J. Nig. Soc. Phys. Sci. 5 (2023) 1075 Journal of the Nigerian Society of Physical Sciences Collocation Method for the Numerical Solution of Multi-Order Fractional Differential Equations G. Ajileyea, A. A. Jamesb,∗ aDepartment of Mathematics and Statistics, Federal University Wukari, Taraba State, Nigeria. b Department of Mathematics and Statistics, American University of Nigeria, Yola, Adamawa State, Nigeria. Abstract This study presents a collocation approach for the numerical integration of multi-order fractional differential equations with initial conditions in the Caputo sense. The problem was transformed from its integral form into a system of linear algebraic equations. Using matrix inversion, the algebraic equations are solved and their solutions are substituted into the approximate equation to give the numerical results. The effectiveness and precision of the method were illustrated with the use of numerical examples. DOI:10.46481/jnsps.2023.1075 Keywords: Keywords: Differential equation, Fractional derivatives, Approximate solution, Power series. Article History : Received: 17 September 2022 Received in revised form: 24 May 2023 Accepted for publication: 25 May 2023 Published: 11 June 2023 © 2023 The Author(s). Published by the Nigerian Society of Physical Sciences under the terms of the Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Communicated by: Tolulope Latunde 1. Introduction In the fields of mathematics, physics, chemistry, and engineering, differential and integral equations involving fractions are of the utmost significance. The use of functional equations, such as ordinary and partial differential equations, is typical when applying mathematics to the modeling of problems arising in the real world. In the early 1900s, Italian mathematician Vito Volterra came up with a whole new sort of equation that came to be known as integro-differential equations in order to investigate the phenomenon of population expansion. In these types of equations, one or more derivatives of the function whose value is unknown is placed under the integral sign. Integro-differential equations can be found in a ∗Corresponding author tel. no: +2348077092831 Email address: adewalejames974@gmail.com (A. A. James ) variety of mathematical formulations of physical phenomena. Additionally, these equations can be found in the modeling of certain phenomena in the fields of science and engineering. For instance, the equations of kinetics that support the kinetic theory of rarefied gases, plasma, radiation transmission, and coagulation are some examples. [1]. Some of the numerical solution of fractional differential equations developed in the literature include: Perturbed collocation method [2], Adomian decompositions method by [3-5], Collocation method by [6-9], Chebyshev- Gelerkin method [10], Bernoulli matrix method [11], Differential transform method [12], Pseudospectral method [13], Bernstein Polynomials method [14, 15], the Mellin transform approach [16]. [17] utilized a numerical approach based on the boubaker polynomial to generate approximate numerical solutions to the multi-order fractional differential equations. Their decision was to use an operational matrix for fractional integration based on boubakar polynomi- 1 Ajileye & James / J. Nig. Soc. Phys. Sci. 5 (2023) 1075 2 als. Collocation approach for the computational solution of fredholm-volterra fractional order of integro-differential equa- tions was presented by [18]. They solved the problem by first obtaining the linear integral form of it and then transforming it into a system of linear algebraic equations by making use of conventional collocation points. In this research, the collocation method is utilized to solve multi-order fractional differential equations of the form Dβy(x) = N∑ j=0 q j(x)D α j y(x) + h(x) (1) subject to the initial condition y( j)(a j) = λ j, j = 0, 1, ..., n − 1, n ∈ N β > αN, (2) where y(x) is the unknown function, Dα j and Dβ are the Ca- puto’s derivative, h(x) is the force known -prior. q j(x) is the known function, a j and λ j are known constants. 2. Basic Definitions In this section, we present certain definitions and fundamental ideas of fractional calculus for the purpose of the formulation of the problem that has been presented. Definition 2.1: The Caputo derivative with order α > 0 of the given function f (x), x ∈ (a, b) is defined as C x D α a y(x) = 1 Γ(m −α) ∫ x a (x − s)m−α−1y(m)(s)d s (3) where m − 1 ≤ α ≤ m, m ∈ N, x > 0 Definition 2.2: Let (an) , n ≥ 0 be a sequence of real numbers. The power series in x with coefficients an is an expression y(x) = a0+a1 x+a2 x 2 +a3 x 3 +· · ·aN x N = N∑ n=0 an x n = φ(x) A(4) where φ(x) = [1 x x2 · · · xN ], A = [a0 a1 · · · aN ]T then y(x, n) = xnA, n = 0(1)N, n ∈ Z+ Definition 2.3: Standard Collocation Method (SCM). This method is used to determine the desired collocation points within an interval. i.e [a,b] and is given by xi = a + (b − a)i N , i = 1, 2, 3, ...N (5) Definition 2.4: Let y(x) be a continuous function, then 0 I β x ( C 0 D β xy(x) ) = y(x) − N∑ k=0 y(k)(0) k! xk (6) where m − 1 < β < 1 Definition 2.5: Let p(s) be an integrable function, then 0 I β x ( p(s)) = 1 Γ(β) ∫ x 0 (x − s)β−1 p(s)d s (7) Definition 2.6: The Riemann -Liouville derivative of order α > 0 with n − 1 < α < n of the power function f (t) = t p−α is given by Dαt p = Γ( p + 1) Γ( p −α + 1) t p−α (8) 3. Mathematical Background In this part, we create a collocation approach for numeri- cally solving multi-order fractional differential equations utiliz- ing power series polynomials as the basis function. Lemma (3.1) (Integral form) Let y(x) be a solution to (1) subject to (2), the integral form is y(x) = W(x) + N∑ j=0 1 Γ(m j −α j) 1 Γ(β) (9) × ∫ x 0 (x − s)β−1q j(s) [∫ s 0 (s − t)m j−α j−1y(m j )(t)dt ] d s where W(x) = N∑ k=0 y(k)(0) k! xk + 1 Γ(β) ∫ x 0 (x − s)β−1h(s)d s Proof. Multiply equation (1) by 0 I β x (.) gives 0 I β x ( Dβy(x) ) = 0 I β x  N∑ j=0 q j(x)D α j y(x) + 0 Iβx (h(x)) (10) using (6) on equation (10) gives y(x) = N∑ k=0 y(k)(0) k! xk + 0 I β x  N∑ j=0 q j(x)D α j y(x)  (11) applying equations (3) and (7) to equation (11) gives y(x) = N∑ k=0 y(k)(0) k! xk + 1 Γ(β) ∫ x 0 (x − s)β−1 (12) ×  N∑ j=0 q j(x) 1 Γ(m j −α j) ∫ s 0 (s − t)m j−α j−1 y(m j )(t)dt  d s Substituting equation (4) into equation (12) gives y(x) = N∑ k=0 y(k)(0) k! xk + 1 Γ(β) ∫ x 0 (x − s)β−1 (13) ×  N∑ j=0 q j(x) 1 Γ(m j −α j) ∫ s 0 (s − t)m j−α j−1 dm j dtm j (φ(t)) dtA  d s 3.1. Method of Solution Collocating at xi in equation (13) gives y(xi) = W(xi) + N∑ j=0 1 Γ(m j −α j) 1 Γ(β) ∫ xi 0 (xi − s) β−1q j(s) × (∫ s 0 (s − t)m j−α j−1 dm j dtm j (φ(t)) dt ) d s A (14) where W(xi) = N∑ k=0 y(k)(0) k! xk + 1 Γ(β) ∫ x 0 (x − s)β−1h(s)d s 2 Ajileye & James / J. Nig. Soc. Phys. Sci. 5 (2023) 1075 3 Simplifying equation (14) gives φ(xi)A = W(xi) +  ∑N j=0 1 Γ(m j −α j) 1 Γ(β) ∫ xi 0 (xi − s)β−1q j(s) × (∫ s 0 (s − t)m j−α j−1 dm j dtm j (φ(t)) dt ) d s  A (15) Factorizing the values of A from equation (15) gives φ(xi)− ∑N j=0 1 Γ(m j −α j) 1 Γ(β) ∫ xi 0 (xi − s)β−1q j(s) × (∫ s 0 (s − t)m j−α j−1 dm j dtm j (φ(t)) dt ) d s  A = W(xi) (16) Equation (16) can be in the form V (xi)A =W(xi) (17) where V (xi) = φ(xi)− N∑ j=0 1 Γ(m j −α j) 1 Γ(β) ∫ xi 0 (xi − s) β−1q j(s) (∫ s 0 (s − t)m j−α j−1 dm j dtm j (φ(t)) dt ) d s (18) and A = [a0 a1 · · · aN ]T Multiplying both sides of equation (17) by V−1(xi) gives A =V−1(xi)W(xi) (19) Lemma (3.2) Let y(x) be approximated by (11) and let L(x) = 0 I β x  N∑ j=0 q j(x)D α j y(x)  (20) If q j(s) = sp j, then L(x; n) = Γ(n + 1)Γ(n −α j + p j + 1) Γ(n −α j + 1)Γ(β + n −α j + p j + 1) × x β+n−α j +p j i A (21) Proof. Applying equation (3) and (7) into equation (20) gives 0 I β x  N∑ j=0 q j(x)D α j y(x)  = N∑ j=0 1 Γ(m j −α j) 1 Γ(β) ∫ xi 0 (x − s)β−1q j(s) [∫ s 0 (s − t)m j−α j−1y(m j )(t)dt ] d s (22) Substituting (8) into (22) gives = N∑ j=0 1 Γ(m j −α j) 1 Γ(β) ∫ x 0 (x − s)β−1S p j (23) [∫ s 0 (s − t)m j−α j−1 ( Γ(n + 1) Γ(n − m j + 1) tn−m j ) dt ] d s A Let s − t = (1 − v)s, then t = vs =⇒ dt dv = s =⇒ dt = sdv, substituting into (23) gives = N∑ j=0 Γ(n + 1) Γ(m j −α j)Γ(n − m j + 1) 1 Γ(β) ∫ x 0 (x − s)β−1S p j (24) [ S n−α j ∫ 1 0 (1 − v)m j−α j−1V n−m j dt ] d s A Simplifying (24), we get L(x; n) = Γ(n + 1)Γ(n −α j + p j + 1) Γ(n −α j + 1)Γ(β + n −α j + p j + 1) xβ+n−α j +p j A(25) Lemma (3.3) Let y(t) be approximated by (9), let C(x) = 0 I β x (h(x)) (26) if h(s) = sm, then C(x) = Γ(m + 1) Γ(β + m + 1) xβ+m Proof. Applying equation (7) into (26) gives 0 I β x (h(x)) = 1 Γ(β) ∫ x 0 (x − s)β−1h(s) d s Substituting for h(s) gives = 1 Γ(β) ∫ x 0 (x − s)β−1 smd s Let x − s = (1 − u)x, s = ux =⇒ d s du = x =⇒ d s = xdu. C(x) = Γ(m + 1) Γ(β + m + 1) xβ+m (27) Lemma (3.4) Let y(x) be the solution of (1) and (2) then the numerical result gives y(x) = φ(xi)V −1(xi) W(xi) (28) where V (xi) = Γ(n + 1)Γ(n −α j + p j + 1) Γ(n −α j + 1)Γ(β + n −α j + p j + 1) x β+n−α j +p j i and W(xi) = − N∑ k=0 y(k)(0) k! xki + Γ(m + 1) Γ(β + m + 1) xβ+mi 3 Ajileye & James / J. Nig. Soc. Phys. Sci. 5 (2023) 1075 4 Proof. Approximate solution of equation (17) is y(x) = φ(x) A From equation (19) A =V−1(xi) W(xi) where V (xi) = Γ(n + 1)Γ(n −α j + p j + 1) Γ(n −α j + 1)Γ(β + n −α j + p j + 1) x β+n−α j +p j i + br+n+1Γ(r + 1) (σ + n + 1) Γ(β + r + 1) xβ+r + Γ(r + σ + n + 2) (σ + n + 1) Γ(β + r + σ + n + 2) xβ+r+σ+n+1 Substituting for A in the approximate solution gives the numer- ical result y(x) = φ(xi)V −1(xi) W(xi) 4. Convergence Analysis In this section, we establish the convergence of the method by substituting the approximate solution into equation (3.0) yN (x) = W(x) + N∑ j=0 1 Γ(m j −α j) 1 Γ(β) (29) × ∫ x 0 (x − s)β−1q j(s) [∫ s 0 (s − t)m j−α j−1y (m j ) N (t)dt ] d s Subtracting (9) from (29) gives EN (x) = yN (x) − y(x). Hence |EN (x)| ≤ 1 Γ(β) ∫ x 0 (x − s)β−1 N∑ j=0 1 Γ(m j −α j) q j(s)∣∣∣∣∣∣ [∫ s 0 (s − t)m j−α j−1 EN (t)dt ] d s ∣∣∣∣∣∣ Therefore ‖EN (xi)‖∞ ‖EN (t)‖∞ ≤ 1 Γ(β) ∫ xi 0 (x − s)β−1∣∣∣∣∣∣∣∣  N∑ j=0 1 Γ(m j −α j) q j(s) (∫ s 0 (s − t)m j−α j−1dt ) ∣∣∣∣∣∣∣∣ d s 5. Numerical Examples In this section, we considered two numerical examples to eval- uate the effectiveness and clarity of the method. A MAPLE 18 program is used to perform the computations. Let yn(x) and y(x) be the approximate and exact solutions respectively. ErrorN = |yn(x) − y(x)| . Example 5.1. [2] Consider multi-order Fractional differential equation . D1.5y(x) = −x−1 D0.5y(x) − x0.5y(x) + f (x) with this condition y ′ (0) = y(0) = 0 and exact solution y(x) = x3 − x2 f (x) = [ 6x ( Γ(3.5) + Γ(2.5) Γ(2.5)Γ(3.5) + x2 6 ) − 2 ( Γ(2.5) + Γ(1.5) Γ(1.5)Γ(2.5) + x2 2 )] x0.5 Solution 1. Comparing with equation (1.1) and Equation (1.2), β = 1.5,α = 0.5 Using N = 4 for illustration, and applying equation (6) gives y(x) = W(x) − 1 Γ(1 − 0.5) 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s−1 (30)[∫ s 0 (s − t)1−0.5−1 Γ(n + 1) Γ(n − 1 + 1) tn−1dt ] d s A − 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s0.5 (sn) d s A where W(x) = N∑ k=0 y(k)(0) k! xk + 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 f (s)d s Substituting (4) into equation (30) gives φ(x) A = W(x) − 1 Γ(1 − 0.5) 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s−1 (31)[∫ s 0 (s − t)1−0.5−1 Γ(n + 1) Γ(n − 1 + 1) tn−1dt ] d s A − 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s0.5 (sn) d s A where W(x) = N∑ k=0 y(k)(0) k! xk + 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 f (s)d s Equation (31) can be in the form τ(x)A =W(x) (32) where τ(x) = φ(x) + 1 Γ(1 − 0.5) 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s−1[∫ s 0 (s − t)1−0.5−1 Γ(n + 1) Γ(n − 1 + 1) tn−1dt ] d s + 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s0.5 (sn) d s 4 Ajileye & James / J. Nig. Soc. Phys. Sci. 5 (2023) 1075 5 collocating at x4 = [ 1 4 2 4 3 4 1 ] and substituting the initial conditions gives τ(xi) ∗A = f (xi)∗ (33) where τi (x) ∗ = 0.0000000000 0.0000000000 0.0000000000 0.0000000000 0.0000000000 0.0470157986 0.2239605405 0.1797773130 0.0750069286 0.0274673600 0.1880631945 0.5184447788 0.7543711011 0.6176863534 0.4482932221 0.4231421877 0.9539764130 1.8295669120 2.1838653930 2.3438648990 0.7522527781 1.6010791410 3.5816739880 5.5056804110 7.7368811360 0 1 0 0 0 1 0 0 0 0  f (x)∗ = [0.0000000000 − 0.1047703844 − 0.1366847478 0.3542984804 1.9240064220] We now solve for the unknown values A making use of matrix inversion results in equation (33): y4 = ( 1.365574320288940 × 10−14 − 2.546407529280260 × 10−12 x −0.999999999275360x2 + 0.999999998413614x3 + 6.644427230639850 × 10−10 x4 ) Example 5.2. [2] Consider multi-order Fractional differential equation . D1.5y(x) + 1 x D0.5y(x) + x 1 2 y(x) = + f (x) with this condition y ′ (0) = y(0) = 0 and exact solution y(x) = −x3 + x2 f (x) = [ 2 ( Γ(2.5) + Γ(1.5) Γ(1.5)Γ(2.5) + x2 2 ) − 6x ( Γ(3.5) + Γ(2.5) Γ(2.5)Γ(3.5) + x2 9 )] x 1 2 Solution 2. Comparing with equation (1.1) and Equation (1.2), β = 1.5,α = 0.5 Using N = 4 for illustration, Applying equation (6) gives y(x) = W(x) − 1 Γ(1 − 0.5) 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s−1(34)[∫ s 0 (s − t)1−0.5−1 Γ(n + 1) Γ(n − 1 + 1) tn−1dt ] d s A − 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s0.5 (sn) d s A Table 1: Exact, approximate and absolute error values for Example 1 x Exact Our methodN=4 errorN=4 error [2]=4 0.0 0.00000000000 1.36557432000e-14 1.3656e-14 5.9232e-13 0.1 -0.900000000e-2 -0.89999999950e-2 5.0000e-12 2.6668e-10 0.2 -0.320000000e-1 -0.31999999980e-1 2.0000e-11 9.6994e-10 0.3 -0.630000000e-1 0.06299999997000 3.0000e-11 1.9604e-09 0.4 -0.960000000e-1 -0.95999999980e-1 2.0000e-11 3.0781e-09 0.5 -0.12500000000 -0.1250000000000 0.0000000 4.1532e-09 0.6 -0.14400000000 -0.1439999999000 1.0000e-10 5.0056e-09 0.7 -0.14700000000 -0.1470000000000 0.0000000 5.4456e-09 0.8 -0.12800000000 -0.1280000001000 1.0000e-10 5.2733e-09 0.9 -0.810000000e-1 -0.81000000160e-1 1.6000e–10 4.2787e-09 1.0 0.00000000000 -2.3555727690e-10 2.3555e-10 2.2421e-09 where W(x) = N∑ k=0 y(k)(0) k! xk + 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 f (s)d s Substituting (4) into equation (34) gives φ(x) A = W(x) − 1 Γ(1 − 0.5) 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s−1(35)[∫ s 0 (s − t)1−0.5−1 Γ(n + 1) Γ(n − 1 + 1) tn−1dt ] d s A − 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s0.5 (sn) d s A where W(x) = N∑ k=0 y(k)(0) k! xk + 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 f (s)d s Equation (35) can be in the form τ(x)A =W(x) (36) where τ(x) = φ(x) + 1 Γ(1 − 0.5) 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s−1[∫ s 0 (s − t)1−0.5−1 Γ(n + 1) Γ(n − 1 + 1) tn−1dt ] d s + 1 Γ(1.5) ∫ x 0 (x − s)1.5−1 s0.5 (sn) d s Collocating at x4 = [ 1 4 2 4 3 4 1 ] and substituting the initial conditions gives τ(xi) ∗A = f (xi)∗ (37) where τi (x) ∗ = 0.5000000000 2.3817583340 1.9118819450 0.7976779168 0.2921077680 0.7071067812 1.9493225120 2.8363918970 2.3224651180 1.6855566990 0.8660254038 1.9524590850 3.7444893700 4.4696155660 4.7970791030 1.0000000000 2.1283791670 4.7612638900 7.3189233350 10.2849485700 1 0 0 0 0 0‘ 1 0 0 0  f (x)∗ = [ 1.1142040280 0.5139267798 -0.7251261978 -2.5576594460 0 0 ] We now solve for the unknown values A making use of matrix inversion results in equation (37); y4 = ( 1.428190898877800 × 10−12 − 2.777014174171200 × 10−10 x +1.000000001121990x2 − 1.000000001716120x3 + 6.387779194483300 × 10−10 x4 ) 5 Ajileye & James / J. Nig. Soc. Phys. Sci. 5 (2023) 1075 6 Table 2: Exact, approximate and absolute error values for Example 2 x Exact Our methodN=4 errorN=4 error [2]=4 0.0 0.00000000000 1.428190899000e-12 1.4281908990000e-12 3.7782e-12 0.1 0.00900000000 0.00899999998200 1.8000000000000e-11 2.4706e-09 0.2 0.03200000000 0.03199999997000 3.0000000000000e-11 1.4306e-08 0.3 0.06300000000 0.06299999997000 3.0000000000000e-11 3.9585e-08 0.4 0.09600000000 0.09599999999000 1.0000000000000e-11 78988e-08 0.5 0.12500000000 0.12499999990000 1.0000000000000e-10 1.2980e-07 0.6 0.14400000000 0.14399999990000 1.0000000000000e-10 1.8590e-07 0.7 0.14700000000 0.14699999980000 2.0000000000000e-10 2.3778e-07 0.8 0.12800000000 0.12799999970000 3.0000000000000e-10 2.7253e-07 0.9 0.08100000000 0.08099999952000 4.8000000000000e-10 2.7386e-07 1.0 0.00000000000 -3.6122208060e-10 3.6122208060000e-10 2.2207e-07 6. Discussion of Results In this section, we discuss the numerical results obtained by applying the derived numerical method to the solved examples. We observed from the result obtained for example 1 as shown in Table 1 that the approximate solution at N=4 gives y4(x) = 1.365574320288940 × 10−14 − 2.546407529280260 × 10−12 x+ 1.000000001121990x2 − 1.000000001716120x3 + 6.387779194483300 × 10−10 x4. The numerical result al- most converges to the exact solution and produces extremely small errors. This demonstrated that our method outperformed the proposed method by Uwaheren et al (2020). The results of the numerical example 2 in Table 2 shows the approximate solution at N=4 as y4(x) = 1.428190898877800 × 10−12 − 2.777014174171200 × 10−10 x + 1.0000001121990x2 − 1.0000001716120x3 + 6.387779194483300 × 10−10 x4. The nu- merical result converge to the exact solution and give better re- sult than the method proposed by Uwaheren et al (2020) at the same value of N. 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