IHJPAS. 36 (3) 2023 427 This work is licensed under a Creative Commons Attribution 4.0 International License Abstract The wavelets have many applications in engineering and the sciences, especially mathematics. Recently, in 2021, the wavelet Boubaker (WB) polynomials were used for the first time to study their properties and applications in detail. They were also utilized for solving the Lane-Emden equation. The aim of this paper is to show the truncated Wavelet Boubaker polynomials for solving variation problems. In this research, the direct method using wavelets Boubaker was presented for solving variational problems. The method reduces the problem into a set of linear algebraic equations. The fundamental idea of this method for solving variation problems is to convert the problem of a function into one that involves a finite number of variables. Different numerical examples were given to demonstrate the applicability and validity of this method using the Matlab program. Also, the results of this technique were compared with the exact solution, and graphs were added to these examples to test the convergence of Wavelet Boubaker polynomials using this method. Keywords: Boubaker wavelets, Calculus of variation problems, Nonlinear programming, Numerical methods. 1. Introduction Many problems arising in mathematical physics and geometry are connected with the calculus of variations, which is determined by finding the maximal and minimal functional functions. The functionals are defined by definite integrals, which include boundary conditions and appear in the mathematical formula, see [2]. Wavelet theory is an emerging field in mathematical research and is applied in a broad range of engineering disciplines. Wavelets are very successful in accurately solving numerical problems. [3] Utilized Legendre wavelets to solve variational problems. [4] Used the Haar wavelet to solve the same problems. [5] Applied direct restarted Pell to solve the problem of variation, then used the spectral method with Chebyshev wavelets in another paper to solve the calculus of variations doi.org/10.30526/36.3.3048 Article history: Received 28 September 2022, Accepted 13 March 2023, Published in July 2023. Ibn Al-Haitham Journal for Pure and Applied Sciences Journal homepage: jih.uobaghdad.edu.iq Direct Method for Variational Problems Using Boubaker Wavelets Eman Hassan Ouda Department of Applied Science, University of Technology- Iraq. Eman.H.Ouda@uotechnology.edu.iq https://creativecommons.org/licenses/by/4.0/ mailto:Eman.H.Ouda@uotechnology.edu.iq mailto:Eman.H.Ouda@uotechnology.edu.iq IHJPAS. 36 (3) 2023 428 [6]. [7-8] using a Spline polynomial with collocation method and [9] used a new technique to find the numerical solution for solving isoperimetric problems. Many researchers have utilized different procedures to solve calculus of variational problems. [10] Developed the new functions for solving the problems of variational, then [11] used a combination of many functions with Bernoulli polynomials. [12] Found the approximate solution for boundary value problems using the wavelet function. [13] Studied moving or fixed boundary Muntz wavelets for solving variation problems. This paper is arranged as follows: in Section 2, Orthogonal Boubaker polynomials and their properties with recurrence relations In Section 3, Boubaker wavelets and their properties In Section 4, the application of Boubaker wavelet polynomials for solving variational problems with some numerical examples has been presented. In Section 5, the convergence test for the introduced method has been studied, and at last the conclusion has been reached. 2. Orthogonal Boubaker polynomials and their properties: Boubaker polynomials haven't been orthogonal, so by applying the Gram-Schmit process to Boubaker polynomials, one can obtain orthogonal Boubaker polynomials, OBm(t). Several papers have been applied with different applications in physics, applied sciences, etc. (see [14], [15]). Orthogonal Boubaker polynomials (OB) of mth degree were presented on the interval [0, 1] by the following equation: [1] π‘‚π΅π‘š(𝑑) = (π‘š!)2 (2π‘š)! βˆ‘ (βˆ’1)π‘š+π‘˜ (π‘š+π‘˜)! (π‘šβˆ’π‘˜)!(π‘˜!)2 π‘‘π‘˜π‘šπ‘˜=0 … (1) A recursive relation of the orthogonal Boubaker polynomial on the interval [0, 1] has given as follows: π‘‚π΅π‘š+1(𝑑) = ((π‘š+1)!)2 (2(π‘š+1))! [ (2π‘š+1)(2π‘š)! (π‘š+1)(π‘š!)2 (2𝑑 βˆ’ 1)π‘‚π΅π‘š(𝑑) βˆ’ (π‘š) 2(π‘šβˆ’1)! (π‘š+1) ((π‘šβˆ’1)!)2 π‘‚π΅π‘šβˆ’1(𝑑)] , π‘š β‰₯2 with 𝑂𝐡0(𝑑) = 1 π‘Žπ‘›π‘‘ 𝑂𝐡1(𝑑) = 1 2 (2𝑑 βˆ’ 1). 3. Boubaker wavelet and their properties: The Boubaker wavelets can be defined as below (see [1]) π‘Šπ΅π‘›,π‘š(𝑑) = {√2π‘š + 1 2 π‘˜ 2 0 π‘œπ‘‘β„Žπ‘’π‘Ÿ 𝑀𝑖𝑠𝑒 (2π‘š)! (π‘š!)2 π‘‚π΅π‘š(2 π‘˜ 𝑑 βˆ’ 𝑛) , 𝑛 2π‘˜βˆ’1 ≀ 𝑑 < 𝑛+1 2π‘˜βˆ’1 … (2) where π‘Šπ΅π‘›π‘š(𝑑) = π‘Šπ΅(π‘š, 𝑛, 𝑑) as Boubaker wavelets so n =0,1, 2, …, 2 k-1 ,k is any positive integer , m =0,1,2…,M and t is normalized time . The first six π‘Šπ΅π‘›π‘š(𝑑) with k=1, are given as follows: IHJPAS. 36 (3) 2023 429 π‘Šπ΅0(𝑑) = 1, π‘Šπ΅1(𝑑) = √3 2 (4𝑑 βˆ’ 3), π‘Šπ΅2(𝑑) = √5 6 (24𝑑2 βˆ’ 36𝑑 + 13) , π‘Šπ΅3(𝑑) = √7 20 (160𝑑3 βˆ’ 360𝑑2 + 264𝑑 βˆ’ 63), π‘Šπ΅4(𝑑) = √9 70 (1120𝑑4 βˆ’ 3360𝑑3 + 3720𝑑2 βˆ’ 1800𝑑 + 321), π‘Šπ΅5(𝑑) = √11 252 (8064𝑑5 βˆ’ 30240𝑑4 + 44800𝑑3 βˆ’ 32760𝑑2 + 11820𝑑 βˆ’ 1683), π‘Šπ΅6(𝑑) = √13 924 (59136𝑑6 βˆ’ 266112𝑑5 + 493920𝑑4 βˆ’ 483840𝑑3 + 263760𝑑2 βˆ’ 75852𝑑 + 8989) . The differential with respect to t of Boubaker wavelet polynomials π‘Šπ΅π‘šΜ‡ (𝑑) is given as follows: π‘Šπ΅0Μ‡ (𝑑) = 0 , π‘Šπ΅1Μ‡ (𝑑) = 2√3 , π‘Šπ΅2Μ‡ (𝑑) = √5 6 (48𝑑 βˆ’ 36) , π‘Šπ΅Μ‡ 3(𝑑) = √7 20 (480𝑑2 βˆ’ 720𝑑 + 264), π‘Šπ΅Μ‡ 4(𝑑) = √9 70 (4480𝑑3 βˆ’ 10080𝑑2 + 7440𝑑 βˆ’ 1800), π‘Šπ΅Μ‡ 5(𝑑) = √11 252 (40320𝑑4 βˆ’ 120960𝑑3 + 134400𝑑2 βˆ’ 65520𝑑 + 11820), π‘Šπ΅Μ‡ 6(𝑑) = √13 924 (354816𝑑5 βˆ’ 1330560𝑑4 + 1975680𝑑3 βˆ’ 1451520𝑑2 + 527520𝑑 βˆ’ 75852) . 4. Boubaker wavelet polynomials for solving variational problems: We demonstrate the application of wavelet Boubaker polynomials to solve some variational problems. A function f (t) is defined over𝐿2[0,1] , the function approximate by wavelet Boubaker polynomials is defined as follows: 𝑓(𝑑) = βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) = 𝑐 𝑇 π‘Šπ΅(𝑑) ∞ 𝑖=0 … (3) where 𝑐𝑖 =βŸ¨π‘“(𝑑), π‘Šπ΅π‘– (𝑑)⟩ and 〈 , βŒͺis inner product on 𝐿 2[0,1]. IHJPAS. 36 (3) 2023 430 If the series in equation (3) is truncated, then equation (3) can be written as 𝑓(𝑑) = βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) = 𝑐 𝑇 π‘Šπ΅(𝑑) 𝑁 𝑖=0 … (4) where c =[c0 , c1, …, cN] T and WBi (t) = [WB0,WB1, …,WBN] T Differentiating equation (4) with respect t, to obtain 𝑓 =Μ‡ 𝑐𝑇 π‘Šπ΅Μ‡ (𝑑) where c =[c0, c1, …, cN] T and π‘Šπ΅Μ‡ (t) = [π‘Šπ΅0Μ‡ ,π‘Šπ΅Μ‡ 1, …,π‘Šπ΅Μ‡ N] T The matrix of derivatives D is given as π‘‘π‘Šπ΅(𝑑) 𝑑𝑑 = π‘Šπ΅Μ‡ (𝑑) = π·π‘Šπ΅(𝑑) Where π‘Šπ΅Μ‡ (𝑑) derivative of wavelet Boubaker functions To demonstrate this procedure, we consider this example of finding the minimum of functional [14] Example1: 𝐽(π‘₯) = ∫ [οΏ½Μ‡οΏ½ 2(𝑑) + 𝑑�̇�(𝑑) + π‘₯2(𝑑)]𝑑𝑑 1 0 , … (5) with two conditions x(0) = 0, π‘₯(1) = 1 4 … (6) and the exact solution π‘₯(𝑑) = βˆ’π‘’ βˆ’π‘‘[(βˆ’1+𝑒 𝑑)(π‘’βˆ’2𝑒 2βˆ’2𝑒 𝑑+𝑒 1+𝑑)] 4(βˆ’1+𝑒 2) Now, we use wavelet Boubaker polynomials of M = 4 and M=6 to approximate the function x(t) Suppose π‘₯(𝑑) = βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) 5 0 or π‘₯(𝑑) = 𝑐 𝑇 π‘Šπ΅(𝑑) … (7) And π‘₯2(𝑑) = 𝑐𝑇 π‘Šπ΅(π‘Šπ΅)𝑇 𝑐 … (8) where c = [c0, c1,… , c5] T and WB (t) = [WB0, WB1, …, WB5], Differentiating equation (7), we get οΏ½Μ‡οΏ½(𝑑) = 𝑐𝑇 𝐷 π‘Šπ΅(𝑑) … (9) and οΏ½Μ‡οΏ½ 2(𝑑) = 𝑐𝑇 𝐷 (π‘Šπ΅)(𝐷 (π‘Šπ΅))𝑇 𝑐 …(10) Substituting equation (7) - (10) into equation (5), we obtain 𝐽(π‘₯) = ∫ [𝑐𝑇 𝐷 (π‘Šπ΅)(𝐷 (π‘Šπ΅)) 𝑇1 0 𝑐 + 𝑑𝑐𝑇 𝐷 (π‘Šπ΅) + 𝑐𝑇 π‘Šπ΅(π‘Šπ΅)𝑇 𝑐]𝑑𝑑 … (11) We can simplify equation (11) to 𝐽(π‘₯) = 1 2 𝑐𝑇 𝐻𝑐 + π‘žπ‘‡ 𝑐 … (12) where 𝐻 = 2 ∫ [𝐷 (π‘Šπ΅)(𝐷 (π‘Šπ΅)) 𝑇1 0 + π‘Šπ΅(π‘Šπ΅)𝑇 ]𝑑𝑑, and π‘žπ‘‡ = ∫ 𝑑 𝐷 (π‘Šπ΅)𝑇𝑑𝑑 1 0 , Equation (7) with boundary conditions (6), can imply π‘₯(0) = 𝑐𝑇 π‘Šπ΅(0) = 0 and π‘₯(1) = 𝑐𝑇 π‘Šπ΅(1) = 1 4 … (13) IHJPAS. 36 (3) 2023 431 We can rewrite equations (12) & (13), as follows Min 𝐽(π‘₯) = 1 2 𝑐𝑇 𝐻𝑐 + π‘žπ‘‡ 𝑐 subject to Fc-b=0 where 𝐹 = [π‘Šπ΅π‘‡ (0) π‘Šπ΅π‘‡(1) ] , 𝑏 = [0 1 4 ] we can find the parameter c using Lagrange equation as π‘βˆ— = βˆ’π»βˆ’1𝑐 + π»βˆ’1𝐹𝑇(πΉπ»βˆ’1𝐹𝑇 )βˆ’1(πΉπ»βˆ’1𝑐 + 𝑏) When M=4 the approximate value of x is x(t)= [0.21463728 ,0.04727207 ,-0.01564479, 0.00192278] WB (t) and when M=6 an approximate value of x is x(t)= [0.21464181, 0.04746694,-0.01585894, 0.00129175, -0.00023926 ,0.00001933] WB (t) Table (1) shows the numerical results for example (1) with M=4 and M=6 compared with exact solution, and graphically in figure (1). Table 1. Results for Example1 t xexact xapp.(t) M=4 M=4 Absolute error xapp.(t) M=6 M=6 Absolute error 0 0.00000000 0.00000000 0.00000000 0.00000000 0.0000000 0.1 0.04195072 0.04180602 0.00014470 0.04195070 0.00000002 0.2 0.07931714 0.07922621 0.00009093 0.07931742 0.00000027 0.3 0.11247322 0.11250475 0.00003152 0.11247334 0.00000011 0.4 0.14175081 0.14188583 0.00013502 0.14175057 0.00000023 0.5 0.16744291 0.16761363 0.00017071 0.16744257 0.00000034 0.6 0.18980668 0.18993234 0.00012566 0.18980660 0.00000007 0.7 0.20906592 0.20908615 0.00002022 0.20906621 0.00000029 0.8 0.22541340 0.22531923 0.00009416 0.22541370 0.00000030 0.9 0.23901272 0.23887579 0.00013693 0.23901258 0.00000014 1.0 0.2500000000 0.25000000 0.00000000 0.25000000 0.00000000 Figure 1. Shows that the Absolute Error when M=6 is very less than M=4 for Example1 IHJPAS. 36 (3) 2023 432 Example 2: Min J= ∫ (οΏ½Μ‡οΏ½2 + π‘₯2) 1 0 𝑑𝑑 with boundary conditions π‘₯(0) = 0 , π‘₯(1) = 1 , and exact solution is π‘₯(𝑑) = 𝑒 π‘‘βˆ’π‘’ βˆ’π‘‘ 𝑒 1βˆ’π‘’βˆ’1 In the same procedure, we can solve example2. Table (2) shows the numerical results for example (2) with M=4 and M=6 compared with exact solution, and graphically in figure (2). When M=4 the approximate value of x is x(t)= [0.70703400,0.32030428,0.03906027, 0.00769113] WB (t) and when M=6 the approximate value of x is x(t)= [0.70703595,0.32001609,0.03929145,0.00870795,0.00060523,0.00007777] WB (t) Table 2. Results for Example2 t xexact M=4 xapp.(t) M=4 Absolute error M=6 xapp.(t) M=6 Absolute error 0 0 0.00000000 0.00000000 0.00000000 0.00000000 0.1 0.08523370 0.08540591 0.00017221 0.08523414 0.00000043 0.2 0.17132045 0.17145031 0.00012986 0.17131995 0.00000049 0.3 0.25912183 0.25910993 0.00001190 0.25912108 0.00000075 0.4 0.34951660 0.34936152 0.00015507 0.34951638 0.00000021 0.5 0.44340944 0.44318181 0.00022762 0.44340990 0.00000046 0.6 0.54174007 0.54154756 0.00019250 0.54174071 0.00000063 0.7 0.64549262 0.64543551 0.00005710 0.64549284 0.00000021 0.8 0.75570548 0.75582241 0.00011693 0.75570520 0.00000027 0.9 0.87348169 0.87368498 0.00020329 0.87348147 0.00000021 1.0 1.00000000 1.00000000 0.00000000 1.00000000 0.00000000 Figure2. Shows that the Absolute Error when M=6 is very less than M=4 for Example2 . IHJPAS. 36 (3) 2023 433 Example 3: Min j= ∫ (οΏ½Μ‡οΏ½2 βˆ’ π‘₯2) 1 0 𝑑𝑑 with boundary conditions π‘₯(0) = 0, π‘₯(1) = 1 , and exact solution is π‘₯(𝑑) = 𝑠𝑖𝑛𝑑 𝑠𝑖𝑛1 Table (3) and figure (3) illustrate the results of example (3). When M=4 the approximate value of x is x(t)= [ 0.80163166, 0.24981351, 0.04537278, 0.00806631] WB (t) and when M =6 the approximate value of x is x(t)= [0.80164433, 0.24944837, 0.04508324, 0.00682505, 0.00070902, 0.00008006] WB (t) Table 3. Results for Example3 t uexact M= 4 uappr (t) M=4 Absolute Error M= 6 uappr.(t) M=6 Absolute Error 0 0.00000000 0.00000000 0.00000000 0.00000000 0.0000000 0.1 0.11864154 0.11885365 0.00021211 0.11862523 0.00001631 0.2 0.23609766 0.23624932 0.00015166 0.23610543 0.00000777 0.3 0.35119476 0.35116260 0.00003216 0.35121956 0.00002480 0.4 0.46278285 0.46256910 0.00021374 0.46280277 0.00001992 0.5 0.56974696 0.56944444 0.00030251 0.56974637 0.00000058 0.6 0.67101835 0.67076422 0.00025412 0.67099789 0.00002045 0.7 0.76558514 0.76550406 0.00008108 0.76556103 0.00002411 0.8 0.85250246 0.85263956 0.00013709 0.85249567 0.00000678 0.9 0.93090186 0.93114634 0.00024447 0.93091791 0.00001604 1.0 1.00000000 1.00000000 0.00000000 1.00000000 0.00000000 Figure 3. Shows that the Absolute Error when M=6 is very less than M=4 for Example3 IHJPAS. 36 (3) 2023 434 5. The Convergence Test of wavelet Boubaker polynomials: By (theorem (1), see [1]) if x(t) is continually defined on [0,1] and Ξ±(t) is the approximate of π›Όβˆ—(t) by applying Boubaker wavelet. Also, suppose that x (t) is bounded by a positive constant that is |π‘₯(𝑑)| < πœ–. Then, the coefficients of x (t) are bounded. The state can be expanded using Wavelet Boubaker polynomials, as: π‘₯𝑖𝑁 (𝑑) = βˆ‘ π‘π‘–π‘˜ π‘Šπ΅π‘˜ (𝑑) 𝑁 π‘˜=1 π‘₯𝑖 (𝑑) = π‘₯𝑖𝑁 (𝑑) + βˆ‘ π‘π‘–π‘˜ π‘Šπ΅π‘˜ (𝑑) ∞ π‘˜=𝑁+1 or π‘₯𝑖 (𝑑) = π‘₯𝑖𝑁 (𝑑) + 𝑒𝑖 (𝑑) …(14) The coefficients in equation (14) is limited by [1] Such that the residual ‖𝑒(𝑑)β€– is less than some πœ€ Where 𝑒(𝑑) = π‘šπ‘Žπ‘₯ {𝑒1(𝑑), 𝑒2(𝑑), … , 𝑒𝑁(𝑑)}, Then, using the convergence test for the technique to the variable x in terms of N proposed 𝐿2norm of π‘₯𝑖 , i=1,2,…,n [∫ (π‘₯(𝑑) βˆ’ π‘₯𝑖𝑁 (𝑑)) 2𝑑𝑑 1 0 ] 1 2 < πœ€π‘– , 𝑖=1,2,…,𝑛 πœ€ = π‘šπ‘Žπ‘₯{πœ€1(𝑑), πœ€2(𝑑), … , πœ€π‘›(𝑑) } , [∫ (π‘₯(𝑑) βˆ’ π‘₯𝑁 (𝑑)) 2𝑑𝑑 1 0 ] 1 2 < πœ€, Using the Boubaker wavelet polynomials for approximating x variables, we get [∫ ( βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) 𝑁+𝑀 𝑖=0 βˆ’ βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) 𝑁 𝑖=0 )2𝑑𝑑 1 0 ] 1 2 < πœ€ = [∫ ( βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) 𝑁+𝑀 𝑖=𝑁+1 )2𝑑𝑑 1 0 ] 1 2 < πœ€ = [∫ ( βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) 𝑁+𝑀 𝑖=𝑁+1 )( βˆ‘ 𝑐𝑖 π‘Šπ΅π‘– (𝑑) 𝑁+𝑀 𝑖=𝑁+1 )𝑑𝑑 1 0 ] 1 2 < πœ€ = βˆ‘ βˆ‘ 𝑐𝑖 𝑐𝑗 𝑁+𝑀 𝑗=𝑁+1 ∫ π‘Šπ΅π‘– (𝑑) 1 0 π‘Šπ΅π‘— (𝑑) 𝑑𝑑 < πœ€ 𝑁+𝑀 𝑖=𝑁+1 Hence, the Wavelet Boubaker polynomials can be reduced to the form IHJPAS. 36 (3) 2023 435 βˆ‘ 𝑐𝑖 2 𝑁+𝑀 𝑖=𝑁+1 < πœ€ when the squares of the remaining coefficients becomes neglected, a favorable approximation to the solution is achieved. 6. 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