Simultaneous approximation of periodic functions and their derivatives (Q1081040)

We continue the investigations, started by the second author [Ukr. Mat. Zh. 33, 356-367 (1981; Zbl 0518.42003) and Investigations in the theory of approximation of functions, Collect. Sci. Works, Kiev 1981, 82-88 (1981; Zbl 0512.42005)], concerning the study of the functionals that characterize the simultaneous approximation of functions and their derivatives by Fourier sums. In particular, the results of these works will be generalized here to the case of more general functionals, defined on the classes of periodic functions that admit fractional differentiation.