On the efficiency of high-order difference schemes for the Schro¨dinger equation

Исследуется ряд двух- и трехслойных разностных схем, построенных на расширенных шаблонах, до восьмого порядка точности для уравнения Шрёдингера. Наряду с многоточечными схемами рассматривается метод коррекции Ричардсона в приложении к схеме четвертого порядка аппроксимации, повышающий порядок точности путем построения линейных комбинаций приближенных решений, полученных на различных вложенных сетках. Проведено сравнение методов по устойчивости, сложности реализации алгоритмов и объему вычислений, необходимых для достижения заданной точности. На основе теоретического анализа и численных экспериментов выявлены методы, наиболее эффективные для практического применения The efficiency of difference methods for solving problems of nonlinear wave optics is largely determined by the order of accuracy. Schemes up to the fourth order of accuracy have the traditional architecture of three-point stencils and standard conditions for the application of algorithms. However, a further increase in the order in the general case is associated with the need to expand the stencils using multipoint difference approximations of the derivatives. The use of such schemes forces formulating additional boundary conditions, which are not present in the differential problem, and leads to the need to invert the matrices of the strip structure, which are different from the traditional tridiagonal ones. An exception is the Richardson correction method, which is aimed at increasing the order of accuracy by constructing special linear combinations of approximate solutions obtained on various nested grids according to traditional structure schemes. This method does not require the formulation of additional boundary conditions and inversion of strip matrices. In this paper, we consider several explicit and implicit multipoint difference schemes up to the eighth order of accuracy for the Schr¨odinger equation. In addition, a simple and double Richardson correction method is also investigated in relation to the classical fourth-order scheme. A simple correction raises the order to sixth and a double correction to eighth. This large collection of schemes is theoretically compared in terms of their properties such as the order of approximation, stability, the complexity of the implementation of a numerical algorithm, and the amount of arithmetic operations required to achieve a given accuracy. The theoretical analysis is supplemented by numerical experiments on the selected test problem. The main conclusion drawn from the research results is that of all the considered schemes, the Richardson-corrected scheme is the most preferable in terms of the investigated properties

Improving the accuracy for numerical solutions of the Ginzburg - Landau equation

Решение актуальной задачи повышения порядка точности разностных методов решения задач нелинейной волоконной оптики выше четвертого путем непосредственного построения сложных схем на расширенных шаблонах сопряжено с усложнением матрицы системы и с затруднениями в постановке дополнительных граничных условий. Кроме того, при таком подходе не происходит одновременное повышение точности также и по эволюционной переменной. В данной работе рассматривается альтернативный путь - применение экстраполяции Ричардсона, которая сводится к построению подходящих линейных комбинаций решений на различных сетках. Этот способ позволяет повышать порядок точности по обеим переменным, избегая при этом проблем с усложнением шаблонов, постановкой дополнительных граничных условий и реализацией алгоритмов. Как средство дополнительного улучшения точности наряду с простыми (однократными) поправками исследуются также двойные поправки на основе экстраполяции Ричардсона. Методика протестирована на нескольких точных решениях уравнения Гинзбурга - Ландау Increasing the order of accuracy for difference methods is an actual problem in nonlinear fiber optics. Computations, which use higher than the fourth order of accuracy by the direct construction of complex circuits on extended templates pose the complication of the system matrix and difficulties in setting additional boundary conditions. In addition, with this approach, there is no simultaneous increase in accuracy for the evolutionary variable. In this paper, we consider an alternative way, namely, application of the Richardson extrapolation, which reduces to construction of suitable linear combinations for solutions on various grids. This method allows improving the order of accuracy for both variables, while avoiding problems associated with the complication of templates, implementation of algorithms and setting additional boundary conditions. Double corrections are also considered to further improve accuracy. The technique was tested on exact solutions of the Ginzburg - Landau equation

Method of additional unknown functions in heat conductivity problems with variable physical properties of the medium

Finding analytical solutions to the problems of thermal conductivity with variable physical properties of the medium by classical analytical methods is very complicated mathematically. The known expressions repre-senting complex infinite series including two types of Bessel functions and gamma-functions are, in fact, numerical as they require a numerical solution to complex transcendental equations with eigenvalues of the boundary problem. Such solutions can hardly be used in engineering applications, especially in cases when a solution to a certain problem is only an intermediate stage in other problems (such as thermoelasticity and control problems, inverse problems, etc.) which can be solved effectively only by finding analytical solutions to the initial problems. Therefore, an urgent problem now is to develop new methods of obtaining analytical solutions to the abovementioned problems, at least approximate ones. The study employed methods of additional boundary conditions and additional unknown functions in the integral method of heat balance. High-precision approximate analytical solutions to the transient heat conduction problem with nonhomogeneous physical properties of the medium for an infinite plate under symmetric boundary conditions of the first type have been obtained. The initial problem for partial differential equations is reduced to two problems in which ordinary differential equations are integrated. Additional boundary conditions are defined in such a way that their fulfillment in accordance with the new method is equivalent to the result of solving the initial partial differential equation at the boundary points and at the temperature perturbation front (for the first stage of the process). By combining methods with finite and infinite heat propagation rate we have been able to obtain high-precision analytical solutions for the whole time range of the unsteady process including its small and ultra small values. The solutions look like simple algebraical polynomials not including special functions (Bessel, Legendre, gamma-functions and others). Since it is not necessary to directly integrate the initial equations by the space variable and to reduce them to ordinary differential equations with additional unknown functions, the considered method can be used for solving complex boundary problems in which differential equations do not allow distinguishing between the variables (into nonlinear, with linear boundary conditions and heat sources, etc.).