Abstract: In approximation theory, statements in which functions from certain classes are approximated by functions from other fixed classes (for example, by polynomials, rational functions, harmonic functions, etc.) and the accuracy of approximation is measured in a certain scale are called direct approximation theorems. Statements where the smoothness class of the approximated function is derived from the known accuracy of approximation of this function by polynomials, rational functions, and harmonic functions are called inverse approximation theorems. It is usually said that some class of generally smooth functions is constructively described in terms of the approximation by polynomials, rational functions, harmonic functions, etc., if functions from this class can be approximated in the chosen scale of the approximation accuracy and if the accuracy of the approximation in this scale yields the belonging of the approximated function to the class under consideration. Since the constructive description of classes of functions is a high-priority area of investigation in approximation theory, there exists a tendency to add inverse statements to the existing direct theorems for some classes of functions. The authors have previously proved the direct theorem concerning the approximation of a set of analytic functions defined on a countable set of continua by entire functions of exponential type. This paper presents the inverse statement. Section 1 assembles definitions and formulations, and Section 2 provides a proof of the main result.

Original languageEnglish
Pages (from-to)366-371
Number of pages6
JournalVestnik St. Petersburg University: Mathematics
Volume54
Issue number4
DOIs
StatePublished - Oct 2021

    Scopus subject areas

  • Mathematics(all)

    Research areas

  • approximation theory, entire functions of exponential type, Hölder classes, inverse theorems

ID: 95016877