The paper deals with topical issues of the modern applied mathematics, in particular, an investigation of approximative properties of Abel–Poisson-type operators on the so-called generalized Hölder’s function classes. It is known, that by the generalized Hölder’s function classes we mean the classes of continuous -periodic functions determined by a first-order modulus of continuity. The notion of the modulus of continuity, in turn, was formulated in the papers of famous French mathematician Lebesgue in the beginning of the last century, and since then it belongs to the most important characteristics of smoothness for continuous functions, which can describe all natural processes in mathematical modeling. At the same time, the Abel-Poisson-type operators themselves are the solutions of elliptic-type partial differential equations. That is why the results obtained in this paper are significant for subsequent research in the field of applied mathematics. The theorem proved in this paper characterizes the upper bound of deviation of continuous -periodic functions determined by a first-order modulus of continuity from their Abel–Poisson-type operators. Hence, the classical Kolmogorov–Nikol’skii problem in A.I. Stepanets sense is solved on the approximation of functions from the classes by their Abel–Poisson-type operators. We know, that the Abel–Poisson-type operators, in partial cases, turn to the well-known in applied mathematics Poisson and Jacobi–Weierstrass operators. Therefore, from the obtained theorem follow the asymptotic equalities for the upper bounds of deviation of functions from the Hölder’s classes of order from their Poisson and Jacobi–Weierstrass operators, respectively. The obtained equalities generalize the known in this direction results from the field of applied mathematics.
In most cases, solutions to problems of the motion of a system of interacting material points are reduced to either ordinary differential equations or partial differential equations. One of the solutions of this type of equations is the so-called generalized Poisson integrals, which in partial cases turn into the well-known Abel-Poisson integrals or biharmonic Poisson integrals. A number of results is known on the approximation of various classes of differentiable periodic and nonperiodic functions by the mentioned above integrals (the so-called Kolmogorov-Nikol’skii problem in the terminology of A.I. Stepanets). Nevertheless, there is a significant drawback practically in all of the solved Kolmogorov-Nikol’skii problems for both Abel-Poisson integrals and Poisson biharmonic integrals from the mathematical modeling (computational experiment) point of view. The core point here is that in most of the previously solved Kolmogorov-Nikol’skii problems for both Abel-Poisson integrals and Poisson biharmonic integrals only the leading and remainder terms of the approximation were obtained, that can significantly affect the accuracy of the computational experiment. In the present paper we obtain exact equalities for approximation of functions from the Sobolev classes by their generalized Poisson integrals. Consequently, the theorem proved in this paper is a generalization and refinement of previously known results characterizing the approximation properties of Abel-Poisson integrals and biharmonic Poisson integrals on the classes of differentiable periodic functions. A peculiarity of the solved in this work problem of approximation for the generalized Poisson integral on the classes of differentiable functions is that the result obtained is successfully written using the well-known Akhiezer-Krein-Favard constants. This fact substantially increases the accuracy of the mathematical modeling result (computational experiment) for a real process described using the generalized Poisson integral. These results can further significantly expand the scope of application of the Kolmogorov-Nikol’skii problems to mathematical modeling.
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