Approximation by a “floating” system of exponentials on classes of smooth periodic functions

The problem of approximating functions by trigonometric polynomials with a given number of harmonics is considered. The exact degree of approximation is determined for three closely related classes of smooth functions: those defined by differential properties, by difference properties, and functions in Besov spaces. In contrast to the classical case, the degrees of approximation for these classes turn out to be different from each other.
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