Vol 59, No 6 (2023)

For continuous and discrete linear autonomous systems, we discuss the possibility of choosing the coefficients of the quadratic Lyapunov function that ensure the validity of the condition of sign-negativity of its first derivative (first difference) with a given margin in the case of multiple roots of the characteristic equation.

Oscillation, rotation, and wandering indicators similar to Lyapunov exponents and adapted to nonlinear systems of differential equations are studied. A variety of both guaranteed and various realizable relationships between linear, spherical, radial, and ball versions of these indicators are listed, and their relationships with similar indicators of the first approximation system are considered

We consider an initial–boundary value problem for a singularly perturbed system of partial differential equations. We pose an inverse problem of determining an unknown initial condition based on additional information about the solution of the initial–boundary value problem. It is proved that using the expansion of the solution of the initial–boundary value problem in the small parameter 
, one can obtain solutions approximating the solution of the inverse problem with order O(e)or O(e).

We study the first and second boundary value problems and the transmission problem for the complex potential of a two-dimensional filtration flow in an anisotropic and inhomogeneous (variable permeability and thickness) porous layer. The flow sources are arbitrary discrete and can generally be located both on the boundaries and outside the boundaries. The boundaries are modeled by arbitrary smooth (piecewise smooth) closed lines, and the flow sources are singularities (isolated singular points) of the complex potential. The presence of a system of sources on the boundaries leads to a fundamentally new generalization (complication) of the boundary conditions, which are characterized by singular functions with isolated singular points. In the case of an anisotropic homogeneous (constant permeability and thickness) layer and rectilinear boundaries, the solutions of the problems are presented in closed form. In the general case, when an arbitrary smooth closed curve models a boundary with sources located on it, a generalized Cauchy type integral for the complex flow potential is used. This permitted reducing the second boundary value problem and the transmission problem to boundary singular integral equations. The problems studied are mathematical models of two-dimensional filtration processes in layered porous media, which are of interest, for example, for the practice of extracting fluids (oil, water) from natural anisotropically heterogeneous soil layers.

We consider the problem of determining the thermomechanical state of a fuel element in a nuclear reactor. A finite element algorithm for solving the thermal problem together with the problem of mechanical contact is described, and a model one-dimensional problem is studied to clarify the main features and a numerical algorithm for solving it. The leading term of the asymptotic expansion of the solution of this problem and a difference scheme for its solution, including iterative methods, are constructed. A cycle of test calculations is carried out to confirm the theoretical estimates. The comparison of calculations for real problems with theoretical predictions shows that the algorithm for solving a multidimensional nonlinear problem qualitatively corresponds to the behavior of one-dimensional calculations.

The parametric identification problem for dynamical systems with rectangular and ellipsoid parameter uncertainty domains is solved for the case in which the experimental data are given in the form of intervals. The state of the considered dynamical systems at each moment of time is a parametric set. An objective function that characterizes the degree of deviation of the parametric sets of states from experimental interval estimates is constructed in the space of parameter uncertainty domains. To minimize the objective function, a sliding window algorithm has been developed, which is related to gradient methods. It is based on an adaptive interpolation algorithm that allows one to explicitly obtain parametric sets of states of a dynamical system within a given parameter uncertainty domain (window). The efficiency and performance of the proposed algorithm are demonstrated.

In even-dimensional phase spaces, we give examples of analytic systems of differential equations that have isolated equilibria and admit nonanalytic first integrals. These integrals are positive definite in a neighborhood of the equilibria, which proves the stability of the equilibria (on the entire time axis). However, such systems of differential equations do not admit nontrivial first integrals in the form of formal power series at all. In particular, the Lyapunov stability of equilibria of analytic systems does not imply their formal stability. In the case of an odd-dimensional phase space, all isolated equilibria are apparently unstable.