388 This work is licensed under a Creative Commons Attribution 4.0 International License IHJPAS. 37 (2) 2024 Ibn Al-Haitham Journal for Pure and Applied Sciences Journal homepage: jih.uobaghdad.edu.iq PISSN: 1609-4042, EISSN: 2521-3407 Firas A. Fawzi*1 , Hadeer M. Globe2 and Nizam G. Ghawadri 3 1,2 Department of Mathematics, Faculty of Computer Science and Mathematics, Tikrit University, Salah Al-Din, Iraq. 3 Doctor of Mathematics, Ministry of Education, Jenin, Palestine. *Corresponding Author. Abstract This paper presents two important contributions to the field of numerical analysis for third-order ordinary differential equations (ODEs). First, a new class of direct implicit Runge-Kutta (RK) processes, called RKTDIO, is introduced as solutions to third-order ODEs. Secondly, it develops the ERKTDIO method, which is an embedded pairwise diagonal implicit RK method. The study begins by introducing the theory of relevant-colored trees and B-series as fundamental concepts. By utilizing the order constraints, two RKTDIO methods are derived: a fifth-order method with three stages and a sixth-order method with four stages. In addition, an embedded method called ERKTDIO6(5) is derived, which has orders six and five. The derivation of the embedded method includes strategies to ensure that the higher-order method achieves high accuracy while the lower- order method provides optimal error estimates. To evaluate the effectiveness of the proposed methods, variable step-size codes are developed and applied to a set of specific third-order problems. The numerical evaluation involves converting the problems into a system of first-order ODEs and comparing the results with existing methods in terms of accuracy and function evaluations. The numerical demonstrations emphasise the superior performance and efficiency of the new methods in solving third-order ODEs. The comparative analysis shows the accuracy achieved by the higher-order method and the improved error estimation of the lower-order method. The results validate the efficacy of the proposed approaches and their potential for practical applications in various domains. Keywords: Third-order ODEs, order conditions, B-series, Relevant-colored trees. 1. Introduction Third-order differential equations can be found in various fields, such as applied sciences, neural network engineering [1,2], fluid dynamics [3] and thin film flow [4]. The aim of this paper is to develop and explain a computational method for solving initial value problems of third-order differential equations. ๐›ผโ€ฒโ€ฒโ€ฒ(๐‘ฅ) = ๐œ‡ (๐‘ฅ, ๐›ผ(๐‘ฅ)), ๐‘ฅ โ‰ฅ ๐‘ฅ0, (1) With initial conditions ฮฑ(xn) = ฮฑn , ฮฒ(xn) = ๐›ผ๐‘› โ€ฒ , ฮณ(xn) = ฮฑ๐‘› โ€ฒโ€ฒ . Received: 13 April 2023 Accepted: 12 June 2023 Published: 20 April 2024 Efficient Embedded Diagonal Implicit Runge-Kutta Method for Directly Solving Third Order ODEs doi.org/10.30526/37.2.3409 https://creativecommons.org/licenses/by/4.0/ https://jih.uobaghdad.edu.iq/index.php/j/index#1609-4042 https://jih.uobaghdad.edu.iq/index.php/j/index#2521-3407 https://orcid.org/my-orcid?orcid=0000-0003-0939-7940 mailto:firasadil01@tu.edu.iq https://orcid.org/0009-0008-2163-2484 mailto:hadeermohamadjalob@gmail.com https://orcid.org/0000-0002-4335-1789 mailto:nizamghawadri@gmail.com IHJPAS. 37 (2) 2024 389 Where (๐‘ฅ)โˆˆ ๐‘…๐‘‘, ๐‘“: ๐‘…๐‘‘ร— ๐‘…๐‘‘ โ†’ ๐‘…๐‘‘ has a continuous value but lacks the first and second derivatives. Numerical solutions are often necessary for third-order differential equations since analytical solutions are often not available. Some researchers have used classical methods to solve higher- order differential equations by converting them into a system of first-order differential equations [5-16,27]. However, this method can be computationally intensive and time consuming. Direct numerical approaches have been proposed to reduce the computation time, but they require a second method to obtain initial values for the numerical solutions. Implicit methods are useful because they can achieve high accuracy with fewer steps, making it easier to find solutions to complex problems. Several researchers have developed embedded Runge-Kutta methods with high algebraic orders for solving third-order differential equations [17-18]. Ismail et. al [19] proposed the Singly Embedded Diagonally Implicit Runge-Kutta (SDIRK) method to combine delay differential equations (DDEs) and compared the computational results. The researchers in [20,21,26] developed a novel embedded explicit and implicit Runge-Kutta method for solving special third and fourth-order problems. The main objective of this work is to develop a new method called RKTDIO, an implicit one-step Runge-Kutta method designed for directly solving specific third-order differential equations. This method is developed using the theory of relevant-colored trees theory and also involves the derivation of embedded diagonally implicit Runge-Kutta methods for the direct integration of certain third-order differential equations. The structure of this paper is as follows: Section 2 provides the formulation concept for the RKTDIO approach for the direct integration of certain third-order ODEs. In Section 3, we develop the novel theory of relevant-colored trees theory and the corresponding B-series theory. Section 4 contains the derivation of the ordering criteria of the RKTDIO method. Section 5 presents the construction of a three-stage RKTDIO approach for order five and a four-stage method for order six. Section 6 describes the derivation of the embedded ERKTDIO6(5) method. To demonstrate the efficiency and effectiveness of the RKTDIO and ERKTDIO6(5) methods compared to the methods currently used in the scientific literature, we give numerical findings in Section 7. Section 8 provides conclusions. 2. The formulation of RKTDIO method By turning it into a system of first-order ODEs, the special third-order IVP (1) can be solved as follows: ( ฮฑ(x) ฮฒ(x) ฮณ(x) ) โ€ฒ = ( ฮฒ(x) ฮณ(x) ฮผ(x, ฮฑ(x)) ) (2) With initial conditions ฮฑ(xn) = ฮฑn , ฮฒ(xn) = ๐›ผ๐‘› โ€ฒ , ฮณ(xn) = ฮฑ๐‘› โ€ฒโ€ฒ. The Runge-Kutta method first-order is used to obtain the following system of equations ฮฑi= ฮฑn + h โˆ‘ aijฮฑโ€ฒj s j=1 , (3) ฮฑโ€ฒi = ๐›ผ๐‘› โ€ฒ + h โˆ‘ aijฮฑโ€ฒโ€ฒj , s j=1 (4) ฮฑ๐‘– โ€ฒโ€ฒ = ฮฑ๐‘› โ€ฒโ€ฒ + h โˆ‘ aij ฮผ(xn + cjh, ฮฑj) ,s j=1 (5) ฮฑn+1 = ฮฑn + h โˆ‘ biฮฑ๐‘– โ€ฒ , (6)s i=1 IHJPAS. 37 (2) 2024 390 ฮฑ๐‘›+1 โ€ฒ = ฮฑ๐‘› โ€ฒ + h โˆ‘ biฮฑ๐‘– โ€ฒโ€ฒs i=1 , (7) ฮฑ๐‘›+1 โ€ฒโ€ฒ = ฮฑ๐‘› โ€ฒโ€ฒ + h โˆ‘ biฮผ s i=1 (xn + cih, ฮฑi) . (8) When we ignore ๐›ผ๐‘– โ€ฒ, ๐›ผ๐‘– โ€ฒโ€ฒ and ฮฑ๐‘– โ€ฒโ€ฒโ€ฒ , from (3) โ€“ (8) we conclude ฮฑi = ฮฑn + h โˆ‘ aij s j=1 ฮฑ๐‘› โ€ฒ + h2 โˆ‘ aij s j,k=1 ฮฑjkฮฑ๐‘› โ€ฒโ€ฒ + h3 โˆ‘ aijajkakl ฮผ(xn + cih, ฮฑl), i = 1, โ€ฆ , s. (9) s j,k,l ๐›ผ๐‘›+1 = ๐›ผ๐‘› + โ„Ž โˆ‘ ๐‘๐‘–๐›ผ๐‘› โ€ฒ + โ„Ž2 โˆ‘ ๐‘๐‘– ๐‘  ๐‘–,๐‘—=1 ๐‘Ž๐‘–๐‘—๐›ผ๐‘› โ€ฒโ€ฒ ๐‘  ๐‘–=1 + โ„Ž3 โˆ‘ ๐‘๐‘–๐‘Ž๐‘–๐‘—๐‘Ž๐‘—๐‘˜ ๐œ‡(๐‘ฅ๐‘› + ๐‘๐‘˜โ„Ž, ๐‘  ๐‘–,๐‘—,๐‘˜=1 ๐›ผ๐‘˜), (10) ๐›ผโ€ฒ๐‘›+1 = ๐›ผ๐‘› โ€ฒ + โ„Ž โˆ‘ ๐‘๐‘–๐›ผ๐‘› โ€ฒโ€ฒ + โ„Ž2 โˆ‘ ๐‘๐‘–๐‘Ž๐‘–๐‘— ๐œ‡๐‘  ๐‘–.๐‘—=1 (๐‘ฅ๐‘› + ๐‘๐‘—โ„Ž๐‘  ๐‘–=1 , ๐›ผ๐‘—), (11) ๐›ผ๐‘›+1 โ€ฒโ€ฒ = ๐›ผ๐‘› โ€ฒโ€ฒ + h โˆ‘ bi ฮผ(xn + cih, s i=1 ฮฑi) . (12) We assume that โˆ‘ aij = ci, s j=1 โˆ‘ aijajk = 1 2 s j,k=1 ci 2 , โˆ‘ bi = 1 s i=1 , โˆ‘ biaij = 1 2 s i,j=1 , โˆ‘ biaij = b๐‘– โ€ฒโ€ฒ , โˆ‘ bjajkakl = b๐‘– โ€ฒโ€ฒ s j,k=1 s j=1 , โˆ‘ aik s k,l,r=1 aklalr = ๏ฟฝฬ‚๏ฟฝ๐‘–๐‘— , ๐‘– = 1, โ€ฆ , ๐‘ . As a result, we are able to solve the specific third-order IVP (1), indicated by the RKTDIO approach, using the following direct integration method. Thus, the following formula is used to represent the s-stage RKTDIO approach for the numerical solution of the IVP (1): ฮฑn+1 = ฮฑn + h๐›ผ๐‘› โ€ฒ + 1 2 h2๐›ผ๐‘› โ€ฒโ€ฒ + h3 โˆ‘ bi s i=1 ฮผ(xn + cih, ฮฑi), (13) ๐›ผโ€ฒ๐‘›+1 = ๐›ผ๐‘› โ€ฒ + h๐›ผ๐‘› โ€ฒโ€ฒ + h2 โˆ‘ b๐‘– โ€ฒ ฮผ(xn + cih, s i=1 ฮฑ๐‘– โ€ฒ) , (14) ๐›ผ๐‘›+1 โ€ฒโ€ฒ = ๐›ผ๐‘› โ€ฒโ€ฒ + h โˆ‘ b๐‘– โ€ฒโ€ฒ s i=1 ฮผ (xn + cih, ฮฑ๐‘– โ€ฒ) , (15) ฮฑi = ฮฑn + cih๐›ผ๐‘› โ€ฒ + 1 2 ci 2h2๐›ผ๐‘› โ€ฒโ€ฒ + h3 + โˆ‘ ๏ฟฝฬ‚๏ฟฝij ฮผ(xn + cih, s i,j=1 ฮฑ๐‘— โ€ฒ). (16) All RKTDIO parameters aij, bi, b๐‘– โ€ฒ, b๐‘– โ€ฒโ€ฒ , and ci are real numbers and i, j = 1, 2, . . . , s. The RKTDIO method (13)โ€“(16) can be expressed in Butcher tableau as follows IHJPAS. 37 (2) 2024 391 : 3. B-series and associated relevant-colored trees The essential definitions of lemmas and associated theorems that are utilized throughout this article will be covered in this part. Definition 3.1 [22,23]: Order of the RKTDIO Method for Third-Order ODEs The RKTDIO method (13)-(16) has order ๐‘ when the third-order ODE (1) with the assumption ฮฑ(xn) = ฮฑn, ฮฑโ€ฒ(xn) = ฮฑโ€ฒ n, ฮฑโ€ฒโ€ฒ(xn) = ฮฑโ€ฒโ€ฒ n hence the local truncation error norms of the exact solution and the first, and second derivatives of the solution must satisfied. ฮฑ(xn + h) โˆ’ ฮฑn+1 = O(hp+1), ฮฑโ€ฒ(xn + h) โˆ’ ฮฑโ€ฒn+1 = O(hp+1), ฮฑโ€ฒโ€ฒ(xn + h) โˆ’ ฮฑโ€ฒโ€ฒ n+1 = O(hp+1), (17) The following autonomous form of third-order IVP must be used in order to derive the algebraic order conditions for the RKTDIO technique (13)โ€“(16). ฮฑ(3)(x) = ฯ(ฮฑ(x)), (18) With initial conditions ฮฑ(xn) = ฮฑn, ฮฑโ€ฒ(xn) = ฮฑโ€ฒ n, ฮฑโ€ฒโ€ฒ(xn) = ฮฑ๐‘› โ€ฒโ€ฒ. By extending IVP (1) with a one-dimensional vector ๐‘ค = ๐‘ฅ, the autonomous problem can be expressed equivalently to the third-order initial value problem (1) as follows: w3 = 0, (19) ฮฑ(3) = ฯ(w, ฮฑ), (20) w(xn) = wn wโ€ฒ(xn) = wโ€ฒ n = 1, wโ€ฒโ€ฒ(xn) = wโ€ฒโ€ฒ n = 0, (21) ฮฑ(xn) = ฮฑn, ฮฑโ€ฒ(xn) = ฮฑโ€ฒ n, ฮฑโ€ฒโ€ฒ(xn) = ฮฑโ€ฒโ€ฒn. (22) Applying RKTDIO method (13)โ€“(16) to the scheme (19)โ€“(22), we obtain Wi = wn + cih wโ€ฒ n + 1 2 ci 2h2 wโ€ฒโ€ฒ n , (23) ฮฑi = ฮฑn + cihฮฑโ€ฒ n + 1 2 ci 2h2 ฮฑโ€ฒโ€ฒ n + h3 โˆ‘ aij ฮผ(Wj, ฮฑj) , s i,j=1 (24) wn+1 = wn + h wโ€ฒ n + 1 2 h2wโ€ฒโ€ฒ n , (25) wโ€ฒ n+1 = wโ€ฒ n + h wโ€ฒโ€ฒ n , (26) wโ€ฒโ€ฒ n+1 = wโ€ฒโ€ฒ n , (27) ฮฑn+1 = ฮฑn + h ฮฑโ€ฒ n + 1 2 h2ฮฑโ€ฒโ€ฒ n + h3 โˆ‘ bi ฮผ(Wi s i=1 , ฮฑi), (28) ฮฑโ€ฒn+1 = ฮฑโ€ฒ n + h ฮฑโ€ฒโ€ฒ n + h2 โˆ‘ bโ€ฒ i ฮผ(Wi, s i=1 ฮฑi) , (29) ๐‘1 โ‹ฎ ๐‘๐‘  ๏ฟฝฬ‚๏ฟฝ11 โ€ฆ ๏ฟฝฬ‚๏ฟฝ1๐‘  โ‹ฎ โ‹ฑ โ‹ฎ ๏ฟฝฬ‚๏ฟฝ๐‘ 1 โ€ฆ ๏ฟฝฬ‚๏ฟฝ๐‘ ๐‘  ๐‘1 โ€ฆ ๐‘๐‘  ๐‘1 โ€ฒ โ€ฆ ๐‘๐‘  โ€ฒ ๐‘1 โ€ฒโ€ฒ โ€ฆ ๐‘๐‘  โ€ฒโ€ฒ IHJPAS. 37 (2) 2024 392 ฮฑโ€ฒโ€ฒ n+1 = ฮฑโ€ฒโ€ฒ n + h โˆ‘ bโ€ฒโ€ฒ i s i=1 ฮผ(Wi, ฮฑi). (30) Substituting Eq. (21) into system of Eqs. (23)โ€“(30), we get Wi = xn + cih, (31) wn+1 = xn + h, (32) wโ€ฒ n+1 = 1, (33) wโ€ฒโ€ฒ n+1 = 0, (34) ฮฑn+1 = ฮฑn + hฮฑโ€ฒ n + 1 2 h2ฮฑโ€ฒโ€ฒ n + h3 โˆ‘ bi s i=1 ฮผ(xn + cih, ฮฑi), (35) ฮฑโ€ฒ n+1 = ฮฑโ€ฒ n + hฮฑโ€ฒโ€ฒ n + h2 โˆ‘ bโ€ฒ i s i=1 ฮผ(xn + cih, ฮฑi), (36) ฮฑโ€ฒโ€ฒ n+1 = ฮฑโ€ฒโ€ฒ n + h โˆ‘ bโ€ฒโ€ฒ i s i=1 ฮผ(xn + cih, ฮฑi), (37) ฮฑi = ฮฑn + cihฮฑโ€ฒ n + 1 2 ci 2h2ฮฑโ€ฒโ€ฒ n + h3 โˆ‘ ๏ฟฝฬ‚๏ฟฝ๐‘–๐‘— ฮผ(xn + cih s i,j=1 , ฮฑj). (38) We find that the system of equations (13)โ€“(16), which is generated by using the RKTDIO approach on the non-autonomous problem (1), is totally similar to Eqs. (35)โ€“ (38). Hence, discussing the numerical solutions of autonomous form is sufficient (18). Hence, the RKTDIO method (13)โ€“(16) can be implemented as follows. ฮฑn+1 = ฮฑn + h ฮฑโ€ฒ n + 1 2 h2ฮฑโ€ฒโ€ฒ n + h3 โˆ‘ bi s i=1 ฮผ(ฮฑi), ฮฑโ€ฒ n+1 = ฮฑโ€ฒ n + hฮฑโ€ฒโ€ฒ n + h2 โˆ‘ bโ€ฒ i s i=1 ฮผ(ฮฑi), ฮฑโ€ฒโ€ฒ n+1 = ฮฑโ€ฒโ€ฒ n + h โˆ‘ bโ€ฒโ€ฒ i s i=1 ฮผ(ฮฑi), ฮฑi = ฮฑn + cihฮฑโ€ฒ n + 1 2 ci 2h2ฮฑโ€ฒโ€ฒ n + h3 โˆ‘ ๏ฟฝฬ‚๏ฟฝ๐‘–๐‘— s i,j=1 ฮผ(ฮฑj). (39) The following elementary differentials are obtained by using the elementary differential notation on the analytical solution ๐›ผ(๐‘ฅ). [15] ฮฑ(1) = ฮฑโ€ฒ, ฮฑ(2) = ฮฑโ€ฒโ€ฒ, ฮฑ(3) = ฮผ, ฮฑ(4) = ฮผโ€ฒฮฑโ€ฒ, ฮฑ(5) = ฮผโ€ฒโ€ฒ(ฮฑโ€ฒ, ฮฑโ€ฒ) + ฮผโ€ฒฮฑโ€ฒโ€ฒ(๐›ผโ€ฒ, ๐›ผโ€ฒ), ฮฑ(6) = 3๐›ผโ€ฒโ€ฒ(๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ) + ฮผโ€ฒโ€ฒโ€ฒ(๐›ผโ€ฒ, ๐›ผโ€ฒ, ๐›ผโ€ฒ) + ฮผโ€ฒฮฑโ€ฒโ€ฒ. (40) These processes very quickly get more difficult as the order increases. The optimum method for overcoming this challenge, according to [25], will be to use a graphical representation with a few modifications for third-order ODEs, denoted by relevant-colored trees. The three sorts of nodes in the relevant-colored trees are "meager," "black ball", and "white ball", and they are connected by arcs. In these trees, we specifically use the end meager node to denote each ๐›ผโ€ฒ, the end black ball node to denote each ๐›ผโ€ฒโ€ฒ, the end white ball to denote each ๐œ‡, and each arc to denote each arc, leaving this node to represent the ๐‘š โˆ’ ๐‘กโ„Ž derivative of ๐œ‡ with respect to ๐›ผ. In addition, the symbols ๐‘ก1 and ๐‘ก2 denote the first-order and the second-order tree and ๐‘ก3 the third-order tree, respectively (see Figure 1) IHJPAS. 37 (2) 2024 393 ๐œ1 = ๐œ2 = ๐œ3 = Figure 1. The relevant-colored trees Here, we'll go through some essential definitions for the relevant-colored trees and related B-series that are necessary for this work. Definition 3.2 [22,23]: Relevant-Colored Trees (RT) and Meager Node The following definitions are repeated for the set of relevant-colored trees (RT): 1- The trees ๐‘ก1, ๐‘ก2 and ๐‘ก3 above are all in RT, and the tree ๐‘ก1 includes only one meager node (known as the root). 2- If ๐œ1, ๐œ2, . . . , ๐œ๐‘š โˆˆ ๐‘…๐‘‡, then ๐œ = [๐œ1, ๐œ2, โ€ฆ , ๐œ๐‘š]3 is the tree obtained by linking the roots ๐œ1, ๐œ2, . . . , ๐œ๐‘š and the root is the โ€˜โ€˜meager nodeโ€™โ€™ ๐‘ก1 is at the bottom. The subscript 3 is to mention that the trees of the roots of ๐œ1, ๐œ2, . . . , ๐œ๐‘š onto the tree ๐‘ก3 contain a chain of three nodes. We will employ the following principles to create the appropriate colored trees: 1- The โ€˜โ€˜meagerโ€™โ€™ node is always the root. 2- A โ€˜โ€˜meagerโ€™โ€™ node has only single child and that child should be โ€˜โ€˜black ballโ€™โ€™. 3- A โ€˜โ€˜black ballโ€™โ€™ node has only single child and that child should be โ€˜โ€˜white ballโ€™โ€™. Definition 3.3 [13,14]: Order Function ๐†(๐‰) for Relevant-Colored Trees (RT) The order ๐œŒ(๐œ) and symmetry ๐œŽ(๐œ) functions are defined recursively as follows: 1- ๐œŒ(๐‘ก1) = 1, ๐œŒ(๐‘ก2) = 2, ๐œŒ(๐‘ก3) = 3, 2- ๐œŽ(๐‘ก1) = 1, ๐œŽ(๐‘ก2) = 1, ๐œŽ(๐‘ก3) = 1, 3- If ๐œ = [๐œ1, ๐œ2, โ€ฆ , ๐œ๐‘š]3 for each ๐œ โˆˆ ๐‘…๐‘‡, then ฯ(ฯ„ ) = 3 + โˆ‘ ฯ(ฯ„i) m i=1 and ๐œŽ(๐œ ) = โˆ ๐œŽ(๐œ๐‘–)(๐‘ฃ1! ๐‘ฃ2!๐‘š ๐‘–=1 โ€ฆ ) where ๐œŒ(๐œ) is the number of nodes of ๐œ, โˆ€ฯ„ โˆˆ RT and ฮฝ1! ฮฝ2! โ€ฆ count equal trees among ๐œ1, ๐œ2, . . . , ๐œ๐‘š. Then we can define the set ๐‘†๐‘Ÿ which consist of every trees ๐‘…๐‘‡ of order ๐‘Ÿ. Definition 3.4 [22,23]: Elementary Differential and B-Series on Relevant-Colored Trees (RT) for the RKTDIO Approach The elementary differential for every tree ๐œ โˆˆ ๐‘…๐‘‡ is a function F(ฯ„): Rd ร— Rd ร— Rd โ†’ Rd , recursively defined on RT as follows 1- ๐•Œ(๐‘ก1)(๐›ผ, ๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ) = ๐›ผโ€ฒ, ๐•Œ(๐‘ก2)(๐›ผ, ๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ) = ๐›ผโ€ฒโ€ฒ, ๐•Œ(๐‘ก3) (๐›ผ, ๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ) = ๐œ‡(๐›ผ), 2- ๐•Œ (๐œ ) = ๐œ‡(๐‘š)(๐›ผ) (๐•Œ (๐œ1)(๐›ผ, ๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ), . . . , ๐•Œ (๐œ๐‘š)(๐›ผ, ๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ)) for ๐œ = [๐œ1, ๐œ2, โ€ฆ , ๐œ๐‘š]3. We expanded these definitions to provide the definition of B-series on the set RT of the relevant- colored trees for the RKTDIO approach, which was motivated by the definitions of B-series on the root trees in [23] and the tri-colored trees in [24]. Definition 3.5 [22,23]: B-Series Representation Let ฮด: RT โˆช {โˆ…} โ†’ Rd be a mapping, then we can give the form of a formal series as follows: B(ฮด, y) = ฮด(โˆ…) + โˆ‘ hฯ(ฯ„) ฯƒ(ฯ„) ฮด(ฯ„)ฯ„โˆˆRT ๐•Œ(ฯ„)(ฮฑ, ฮฑโ€ฒฮฑโ€ฒโ€ฒ) , (41) Which is called the B-series. IHJPAS. 37 (2) 2024 394 We present the following crucial lemma that is important to this derivation in order to accomplish the main goal of this research, which is the derivation of the order conditions of the RKTDIO technique. Lemma 3.1 [22,23]: Let ๐›ฟ be a function ๐›ฟ โˆถ ๐‘…๐‘‡ โˆช {โˆ…} โ†’ ๐‘…๐‘‘ with ๐›ฟ(โˆ…) = 1. Thus โ„Ž3๐œ‡(๐ต(๐›ฟ, ๐›ผ)) is also a B-series โ„Ž3๐œ‡(๐ต(๐›ฟ, ๐›ผ)) = ๐ต(๐›ฟโ€ฒ, ๐›ผ) where ๐›ฟโ€ฒ(โˆ…) = 0, ๐›ฟโ€ฒ(๐‘ก1) = 0, ๐›ฟโ€ฒ(๐‘ก2) = 0, ๐›ฟโ€ฒ(๐‘ก3) = 1, and for ๐œ = [๐œ1, ๐œ2, โ€ฆ , ๐œ๐‘š]3 โˆˆ ๐‘…๐‘‡ , ๐›ฟโ€ฒ(๐œ) = ๐›ฟ(๐œ1 ),. . . ๐›ฟ(๐œ๐‘š). Lemma 3.2 [22,23]: If we suppose that the analytic solution of (18) is a B-series ๐ต(๐œ—, ๐›ผ0) with a real function ๐œ— which is defined on ๐‘…๐‘‡ โˆช {โˆ…}, then ๐œ—(โˆ…) = 1, ๐œ—(๐‘ก1) = 1, ๐œ—(๐‘ก2) = 1, ๐œ—(๐‘ก3) = 1, And ๐œ = [๐œ1, ๐œ2, โ€ฆ , ๐œ๐‘š]3 โˆˆ ๐‘…๐‘‡, we have ๐œ—(๐œ) = 1 ๐œŒ(๐œ)(๐œŒ(๐œ) โˆ’ 1)(๐œŒ(๐œ) โˆ’ 2) (๐œ—(๐œ1),. . . ๐œ—(๐œ๐‘š)). Proposition 3.2.1 [22,23]: The density ๐œŽ(๐œ) is the non negative integer factors defined on trees ๐‘…๐‘‡ , โˆ€๐œ โˆˆ ๐‘…๐‘‡ satisfy 1- ๐œŽ(๐‘ก1) = 1, ๐œŽ(๐‘ก2) = 2 , ๐œŽ(๐‘ก3) = 6, 2- with ๐œ = [๐œ1, ๐œ2, โ€ฆ , ๐œ๐‘š]3, we have ๐œŽ(๐œ) = ๐œŒ(๐œ)(๐œŒ(๐œ) โˆ’ 1)(๐œŒ(๐œ) โˆ’ 2)(๐œŽ(๐œ1), โ€ฆ , ๐œŽ(๐œ๐‘š)). Proposition 3.2.2 [22,23]: The non-negative integer ํœ€(๐œ), โˆ€๐œ โˆˆ ๐‘…๐‘‡ satisfy 1- ๐œ–(๐‘ก1) = 1, ํœ€(๐‘ก2) = 1 , ๐œ–(๐‘ก3) = 1, 2- For the tree ๐œ = [๐œ1 ๐‘ฃ1 , โ€ฆ , ๐œ๐‘š ๐‘ฃ๐‘š] 3 โˆˆ ๐‘…๐‘‡, with distinct ๐œ๐‘– we have ํœ€(๐œ) = (๐œŒ(๐œ) โˆ’ 3)! โˆ 1 ๐‘ฃ๐‘– ( ๐œ€(๐œ๐‘–) ๐œŒ(๐œ๐‘–)!) ๐‘ฃ๐‘– ,๐‘š ๐‘–=1 where ๐‘ฃ๐‘– count similar tree of ๐œ๐‘–, ๐‘– = 1, . . . , ๐‘š. Therefore we can represent B-series (41) as follows: ๐ต(๐›ฟ, ๐›ผ) = ๐›ฟ(โˆ…) + โˆ‘ โ„Ž๐œŒ(๐œ) ๐œŒ(๐œ)! ๐›ฟ(๐œ) ํœ€(๐œ) ๐œŽ(๐œ) ๐œ‡(๐œ)(๐›ผ, ๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ).๐œโˆˆ๐‘…๐‘‡ (42) The previous analysis results in the following theorem. Theorem 3.1 [22,23]: The exact solution of (18) is a B-series ๐›ผ(๐‘ฅ0 + โ„Ž) = โˆ‘ โ„Ž๐œŒ(๐œ) ๐œŒ(๐œ)!๐œโˆˆ๐‘…๐‘‡ ํœ€(๐œ)๐•Œ(๐œ)(๐›ผ0, ๐›ผ0 โ€ฒ , ๐›ผ0 โ€ฒโ€ฒ), (43) And the first and second derivatives have the following B-series respectively, ๐›ผโ€ฒ(๐‘ฅ0 + โ„Ž) = โˆ‘ โ„Ž๐œŒ(๐œ)โˆ’1 (๐œŒ(๐œ)โˆ’1)!๐œโˆˆ๐‘…๐‘‡ ํœ€(๐œ)๐•Œ(๐œ)(๐›ผ0, ๐›ผ0 โ€ฒ , ๐›ผ0 โ€ฒโ€ฒ), (44) ๐›ผโ€ฒโ€ฒ(๐‘ฅ0 + โ„Ž) = โˆ‘ โ„Ž๐œŒ(๐œ)โˆ’2 (๐œŒ(๐œ)โˆ’2)!๐œโˆˆ๐‘…๐‘‡/[๐‘ก1] ํœ€(๐œ)๐•Œ(๐œ)(๐›ผ0, ๐›ผ0 โ€ฒ , ๐›ผ0 โ€ฒโ€ฒ) . (45) Lemma 3.3 [22,23]: We can calculate the function ๐œ‚๐‘–(๐œ) on โˆˆ ๐‘…๐‘‡/[๐‘ก1 , ๐‘ก2 ] recursively as 1- ๐œ‚๐‘–(๐‘ก3) = 1, 2- For the tree ๐œ = [๐œ1 ๐‘ฃ1 , ๐‘ก2 ๐‘ฃ2 , ๐‘ก3 ๐‘ฃ3 , . . . , ๐œ๐‘š ๐‘ฃ๐‘š]3 โˆˆ ๐‘…๐‘‡, with distinct ๐œ๐‘– , ๐‘– = 1, . . . , ๐‘š and different ๐‘ก1, and ๐‘ก2, ๐œ‚๐‘— = 1 2๐‘ฃ2 ๐‘๐‘— ๐‘ฃ1+2๐‘ฃ2 โˆ (โˆ‘ ๏ฟฝฬ‚๏ฟฝ๐‘—๐‘˜๐œ‚๐‘˜(๐œ๐‘–))๐‘ฃ๐‘–๐‘  ๐‘˜=1 ๐‘š ๐‘–=3 . Now, we denote the vector ๐œ‚(๐œ) = (๐œ‚1(๐œ)(๐œ),. . . , ๐œ‚๐‘ (๐œ)(๐œ))๐‘‡, โˆ€๐›ผ โˆˆ ๐‘…๐‘‡\ {๐‘ก1, ๐‘ก2 }. the initial weight associated to ๐›ผ๐‘›+1 is indicated by ๐œ•(๐œ) and is defined as follows: ๐œ•(๐œ) = โˆ‘ ๐‘ ๐‘– ๐œ‚๐‘–(๐œ)๐‘  ๐‘–=1 = ๐‘ ๐‘‡ ๐œ‚(๐œ), ๐œ•โ€ฒ(๐œ) is indicated to the initial weight associated with ๐›ผโ€ฒ ๐‘›+1 and is defined as follows: ๐œ•โ€ฒ(๐œ) = โˆ‘ ๐‘ ๐‘–โ€ฒ ๐œ‚๐‘–(๐œ)๐‘  ๐‘–=1 = ๐‘ โ€ฒ๐‘‡ ๐œ‚(๐œ), and the initial weight associated with ๐›ผโ€ฒโ€ฒ ๐‘›+1 indicated by ๐œ•โ€ฒโ€ฒ(๐œ) and is defined as follows: IHJPAS. 37 (2) 2024 395 ๐œ•โ€ฒโ€ฒ(๐œ) = โˆ‘ ๐‘ ๐‘–โ€ฒโ€ฒ ๐œ‚๐‘–(๐œ)๐‘  ๐‘–=1 = ๐‘ โ€ฒโ€ฒ๐‘‡ ๐œ‚(๐œ). As a result, we obtain the following fundamental theorem for the numerical solution and RKTDIO technique numerical derivatives. Theorem 3.2 [22,23]: When we apply the RKTDIO method (39) on the autonomous problem (18) yields the numerical solution ๐›ผ๐‘›+1 and numerical derivatives ๐›ผโ€ฒ๐‘›+1, ๐›ผโ€ฒโ€ฒ๐‘›+1 which have the B- series as follows: ๐›ผ๐‘›+1 = ๐›ผ๐‘› + โ„Ž๐›ผ๐‘› โ€ฒ + 1 2 โ„Ž2๐›ผ๐‘› โ€ฒโ€ฒ + โˆ‘ โ„Ž๐œŒ(๐œ) ๐œŒ(๐œ)!๐œโˆˆ ๐‘…๐‘‡ {๐‘ก1,๐‘ก2} ํœ€(๐œ)๐œŽ(๐œ)๐œƒ(๐œ). ๐•Œ(๐œ)(๐›ผ0, ๐›ผ0 โ€ฒ , ๐›ผ0 โ€ฒโ€ฒ), (45) ๐›ผ๐‘›+1 โ€ฒ = ๐›ผ๐‘› โ€ฒ + โ„Ž๐›ผ๐‘› โ€ฒโ€ฒ + โˆ‘ โ„Ž๐œŒ(๐œ)โˆ’1) ๐œŒ(๐œ)!๐œโˆˆ๐‘…๐‘‡/{๐‘ก1,๐‘ก2} ํœ€(๐œ)๐œŽ(๐œ)๐œƒโ€ฒ(๐œ). ๐•Œ(๐œ)(๐›ผ0, ๐›ผ0 โ€ฒ , ๐›ผ0 โ€ฒโ€ฒ), (46) ๐›ผ๐‘›+1 โ€ฒโ€ฒ = ๐›ผ๐‘› โ€ฒโ€ฒ + โˆ‘ โ„Ž๐œŒ(๐œ)โˆ’2) ๐œŒ(๐œ)!๐œโˆˆ๐‘…๐‘‡/{๐‘ก1,๐‘ก2} ํœ€(๐œ)๐œŽ(๐œ)๐œƒโ€ฒโ€ฒ(๐œ). ๐•Œ(๐œ)(๐›ผ0, ๐›ผ0 โ€ฒ , ๐›ผ0 โ€ฒโ€ฒ) . (47) 4. Algebraic order conditions We arrive at this paper's main contributionโ€”the order conditions of the RKTDIO method-through Theorems 3.1 and 3.2. In Table 1, the relevant-colored trees of orders up to six are listed together with the accompanying function values. Theorem 4.1: The RKTDIO method has order ๐‘ (๐‘ โ‰ฅ 3) if and only if it satisfies the following conditions. 1- ๐œƒ(๐œ) = 1 ๐œŽ(๐œ) , ๐œ โˆˆ โ‹ƒ ๐‘ ๐‘Ÿ ๐‘ ๐‘Ÿ=4 , (48) 2- ๐œƒโ€ฒ(๐œ) = ๐œŒ(๐œ) ๐œŽ(๐œ) , ๐œ โˆˆ โ‹ƒ ๐‘ ๐‘Ÿ ๐‘+1 ๐‘Ÿ=4 , (49) 3- ๐œƒโ€ฒโ€ฒ(๐œ) = ๐œŒ(๐œ)(๐œŒ(๐œ)โˆ’1) ๐œŽ(๐œ) , ๐œ โˆˆ โ‹ƒ ๐‘ ๐‘Ÿ ๐‘+2 ๐‘Ÿ=4 . (50) Even though some of the trees in the set RT provide the same order criteria and pertain to various elementary differentials, it is still unnecessary. In general, the following corollary, which may be derived from the definition of density and order, and from Lemma 3.3, can be used to overcome the similarity between the order criteria. Table 1. lists elementary differentials, relevant-colored trees of up to six orders, and related functions. Order ๐‰ Tree ๐œถ(๐‰) Density ๐’(๐‰) Elementary 0 โˆ… โˆ… 1 1 ๐›ผ 1 ๐‘ก1 1 1 ๐›ผโ€ฒ 2 ๐‘ก2 1 2 ๐›ผโ€ฒโ€ฒ 3 ๐‘ก3 1 6 ๐œ‡ 4 ๐œ41 1 24 ๐‘ ๐œ‡โ€ฒ๐›ผโ€ฒ IHJPAS. 37 (2) 2024 396 5 ๐œ51 1 60 ๐‘2 ๐œ‡โ€ฒ๐›ผโ€ฒโ€ฒ 5 ๐œ52 1 120 1 2 ๐‘2 ๐œ‡โ€ฒโ€ฒ(๐›ผโ€ฒ, ๐›ผโ€ฒ) 6 ๐œ61 3 120 ๐‘3 ๐œ‡โ€ฒโ€ฒ(๐›ผโ€ฒ, ๐›ผโ€ฒโ€ฒ) 6 ๐œ62 1 240 1 2 ๐‘3 ๐œ‡โ€ฒโ€ฒโ€ฒ(๐›ผโ€ฒ, ๐›ผโ€ฒ, ๐›ผโ€ฒ) 6 ๐œ63 1 720 1 6 ๐‘3 ๐œ‡โ€ฒ๐›ผโ€ฒโ€ฒ The order criteria for the RKTDIO technique up to the sixth order can be expressed as follows using Theorem 4.1 as a foundation: Order condition for ๐œถ order 3 โˆ‘ ๐‘˜๐‘– = 1 6 (51) Order 4 โˆ‘ ๐‘˜๐‘– ๐‘ ๐‘– = 1 24 (52) Order 5 โˆ‘ ๐‘˜๐‘–๐‘ ๐‘– 2 = 1 60 (53) Order 6 โˆ‘ ๐‘˜๐‘–๐‘ ๐‘– 3 = 1 120 , โˆ‘ ๐‘˜๐‘–๐‘Ž๐‘–๐‘— = 1 720 (54) Order condition for ๐œถโ€ฒ Order 2 โˆ‘ ๐‘˜โ€ฒ๐‘– = 1 2 (55) Order 3 โˆ‘ ๐‘˜๐‘–โ€ฒ ๐‘ ๐‘– = 1 6 (56) Order 4 IHJPAS. 37 (2) 2024 397 โˆ‘ ๐‘˜๐‘–โ€ฒ๐‘ ๐‘– 2 = 1 12 (57) Order 5 โˆ‘ ๐‘˜๐‘–โ€ฒ๐‘ ๐‘– 3 = 1 20 , โˆ‘ ๐‘˜๐‘–โ€ฒ๐‘Ž๐‘–๐‘— = 1 120 (58) Order 6 โˆ‘ ๐‘˜๐‘–โ€ฒ๐‘ ๐‘– 4 = 1 30 , โˆ‘ ๐‘˜๐‘–โ€ฒ๐‘Ž๐‘–๐‘—๐‘ ๐‘— = 1 720 , โˆ‘ ๐‘˜๐‘–โ€ฒ๐‘ ๐‘—๐‘Ž๐‘–๐‘— = 1 180 (59) Order condition for ๐œถโ€ฒโ€ฒ Order 1 โˆ‘ ๐‘˜๐‘– โ€ฒโ€ฒ = 1 (60) Order 2 โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ ๐‘ ๐‘– = 1 2 (61) Order 3 โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘ ๐‘– 2 = 1 3 (62) Order 4 โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘ ๐‘– 3 = 1 4 , โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘Ž๐‘–๐‘— = 1 24 (63) Order 5 โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘ ๐‘– 4 = 1 5 , โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘Ž๐‘–๐‘—๐‘ ๐‘— = 1 120 , โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘ ๐‘—๐‘Ž๐‘–๐‘— = 1 30 (64) Order 6 โˆ‘ ๐‘˜๐‘– โ€ฒโ€ฒ๐‘ ๐‘– 2๐‘Ž๐‘–๐‘— = 1 36 , โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘ ๐‘– 5 = 1 6 , โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘Ž๐‘–๐‘—๐‘ ๐‘— 2 = 1 360 , โˆ‘ ๐‘˜๐‘–โ€ฒโ€ฒ๐‘ ๐‘–๐‘Ž๐‘–๐‘—๐‘ ๐‘— = 1 144 (65) 5. The Construction of RKTDIO Method When creating implicit RKTDIO methods, the order conditions listed in Section 3.2 must be met. For the ๐‘ž-order RKTDIO method, the local truncated error is defined as follows: โ€–๐ฟ๐‘” (๐‘ž+1)โ€–2 = (โˆ‘ (๐ฟ๐‘– (๐‘ž+1) )2 + (โˆ‘ (๐ฟ๐‘–โ€ฒ (๐‘ž+1) )2 + (โˆ‘ (๐ฟ๐‘–โ€ฒโ€ฒ (๐‘ž+1) )2๐‘›โ€ฒโ€ฒ ๐‘ž+1 ๐‘–=1 ๐‘›โ€ฒ ๐‘ž+1 ๐‘–=1 ๐‘›๐‘ž+1 ๐‘–=1 (66) Where ๐ฟ(๐‘ž+1) , ๐ฟโ€ฒ(๐‘ž+1), ๐ฟโ€ฒโ€ฒ(๐‘ž+1) the local truncation error are terms respectively, ๐ฟ๐‘” (๐‘ž+1) is the global local truncation error. 5.1 A Three-Stage Fifth-Order RKTDIO Method In this subsection, the derivation of the three-stage RKTDIO technique of order five by using the algebraic order conditions up to order five will be considered. The resulting system consists of 16 nonlinear equations with 16 unknown variables, solving the system simultaneously, and assuming ๐‘Ž11 = ๐‘Ž22 and ๐‘Ž22 = ๐‘Ž33 ๐‘Ž21 = ๐‘Ž21, ๐‘Ž31 = ๐‘Ž21, ๐‘Ž32 = 3 20 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“ (5 โˆ’ ๐‘ง2 โˆ’ 3), ๐‘Ž33 = โˆ’ 5 9 ๐‘Ž21 โˆ’ 1 24 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(5 โˆ’ ๐‘ง2 โˆ’ 3) + 1 24 , ๐‘1 = 2 9 , ๐‘2 = 1 36 200 ๐‘Ž21๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(5โˆ’๐‘ง2โˆ’3)+15๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(5โˆ’๐‘ง2โˆ’3)+200 ๐‘Ž21+9 40 ๐‘Ž21+3 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“( 5โˆ’๐‘ง2โˆ’3) , ๐‘3 = โˆ’ 1 36 200 ๐‘Ž21๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(5โˆ’๐‘ง2โˆ’3)โˆ’15๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(5โˆ’๐‘ง2โˆ’3)+200 ๐‘Ž21+9 40 ๐‘Ž21+3 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“( 5โˆ’๐‘ง2โˆ’3) , ๐‘1 = 1 2 , ๐‘2 = 1 2 โˆ’ 1 2 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“( 5 โˆ’ ๐‘ง2 โˆ’ 3), ๐‘3 = 1 2 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“( 5 โˆ’ ๐‘ง2 โˆ’ 3) + 1 2 , ๐‘‘1 = 1 18 , ๐‘‘2 = 5 72 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“( 5 โˆ’ ๐‘ง2 โˆ’ 3) + 1 18 , ๐‘‘3 = โˆ’ 5 72 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“( 5 โˆ’ ๐‘ง2 โˆ’ 3) + 1 18 , ๐‘”1 = 4 9 , ๐‘”2 = 5 18 , ๐‘”3 = 5 18 . Next, we minimize the truncation error term by using minimize command in Maple. Thus, for the optimized value of coefficients in fractional we chose ๐‘Ž21 = โˆ’ 1 125 with this value โ€–๐œ‹๐‘” (5)โ€– 2 = IHJPAS. 37 (2) 2024 398 0.001366343866. Finally, all the parameters of three-stage fifth-order RKTDIO approach that will be denoted as RKTDIO5 can be written as follows (see Table 2): Table 2. The RKTDIO5 Method 1 2 83 1800 โˆ’ โˆš15 120 1 2 โˆ’ โˆš15 10 โˆ’ 1 125 83 1800 โˆ’ โˆš15 120 1 2 + โˆš15 10 โˆ’ 1 125 3โˆš15 100 83 1800 โˆ’ โˆš15 120 1 18 1 18 + โˆš15 72 1 18 โˆ’ โˆš15 72 2 9 5 36 + โˆš15 36 5 36 โˆ’ โˆš15 36 4 9 5 18 5 18 5.2 A Four-Stage RDTDIO Method of Order Six For the four-stage RKTDIO technique of order six, the algebraic conditions up to order six will be solved. The resulting system consists of 25 nonlinear equations with 23 unknown variables, and using simplifying assumption ๐‘๐‘– โ€ฒ = ๐‘๐‘– โ€ฒโ€ฒ(1 โˆ’ ๐‘๐‘–), ๐‘– = 1, โ€ฆ , ๐‘ , and supposing ๐‘2 โ€ฒโ€ฒ = 0, and ๐‘2 โ€ฒโ€ฒโ€ฒ = 0 ๐‘Ž21 = 3 20 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“ (10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1) โˆ’ 3 80 , ๐‘Ž31 = โˆ’ 1 40 , ๐‘Ž32 = 11 80 โˆ’ 3 20 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1), ๐‘Ž41 = 1 40 , ๐‘Ž42 = โˆ’ 1 40 , ๐‘Ž43 = 3 20 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1) โˆ’ 3 80 , ๐‘Ž44 = 1 48 , ๐‘1 = 2 9 , ๐‘2 = 0, ๐‘3 = 5 18 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1), ๐‘4 = โˆ’ 5 18 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1) + 5 36 , ๐‘1 = 1 2 , ๐‘2 = ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1), ๐‘3 = 1 โˆ’ ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1), ๐‘4 = ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1), ๐‘‘1 = 1 18 , ๐‘‘2 = ๐‘‘2, ๐‘‘3 = 5 36 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1) โˆ’ 1 72 , ๐‘‘4 = โˆ’๐‘‘2 โˆ’ 5 36 ๐‘…๐‘œ๐‘œ๐‘ก ๐‘‚๐‘“(10 โˆ’ ๐‘ง2 โˆ’ 10 โˆ’ ๐‘ง + 1) โˆ’ 1 8 , ๐‘”1 = 4 9 , ๐‘”2 = 0, ๐‘”3 = 5 18 , ๐‘”4 = 5 18 Lastly, all the parameters of four-stage sixth-order RKTDIO method indicated by RKTDIO6 can be written as follows: Table 3. The RKTDIO6 Method. 1 2 1 48 1 2 โˆ’ โˆš15 10 3 80 โˆ’ 3โˆš15 200 1 48 1 2 + โˆš15 10 โˆ’ 1 40 1 16 + 3โˆš15 200 1 48 1 2 โˆ’ โˆš15 10 1 40 โˆ’ 1 40 3 80 โˆ’ 3โˆš15 200 1 18 + โˆš15 72 IHJPAS. 37 (2) 2024 399 1 18 0 1 18 โˆ’ โˆš15 72 1 18 โˆ’ โˆš15 72 2 9 0 5 36 โˆ’ โˆš15 36 5 36 + โˆš15 36 4 9 0 5 18 5 18 6. Derivation Embedded ERKTDIO6(5) Method The RKTDIO method with ๐”ฐ-stages for solving equation (1) is presented in its general form. Subsequently, the development of the embedded pair RK approach is discussed, which is an area of active research aimed at improving existing codes. To estimate the minimum error, implicit RKTDIO techniques are used, employing pairs of ๐’ซ(๐’ฌ) orders in the values of step size codes. These methods are based on the ๐’ซ-order method (๐ถ, ๐ด, ๐‘, ๐‘โ€ฒ, ๐‘โ€ฒโ€ฒ) and the ๐’ฌ-order method (๐ถ, ๐ด, ๏ฟฝฬˆ๏ฟฝ, ๏ฟฝฬˆ๏ฟฝโ€ฒ, ๏ฟฝฬˆ๏ฟฝโ€ฒโ€ฒ), and can be represented using the Butcher Tabular notation. The embedded pair can be initialized in the following manner: C A ๐‘ ๐‘‡ ๐‘โ€ฒ ๐‘‡ ๐‘โ€ฒโ€ฒ ๐‘‡ ๏ฟฝฬˆ๏ฟฝ ๐‘‡ ๏ฟฝฬˆ๏ฟฝโ€ฒ ๐‘‡ ๏ฟฝฬˆ๏ฟฝโ€ฒโ€ฒ ๐‘‡ The main objective of constructing the embedded pair of implicit RKTDIO techniques is to obtain a single error estimate that can be used in step-size approaches. This is achieved by improving the existing pairs and local error estimates and then restricting the step size ๐“€. ๐’ฝ๐‘›+1 = 0.9๐“€๐‘›( ๐‘‡๐‘œ๐‘™ ๐ฟ๐‘‡๐ธ ) 1 ๐’ฌ+1 (67) A safety factor of 0.9 is used to determine the local error estimate at each step, and ๐‘‡๐‘œ๐‘™ represents the maximum allowable local error that ensures the necessary accuracy. If the local truncation error (LTE) is less than or equal to ๐‘‡๐‘œ๐‘™, the step is accepted and the higher order method (or local extrapolation) is used, where a more accurate approximation is applied to drive the integration and update ๐“€ using Equation (67). On the other hand, if LTE is greater than ๐‘‡๐‘œ๐‘™, the step is rejected and the step size ๐“€ is reduced by half. The RKTDIO method is an embedded Runge- Kutta method developed for solving third-order ODEs. To ensure high accuracy for the higher- order method and the most accurate error estimates for the lower-order methods, fractions were used to develop orders 6 and 5, respectively. The step size ๐“€ plays a crucial role in obtaining accurate results and can be doubled to achieve this goal. The embedded RKTDIO6(5) method is derived in Table 4 for this study. In RKTDIO6(5), the ๐ด and ๐ถ values is computed from the 6th-order solution then derived the four-stage 5rd-order embedded equation. Solving of the eqs. (51-53), (55-58), and (60-64) IHJPAS. 37 (2) 2024 400 simultaneously then the solution for ๏ฟฝฬˆ๏ฟฝ and ๏ฟฝฬˆ๏ฟฝโ€ฒ while ๏ฟฝฬˆ๏ฟฝโ€ฒโ€ฒ have the same values as the 6th -order. The solutions are obtained as ๏ฟฝฬˆ๏ฟฝ1 โ€ฒ = 2 9 , ๏ฟฝฬˆ๏ฟฝ2 โ€ฒ = 0, ๏ฟฝฬˆ๏ฟฝ3 โ€ฒ = 5 36 โˆ’ โˆš15 36 , ๏ฟฝฬˆ๏ฟฝ4 โ€ฒ = 5 36 + โˆš15 36 , ๏ฟฝฬˆ๏ฟฝ1 = 1 18 , ๏ฟฝฬˆ๏ฟฝ2 = 1 18 + โˆš15 72 , ๏ฟฝฬˆ๏ฟฝ3 = 1 18 โˆ’ โˆš15 72 , ๏ฟฝฬˆ๏ฟฝ4 = ๏ฟฝฬˆ๏ฟฝ4, ๏ฟฝฬˆ๏ฟฝ1 โ€ฒโ€ฒ = 4 9 , ๏ฟฝฬˆ๏ฟฝ2 โ€ฒโ€ฒ = 0, ๏ฟฝฬˆ๏ฟฝ3 โ€ฒโ€ฒ = 5 18 , ๏ฟฝฬˆ๏ฟฝ4 โ€ฒโ€ฒ = 5 18 . The following simplifying assumption is used in order to reduce the number of equations to be solved: ๐‘๐‘– โ€ฒ = ๐‘๐‘– โ€ฒโ€ฒ(1 โˆ’ ๐‘๐‘–), ๐‘– = 1, โ€ฆ , ๐”ฐ. (68) Initially, we have 16 nonlinear equations with 12 unknown variables that we need to find a solution for. However, as the number of equations is greater than the number of unknowns, there is no solution. To tackle this issue, we make the simplifying assumption (68) which reduces the number of equations to 12 with 11 unknowns, enabling us to solve the system. We choose ๏ฟฝฬˆ๏ฟฝ = 1 10 as the free parameter, and as a result, we can express the coefficients of the 4-stage embedded ERKTDIO6(5) technique. Table 4: Table of ERKTDIO6(5) method. 7. Numerical Experiments 7.1 Constant Methods In order to evaluate the performance of the new RKTDIO methods to the established RK methods in the scientific literature, a series of test problems are addressed in this part. The following methods have been selected for comparison: ๏‚ท RKTDIO5: the implicit RKTDIO method of order five with three-stage derived in this paper. ๏‚ท RKTDIO6: the implicit RKTDIO method of order six with four-stage derived in this paper. IHJPAS. 37 (2) 2024 401 ๏‚ท DITRKM5: three-stage fifth-order implicit RK derived by [25]. ๏‚ท Radau I: three-stage fifth-order implicit RK method presented in [28]. ๏‚ท Radau IA: three-stage fifth-order implicit RK method presented in [24]. ๏‚ท Lobatto III: The sixth-order four-stage implicit Rungeโ€“Kutta method as given by [30]. ๏‚ท Lobatto IIIB: The sixth-order four-stage implicit Rungeโ€“Kutta method as given by [29]. Problem (1): Consider a nonhomogeneous linear ODE given in [22] ฮฑโ€ฒโ€ฒโ€ฒ(x) = ฮฑ(x) + cos(x), With ๐›ผ(0) = 0 , ๐›ผโ€ฒ(0) = 0, ๐›ผโ€ฒโ€ฒ(0) = 1 Where x โˆˆ [0,1], and analytic solution ๐›ผ(๐‘ฅ) = (๐‘’๐‘ฅโˆ’cos(๐‘ฅ)โˆ’sin(๐‘ฅ)) 2 . Problem (2): Consider the nonhomogeneous nonlinear ODE ฮฑโ€ฒโ€ฒโ€ฒ(x) = (ฮฑ(x))2 + cos2(x) โˆ’ cos(x) โˆ’ 1 , With ๐›ผ(0) = 0, ๐›ผโ€ฒ(0) = 1, ๐›ผโ€ฒโ€ฒ(0) = 1 ๐‘คโ„Ž๐‘’๐‘Ÿ๐‘’ 0 โ‰ค ๐‘ฅ โ‰ค 2 , and analytic solution ๐›ผ(๐‘ฅ) = sin (๐‘ฅ). Problem (3): The nonhomogeneous nonlinear ODEs is considered as ฮฑโ€ฒโ€ฒโ€ฒ(x) = 8( ๐›ผ2(๐‘ฅ) ๐‘’2๐‘ฅ ) With ๐›ผ(0) = 1, ๐›ผโ€ฒ(0) = 2 , ๐›ผโ€ฒโ€ฒ(0) = 4 ๐‘คโ„Ž๐‘’๐‘Ÿ๐‘’ 0 โ‰ค ๐‘ฅ โ‰ค 1 , and analytic solution ๐›ผ(๐‘ฅ) = ๐‘’2๐‘ฅ . Figure 2. Accuracy curve for RKTDIO5, DITRKM5, Radau I, Radau IA with โ„Ž = 0.1, 0.05, 0.025, 0.00125, 0.00625 for Problem1. IHJPAS. 37 (2) 2024 402 Figure3. Accuracy curve for RKTDIO5, DITRKM5, Radau I, Radau IA with โ„Ž = 0.1, 0.05, 0.025, 0.00125, 0.00625 for Problem2. Figure4. Accuracy curve for RKTDIO5, DITRKM5, Radau I, Radau IA with โ„Ž = 0.1, 0.05, 0.025, 0.00125, 0.00625 for Problem3. Figure 5. Accuracy curve for RKTDIO6, Lobattoo III, Lobattoo IIIB with โ„Ž = 0.1, 0.05, 0.025, 0.00125, 0.00625 for Problem1. IHJPAS. 37 (2) 2024 403 Figure 6. Accuracy curve for RKTDIO6, Lobattoo III, Lobattoo IIIB with โ„Ž = 0.1, 0.05, 0.025, 0.00125, 0.00625 for Problem2. Figure 7. Accuracy curve for RKTDIO6, Lobattoo III, Lobattoo IIIB with โ„Ž = 0.1, 0.05, 0.025, 0.00125, 0.00625 for Problem3. 7.2 Variable Method This subsection will apply the new embedded method to the same third-order differential equation problems in the previous subsection. The following implicit diagonally RK method is selected for the numerical comparisons. The approximation results are illustrated in the tables below for solving problems. The following abbreviations will be used in the tables: ๏‚ท ๐‘ป๐’๐’: Tolerance. ๏‚ท Method: method employed step sizes between two points or positions. ๏‚ท F. N: number of the function call. ๏‚ท STEP: The number of successful steps. ๏‚ท FSTEP: The number of failed steps. ๏‚ท Time: execution time. ๏‚ท ERKTDIO6(5): The novel embedded 6(5) derived in this study. ๏‚ท EDITRK5(4): The embedded diagonally implicit 5(4) RK derived in [7]. IHJPAS. 37 (2) 2024 404 Table 4. Comparisons of number of function call and Time of ERKTDIO6(5) and EDITRK 5(4) with h = 10โˆ’6, 10โˆ’8, 10โˆ’10 for the problem 1. FSTEP Step Time No. of Function Call Method ๐‘ป๐‘ถ๐‘ณ(๐“ด) 2 5 0.025 270 ERKTDIO6(5) 10โˆ’6 0 13 0.033 421 EDITRK 5(4) 2 16 0.038 1210 ERKTDIO6(5) 10โˆ’8 0 40 0.047 1962 EDITRK 5(4) 2 72 0.060 6353 ERKTDIO6(5) 10โˆ’10 1 128 0.074 9105 EDITRK 5(4) Table 5. Comparisons of number of function call and Time of EDITRKM 4(3) and EDITRKM 5(4) with h = 10โˆ’2, 10โˆ’4, 10โˆ’6 for the problem 2. FSTEP Step Time No. of Function Call Method ๐‘ป๐‘ถ๐‘ณ(๐“ด) 0 3 0.032 183 ERKTDIO6(5) 10โˆ’6 0 7 0.062 166 EDITRK 5(4) 0 8 0.045 662 ERKTDIO6(5) 10โˆ’8 0 22 0.083 759 EDITRK 5(4) 0 29 0.080 2533 ERKTDIO6(5) 10โˆ’10 1 69 0.119 3495 EDITRK 5(4) Table 6. Comparisons of number of function call and Time of EDITRKM 4(3) and EDITRKM 5(4) with h = 10โˆ’2, 10โˆ’4, 10โˆ’6 for the problem 3. FSTEP Step Time No. of Function Call Method ๐‘ป๐‘ถ๐‘ณ(๐“ด) 1 97 0.012 466 ERKTDIO6(5) 10โˆ’6 0 29 0.035 882 EDITRK 5(4) 1 315 0.031 2511 ERKTDIO6(5) 10โˆ’8 1 152 0.059 4101 EDITRK 5(4) 1 1260 0.075 13325 ERKTDIO6(5) 10โˆ’10 2 725 0.095 19044 EDITRK 5(4) IHJPAS. 37 (2) 2024 405 Problem 1 Problem 2 Problem 3 Figure 8: Accuracy curve for ERKTDIO6(5) and EDITRK5(4) with โ„Ž = 10โˆ’6, 10โˆ’8, 10โˆ’10 . IHJPAS. 37 (2) 2024 406 8. Discussions The objective of our research was to directly solve third-order ordinary differential equations (ODEs) using the RKTDIO approach, which is based on the algebraic theory of order conditions. Previous research has primarily concentrated on defining algebraic order conditions and B-series theory to solve first- and second-order ordinary differential equations (ODEs). However, we were inspired to expand upon these ideas and create the RKTDIO formula specifically designed for third-order ODEs. As a result, the RKTDIO5 and RKTDIO6 methods were developed, offering improved computing efficiency and accuracy for solving certain third-order ODEs as compared to other current approaches. In addition, we shared the results of our study on the embedded diagonal implicit type Runge- Kutta technique (ERKTDIO). We assessed the performance of our method (ERKTDIO6(5)) in comparison to other approaches by examining the decimal logarithm of the highest time curve and the logarithm of the function call estimates obtained from Tables 4-6. We utilised three separate test problems and calculated the logarithm of the time curve utilising various tolerance values. Figure 10 was generated using the numerical data obtained from Tables 4-6. It presents the number of successful and failed steps that occurred during the calculations. Our work has enhanced and refined the method by transitioning from an explicit to an implicit approach and from a direct to a diagonal scheme. 9. Conclusions Ultimately, our research has made a significant contribution to the progress of numerical methods for solving third-order ordinary differential equations. The introduction of the RKTDIO5 and RKTDIO6 techniques provides improved computational efficiency and precision for particular third-order ordinary differential equations (ODEs). Furthermore, our examination of the ERKTDIO technique showcases its efficacy in comparison to alternative methods, especially in terms of the number of successful steps in computations. Our methods provide the potential for enhanced solutions of third-order ODEs by adopting implicit and diagonal schemes. In summary, these discoveries expand the current understanding of numerical analysis and computational mathematics, offering useful insights for future investigations in this field. Acknowledgment The authors have greatly appreciated the referees for their valuable comments and suggestions for improving the paper. 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