Es. Belinskii, STRONG SUMMABILITY OF FOURIER-SERIES OF THE PERIODIC-FUNCTIONS FROM H-P (0-LESS-THAN-P-LESS-THAN-OR-EQUAL-TO-1), Constructive approximation, 12(2), 1996, pp. 187-195
Exact estimates of the rate of approximation by the strong Riesz means
are given, a new formula for the K-functional of the couple of spaces
is found, two-sided estimates of approximation are proved.