A Two-Step Modified Explicit Hybrid Method with Step-Size-Dependent Parameters for Oscillatory Problems

A new two-step modified explicit hybrid method with parameters depending on the step-size is constructed. This method is derived using the coefficients from a sixth-order explicit hybrid method with extended interval of absolute stability and then imposed each stage of the modified formula to exactly integrate the differential equations with solutions that can be expressed as linear combinations of sinwx and coswx, where w is the known frequency. Numerical results show the advantage of the new method for solving oscillatory problems.


Introduction
Second-order ordinary differential equations are important tools for modelling physical phenomena in science and engineering.
is paper is concerned with the numerical solution of the second-order ordinary differential equations of the form having oscillatory solutions. ese problems can be numerically solved by general-purpose methods or any other methods specially adapted to the structure of the intended problem. In the case of adapted numerical methods, particular algorithms have been proposed by several authors, see [1][2][3] to solve these classes of problems.
Franco [4] has established the following class of explicit hybrid methods: a ij f x n + c j h, Y j , i � 3, . . . , s, where h is the step-size while f n− 1 and f n represent f(x n− 1 , y n− 1 ) and f(x n , y n ), respectively. e associated Butcher tableau for this class of methods is given by Kalogiratou et al. [5] have modified each stage of the explicit hybrid methods and the improved version is e coefficients σ i and μ i are functions of v � wh, where w is the known frequency of the second-order problems.
e Butcher tableau for the modified hybrid method is given by We develop a sixth-order explicit hybrid method with extended interval of absolute stability based on the class of explicit hybrid methods (2). Using the coefficients from the sixth-order hybrid method, we develop a new modified hybrid method. e construction of hybrid methods is described in Section 2. In Section 3, we give the stability analysis of the class of modified hybrid method (5). Numerical results are presented in Section 4 for several secondorder problems.

Construction of Hybrid Methods
In this section, we derive the new method with four stages.

Sixth-Order Explicit Hybrid Method.
Consider the explicit hybrid methods (2). e associated Butcher tableau for a class of four-stage explicit hybrid methods is given by e sixth-order explicit hybrid method must satisfy the order conditions for a sixth-order hybrid method as stated in [6]. Solving the order conditions, we obtain Next, we choose the free parameter c 3 � (7731/10000) to maximize the interval of absolute stability. For detail explanation on stability properties of hybrid methods, refer [4].
e resulting method has a phase-lag of order 6 and a dissipation error of order 7. e interval of absolute stability of this method is (0, 4.54).

e New Method with Parameters Depending on
Step-Size.
is method is derived using the coefficients from the sixth-order explicit hybrid method in Section 2.1. Consider the modified four-stage explicit hybrid method represented by this tableau: (9) Associate each formula stage of the modified fourstage explicit hybrid method with the following linear operators: (10) Using the coefficients from the sixth-order explicit hybrid method and imposing the linear operators to exactly integrate the set {sin(wx) and cos(wx)}, we get and For small v, coefficients σ i and μ i may cause heavy calculations which lead to inaccuracy; hence, it is often convenient to use Taylor expansions for the coefficients. e resulting method is denoted by MEHM6.

Stability Analysis
In this section, we present the stability analysis of the modified hybrid method. Assume that H = λh, e = (1, 1, . . . ,1) T , σ(v) = (0, 0, σ 3 , σ 4 , . . ., σ s ) T , and µ(v) = (0, 0, µ 3 , µ 4 , . . . , µ s ) T . Employing the hybrid methods defined by (5) to the standard equation gives us where and the symbol "×" denotes component-wise multiplication. e characteristic polynomial which determines the solution (12) is Definition 1 (see [5]). For the hybrid methods corresponding to the characteristic equation (13) and v � wh, the region in the H-v plane, such that Journal of Mathematics is called the region of absolute stability of the method. e region of stability of the new method is shown in Figure 1.
It is observed that, if v � 0, then the region of absolute stability collapses into the interval of absolute stability.

Numerical Results
e new and existing methods are coded using Microsoft Visual C++ version 6.0 software and applied to some special second-order problems to provide numerical comparisons of the accuracy and execution time of the methods. e accuracy of the methods is measured by maximum global errors, while execution time (in seconds) is measured after the computation of the starting values. For all codes, the starting values are computed using the exact solution formula of each problem. e abbreviations of the codes are as follows: (i) MEHM6: the modified sixth-order explicit hybrid method with four stages derived in this paper. (ii) TRIMHLI: trigonometrically fitted multistep hybrid method proposed in [7]. (iii) TRIEFW: two-step trigonometrically fitted explicit hybrid method with four stages derived in [8].
Tables 1-5 show the numerical results of the new and existing methods for solving several second-order problems.
with e being the eccentricity of the orbit. e theoretical solution of this problem is where R satisfies the Kepler's equation x � R − e sin(R). In this paper, the eccentricity value is chosen to be e � 0. For all codes, v � h is used.
Problem 4 (nonlinear oscillatory problem) Problem 5 (the almost periodic problem)  It is observed from Table 1 that MEHM6 solves Problem  1 with very close accuracy to TRIMHL1. From the results in  Tables 3 and 5, MEHM6 gives the best accuracy as compared  to the other codes for most of the step-sizes, while in Table 2, MEHM6 is the most accurate for bigger step-sizes. For smaller step-sizes, the accuracy of MEHM6 is close to TRIEFW as shown in Table 2. Table 4 shows that both MEHM6 and TRIEFW codes have the same order of accuracy for all step-sizes.
On the other hand, TRIMHLI has the shortest execution time for all problems considered. is is mainly due to the fact that TRIMHLI has more starting values than that for MEHM6. Hence, less number of integration steps is needed by TRIMHLI to advance the computation as compared to MEHM6.

Conclusions
In this paper, a new two-step modified explicit hybrid method is developed where each stage of the modified method exactly integrates differential equations with solutions that are linear combinations of sin(wx) and cos(wx). From the numerical results, the new method gives the best accuracy when compared with the multistep methods in [7,8], particularly for linear oscillatory and almost periodic problems. e new method has two starting values, but the execution time is nevertheless acceptable. Hence, the new method is as competitive as the existing methods for solving oscillatory problems.

Data Availability
e maximum global error and execution time data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.