown coefficients. Im- posing condition for the construction of the proposed class of methods are: y(xn+r ) = yn+r, r = 0, v (4) y′(xn+r ) = fn+r, r = 0(v)k (5) y′′(xn+r ) = gn+r, r = k (6) Equations (4) - (6) will lead to a system of 4k + 2 equations. These equations are solved simultaneously to obtain b j and the values of b j are substituted into (3) to form the Continuous Hy- brid Method (CHM) expressed in the form y(t) = k−v∑ j=0 α j(t)yn+ j + h k∑ j=0 β j(t) fn+ j + h 2φk(t)gn+k (7) where t = x−xn+k−1h and α j(t), β j(t), j = 0(v)k and φk(t) are the continuous coefficients. The main scheme is generated evaluat- ing CHM (7) at xn+k while the additional scheme is generated from the second derivative of (7) at xn+v of the form h2y′′(t) = k−v∑ j=0 α j(t)yn+ j + h k∑ j=0 β j(t) fn+ j + h 2φk(t)gn+k (8) The developed main and additional schemes are combined and implemented simultaneously as a block hybrid multistep method BHMM for the numerical integration of IVPs (1). Following the steps discussed above, the Continuous Hy- brid Method (CHM) for k=1 is equation (7) y(t) = k−v∑ j=0 α j(t)yn+ j + h k∑ j=0 β j(t) fn+ j + h 2φk(t)gn+k where  α0(t) α 1 2 (t)  = [ 1 0 −48023 128023 −120023 384230 0 48023 −128023 120023 −38423 ]  t0 t1 t2 t3 t4 t5  (1)  β0(t) β 1 2 (t) β1(t)  =  0 1 −13123 265 23 − 224 23 68 23 0 0 −12823 464 23 − 504 23 176 23 0 0 1923 − 89 23 128 23 − 52 23   t0 t1 t2 t3 t4 t5  (2) [ φ1(t) ] = [ 0 0 − 746 17 23 − 26 23 12 23 ]  t0 t1 t2 t3 t4 t5  (3) Interpolating (7) at x = xn+1 to generate the main method at k = 1 becomes yn+1 = 7yn 23 + 16yn+ 12 23 + h fn 23 + 8h fn+ 12 23 + 6h fn+1 23 − h2gn+1 46 (9a) Equation (8), the second derivative of (7) for k = 1 is h2y′′(t) = k−v∑ j=0 α j(t)yn+ j + h k∑ j=0 β j(t) fn+ j + h 2φk(t)gn+k where  α0(t) α 1 2 (t)  = [ −96023 768023 −1440023 768023960 23 − 7680 23 14400 23 − 7680 23 ] t0 t1 t2 t3 (4)  β0(t) β 1 2 (t) β1(t)  =  − 262 23 1590 23 − 2688 23 1360 23 − 256 23 2784 23 − 6048 23 3520 23 38 23 − 534 23 1536 23 − 1040 23   t0 t1 t2 t3  (5) 161 Akinnukawe & Muka / J. Nig. Soc. Phys. Sci. 2 (2020) 160–165 162 [ φ1(t) ] = [ − 7 23 102 23 − 312 23 240 23 ]  t0 t1 t2 t3  (6) Interpolating(8) at x = xn+1/2, the additional method at k = 1 becomes h2gn+ 12 = 240yn 23 − 240yn+ 12 23 + 31h fn 23 + 64h fn+ 12 23 + 25h fn+1 23 − 4h2gn+1 23 (9b) The discrete hybrid methods (9a)and (9b) together forms the One-step Block Hybrid Multistep Methods (BHMM). The BHMM can be presented in a matrix block form as A(1)Y$ = A(0)Y$−1 + hB(1) F$ + hB(0) F$−1 + h 2C(1)G$ (10) where Y$ =  yn+ 12 yn+1  ; Y$−1 =  yn− 12 yn  ; F$ =  fn+ 12 fn+1  ; F$−1 =  fn− 12 fn  ; G$ =  gn+ 12 gn+1  The 2 by 2 matrices A(0), A(1), B(0), B(1), C(1), D(1) of the BHMM (9) are defined as follows A(1) =  240 23 0 − 16 23 1  A(0) =  0 24023 0 723  B(1) =  64 23 25 23 8 23 6 23  B(0) =  0 3123 0 123  C(1) =  −1 − 423 0 − 146  3. Analysis of BHMM 3.1. Order and Error Constant of the Method Following Lambert [13] and Fatunla [18], a method was proposed for finding the order p and error constant Wp+1 of the block method (9) by first expanding y−, f− and g− functions by Taylors series expansion about x and then comparing the coef- ficients of h. It is established from our calculation that one-step Block Hybrid Multistep Method have order and error constants as p = (5, 5)T and Wp+1 = ( 13 44160 , 1 66240 ) T respectively where T is transpose. 3.2. Zero Stability A numerical method is said to be zero-stable if the roots R j, j = 1, 2, . . . , N of the first characteristic polynomial ρ(R) satisfies |R j| ≤ 1, j = 1, . . . , N and those roots with |R j| = 1 is simple (see Lambert [3]). Applying the above conditions to the derived block method, the first characteristic polynomial ρ(R) = 0 is calculated as ρ(R) = det(RA(1) − A(0)) = 240 23 R(R − 1) The BHMM is found to be zero-stable since ρ(R) = 0 satisfies |R j| ≤ 1, j = 1, 2. 3.3. Convergence According to Henrici [19], a numerical method converges if it is consistent and zero-stable. Since BHMM (9) is of order 5 > 1, then it is consistent and we have established earlier that the method satisfies the conditions of zero-stability. Therefore, the block method (9) converges. 3.4. Stability of BHMM Applying the BHMM to the test equation y′ = λy, λ ≤ 0 we obtain Y$ = Q(z)Y$−1, z = λh where Q(z) is the amplification matrix given by Q(z) = A(0) + zB(0) A(1) + zB(1) + z2C(1) Q(z) has eigenvalues (ζ1,ζ2) = (0,ζ2). The dominant eigen- value ζ2 is the stability function with real coefficient as ζ2 = 10.4348 + 4.17391z + 0.652174z2 + 0.0434783z3 10.4348 − 6.26087z + 1.69565z2 − 0.26087z3 + 0.0217391z4 The stability function is used to plot the Region of Absolute Stability (RAS) of the BHMM (see Figure 1). The proposed method BHMM is L-stable since the RAS covers the entire left plane of the graph (A-stable) and the limit of the stability func- tion ζ2 is zero as z →∞ 162 Akinnukawe & Muka / J. Nig. Soc. Phys. Sci. 2 (2020) 160–165 163 Figure 1. Stability Region of Block Hybrid Multistep Method 4. Numerical Results The following problems are considered to examine the ac- curacy and computational efficiency of BHMM. The computa- tions were carried out using MATHEMATICA 9.0 software. The following acronyms are used: • ES = Exact Solution • ESDMM = Error in SDMM • EMHIRK = Error in MHIRK • EBHMM = Error in BHMM • EESDMM = Error in ESDMM Problem 1: Consider the non-linear IVP y′1 = λy1 + y 2 2 y′2 = −y2 with initial conditions y1(0) = −1 λ + 2 , y2(0) = 1, where λ = 104 and the exact solution of the IVP is given as y1(x) = − e−2x λ + 2 , y2(x) = e −x In Table 1, the numerical results obtained using BHMM with h = 10−1 compares favourably with MHIRK method [10] with h = 10−1 and is superior to that of Hojjati et al. [20] that used h = 10−4. Problem 2: Consider the following stiff problem arising from Chemical Kinetic reactions in a Chemistry experiment. y′1 = −0.013y1 − 1000y1y2 − 2500y1y3 y′2 = −0.013y1 − 1000y1y2 y′3 = −2500y1y3 with initial conditions y1(0) = 0, y2(0) = 1, y3(0) = 1 The computed result at x = 2.0 from the new method BHMM is compared with those of Ismail and Ibrahim (ESDMM [21]), Hojjati et al. SDMM [20], Akinfenwa et al. MHIRK [10]. The step length h = 0.0125 was used for MHIRK method and the new method BHMM and compared with Ismail and Ibrahim ESDMM [21], Hojjati et al. SDMM [20] with step length h = 0.001. The result obtained with the new method BHMM is superior to others. See Table 2 for the numerical re- sults. Problem 3: Consider the non-linear system y′1 = −2y1 + y2 + 2sin x y′2 = 998y1 − 999y2 + 999(cos x − sin x) with initial conditions y1(0) = 2, y2(0) = 3, 0 ≤ x ≤ 10 with analytical solution given as y1(x) = 2e −x + sin x, y2(x) = 2e −x + cos x 163 Akinnukawe & Muka / J. Nig. Soc. Phys. Sci. 2 (2020) 160–165 164 Table 1. Comparison of Methods for Problem 1 x yi ES ESDMM[20] EMHIRK[10] EBHMM 3 y1 −0.2478257E − 06 2.478147E − 11 3.06450E − 15 5.00564E − 16 y2 0.4978707E − 01 2.471093E − 06 3.07825E − 10 5.02813E − 11 5 y1 −0.4539085E − 08 3.450217E − 14 9.35475E − 17 1.52787E − 17 y2 0.6737946E − 02 2.304573E − 08 6.94326E − 11 1.13414E − 11 10 y1 0.3059023E − 12 3.456372E − 18 8.49412E − 21 3.75372E − 20 y2 0.3059023E − 04 3.150734E − 10 9.35666E − 13 1.52836E − 13 Table 2. Comparison of Methods for Problem 2 x yi ES EESDMM[21] ESDMM[20] EMHIRK[10] EBHMM 2.0 y1 −0.361693316929E − 5 0.82E − 10 0.52E − 13 0.110E − 13 2.919E − 15 y2 0.9815029948230 0.61E − 05 0.19E − 08 0.220E − 08 5.586E − 10 y3 1.018493388244 0.57E − 05 0.63E − 08 0.220E − 08 5.584E − 10 The numerical results of Problem 3 are shown in Table 3 using step size h = 10−3. The derived method integrated the problem efficiently that the numerical solution is close to that of the analytical solution. Table 3. The absolute error for Problem 3 x yi ES EBHMM 0.25 y1 1.805005525 4.50751E − 14 y2 2.526513988 4.84057E − 14 0.5 y1 1.692486858 9.85878E − 14 y2 2.090643881 9.81437E − 14 1.0 y1 1.577229867 9.45910E − 14 y2 1.276061188 9.54792E − 14 2.0 y1 1.179967993 1.68310E − 13 y2 −0.145476270 1.68365E − 13 4.0 y1 −0.720171217 2.21378E − 13 y2 −0.617012343 2.23044E − 13 6.0 y1 −0.274457993 1.01363E − 13 y2 0.9651277911 1.01474E − 13 8.0 y1 0.9900291719 1.93401E − 13 y2 −0.1448291085 1.94650E − 13 10.0 y1 −0.5439303110 6.10623E − 13 y2 −0.8389807292 6.09068E − 13 Problem 4: A stiff system of Initial Value Problems y′1 = −8y1 + 7y2 y′2 = 42y1 − 43y2 with initial conditions y1(0) = 1, y2(0) = 8, x ∈ [0, 15] with exact solution given as y1(x) = 2e −x − e−50x, y2(x) = 2e −x + 6 − e−50x Problem 4 was integrated using the step size of h = 10−4 to aid in comparing with other methods in literature as shown in Tables 4 and 5. It is discovered that BHMM has better accuracy than the others compared with. Table 4. Numerical Results for Problem 4 x yi ES EBHMM 3 y1 9.9574136 × 10−2 2.68577 × 10−13 y2 9.9574136 × 10−2 2.65843 × 10−13 6 y1 4.9575044 × 10−3 1.68580 × 10−14 y2 4.9575044 × 10−3 1.80611 × 10−14 9 y1 2.4681961 × 10−4 7.57646 × 10−15 y2 2.4681961 × 10−4 5.43191 × 10−15 12 y1 1.2288424 × 10−5 2.10193 × 10−15 y2 1.2288424 × 10−5 2.54783 × 10−15 15 y1 6.1180464 × 10−7 2.29273 × 10−14 y2 6.1180464 × 10−7 1.87085 × 10−14 5. Conclusion A one-step L-stable Block Hybrid Multistep Method (BHMM) of order five was developed via interpolation and collocation techniques. The BHMM has the advantage of being self-starting and its L-stable in nature as displayed in figure 1. The block method possesses high accuracy as shown in Tables (1) − (5) where it was compared with some existing methods. The method satisfies the zero-stability, consistency and convergence condi- tions. BHMM has proved efficient for solving first-order Initial Value Problems. 164 Akinnukawe & Muka / J. Nig. Soc. Phys. 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