


Vol 59, No 2 (2019)
- Year: 2019
- Articles: 15
- URL: https://journal-vniispk.ru/0965-5425/issue/view/11229
Article
Numerical Analysis of Initial-Boundary Value Problem for a Sobolev-Type Equation with a Fractional-Order Time Derivative
Abstract
The paper is concerned with initial-boundary value problems for a Sobolev-type equation with a Gerasimov–Caputo fractional derivative with memory effect. A priori estimates of the solutions are obtained in the differential and difference forms, which imply their uniqueness and stability with respect to the initial data and the right-hand side, as well as the convergence of the solution of the difference problem to the solution of the differential problem.



A New Algorithm for a Posteriori Error Estimation for Approximate Solutions of Linear Ill-Posed Problems
Abstract
A new algorithm for a posteriori estimation of the error in solutions to linear operator equations of the first kind in a Hilbert space is proposed and justified. The algorithm reduces the variational problem of a posteriori error estimation to two special problems of maximizing smooth functionals under smooth constraints. A finite-dimensional version of the algorithm is considered. The results of a numerical experiment concerning a posteriori error estimation for a typical inverse problem are presented. It is shown experimentally that the computation time required by the algorithm is less, on average, by a factor of 1.4 than in earlier proposed methods.



Tensor Trains Approximation Estimates in the Chebyshev Norm
Abstract
A new elementwise bound on the cross approximation error used for approximating multi-index arrays (tensors) in the format of a tensor train is obtained. The new bound is the first known error bound that differs from the best bound by a factor that depends only on the rank of the approximation \(r\) and on the dimensionality of the tensor \(d\), and the dependence on the dimensionality at a fixed rank has only the order \({{d}^{{{\text{const}}}}}\) rather than constd. Thus, this bound justifies the use of the cross method even for high dimensional tensors.



Optimization Method for Axisymmetric Problems of Electric Cloaking of Material Bodies
Abstract
Inverse problems of designing spherical shells intended for cloaking material bodies in a stationary electric field are studied. By applying an optimization method, these problems are reduced to optimization ones in which the role of controls is played by components of the diagonal electrical conductivity tensor of the shell’s anisotropic material. The solvability of the direct and optimization problems is proved. A numerical algorithm based on particle swarm optimization is proposed for solving optimization problems, and numerical results are discussed.



Multimethod Optimization of Control in Complicated Applied Problems
Abstract
An algorithm consisting of gradient and quasilinearization iterations is constructed for obtaining a high-accuracy numerical solution of a boundary value problem. An “ideal” solution of a multiobjective optimal control problem is produced by applying primal and dual algorithms, which ensure an efficient search for both scalarization coefficients and an optimal control. The efficiency of the proposed multimethod algorithms is demonstrated by soling application problems.



Algorithm for Solving the Cauchy Problem for One Infinite-Dimensional System of Nonlinear Differential Equations
Abstract
The Cauchy problem for an infinite-dimensional system of nonlinear evolution equations, which is a generalization of the Langmuir chain, is considered. The global solvability of the problem in the class of rapidly decreasing functions is established. By the inverse spectral method, an algorithm for constructing the solution is obtained.



On the Solvability of a Boundary Value Problem for the Ordinary Fredholm Integrodifferential Equation with a Degenerate Kernel
Abstract
The problems of the existence and construction of solutions of a nonlocal boundary value problem for the homogeneous second-order Fredholm integrodifferential equation with a degenerate kernel and with two spectral parameters are considered. The singularities arising from the definition of arbitrary (unknown) constants are studied. The values of the spectral parameters are calculated and the solvability of the boundary value problem is established. The corresponding theorems are proven. Meaningful examples are provided.



Estimates in Hölder Classes for the Solution of an Inhomogeneous Dirichlet Problem for a Singularly Perturbed Homogeneous Convection–Diffusion Equation
Abstract
An inhomogeneous Dirichlet boundary value problem for a singularly perturbed homogeneous convection–diffusion equation with constant coefficients is considered in a half-plane. Convection is assumed to be directed orthogonally to the half-plane boundary away from it. Assuming that the boundary function is from the space \({{C}^{{2,\lambda }}}\), \(0 < \lambda < 1\), an unimprovable estimate for the solution bounded at infinity is obtained in the appropriate Hölder norm.



Alternating Triangular Schemes for Second-Order Evolution Equations
Abstract
Schemes of the Samarskii alternating triangular method are based on splitting the problem operator into two operators that are conjugate to each other. When the Cauchy problem for a first-order evolution equation is solved approximately, this makes it possible to construct unconditionally stable two-component factorized splitting schemes. Explicit schemes are constructed for parabolic problems based on the alternating triangular method. The approximation properties can be improved by using three-level schemes. The main possibilities are indicated for constructing alternating triangular schemes for second-order evolution equations. New schemes are constructed based on the regularization of the standard alternating triangular schemes. The features of constructing alternating triangular schemes are pointed out for problems with many operator terms and for second-order evolution equations involving operator terms for the first time derivative. The study is based on the general stability (well-posedness) theory for operator-difference schemes.



Classical and Generalized Solutions of a Mixed Problem for a Nonhomogeneous Wave Equation
Abstract
A.N. Krylov’s ideas concerning convergence acceleration for Fourier series are used to obtain explicit expressions for the classical solution of a mixed problem for a nonhomogeneous equation and explicit expressions for a generalized solution in the case of arbitrary summable \(q(x)\), \(\varphi (x)\), \(\psi (x)\), and \(f(x,t)\).



Simulation of Mean Curvature Flows on Surfaces of Revolution
Abstract



Solution of the Equation for Oil Spills Spreading over the Sea Surface by the Characteristic Method
Abstract
A model of oil spill spreading described by partial differential equations based on the balance of the forces acting on an axisymmetric slick is proposed. The aim of the work is to construct a numerical solution method capable of correctly describing the motion of the spill boundary. The originality of the proposed approach lies in the fact that, on the one hand, it offers a technique for the numerical solution of the spill spread equation on the basis of a characteristic method that can describe the motion of the spill boundary and, on the other hand, the numerical solution is shown to agree with Fay’s formulas based on observations.



On the Calculation of the Interaction Potential in Multiatomic Systems
Abstract
A numerical method for finding the potential of a multiatomic system in the real space is proposed. A distinctive feature of this method is the decomposition of the electron density \(\rho \) and the potential \(\varphi \) into two parts \(\rho = {{\rho }_{0}} + \hat {\rho }\) and \(\varphi = {{\varphi }_{0}} + \hat {\varphi }\), where \({{\rho }_{0}}\) is the sum of the spherical atom densities and the potential \({{\varphi }_{0}}\) is generated by the density \({{\rho }_{0}}\). The potential \(\hat {\varphi }\) is found by solving Poisson’s equation. The boundary conditions are obtained by expanding the reciprocal distance between two points in a series in Legendre polynomials. To improve the accuracy of the method, the computation domain is decomposed into Voronoi polyhedra, and asymptotic estimates of iterations are used when the characteristic function is replaced by its smooth approximations. Poisson’s equation is numerically solved using the two-grid method and the Fourier transform. An estimate \(O({{h}^{{\gamma - 1}}})\), where \(h\) is the grid size and \(\gamma \) is a fixed number greater than one, is obtained for the accuracy of the method. The error of the method is analyzed using a two-atom problem as an example.



Method for Computing the Velocity of an Electromagnetic Wave Propagating in a Medium with Inclusions at a Low Particle Concentration
Abstract



On the Representation of Electromagnetic Fields in Discontinuously Filled Closed Waveguides by Means of Continuous Potentials
Abstract
A closed waveguide of a constant cross section \(S\) with perfectly conducting walls is considered. It is assumed that its filling is described by function \(\varepsilon \) and \(\mu \) invariable along the waveguide axis and piecewise continuous over the waveguide cross section. The aim of the paper is to show that, in such a system, it is possible to make a change of variables that makes it possible to work only with continuous functions. Instead of discontinuous transverse components of the electromagnetic field \({\mathbf{E}}\), it is proposed to use potentials \({{u}_{e}}\) and \({{v}_{e}}\) related to the field as \({{{\mathbf{E}}}_{ \bot }} = \nabla {{u}_{e}} + \tfrac{1}{\varepsilon }\nabla {\kern 1pt} '{{v}_{e}}\) and, instead of discontinuous transverse components of the magnetic field \({\mathbf{H}}\), to use the potentials \({{u}_{h}}\) and \({{v}_{h}}\) related to the field as \({{{\mathbf{H}}}_{ \bot }} = \nabla {{v}_{h}} + \tfrac{1}{\mu }\nabla {\kern 1pt} '{{u}_{h}}\). It is proven that any field in the waveguide admits the representation in this form if the potentials \({{u}_{e}},{{u}_{h}}\) are elements of the Sobolev space \(\mathop {W_{2}^{1}}\limits^0 (S)\) and \({{v}_{e}},{{v}_{h}}\) are elements of the space \(W_{2}^{1}(S)\).


