STEPANETS, O. I. Approximation of $\bar {\psi} - integrals$−integrals of periodic functions by Fourier sums (small smoothness). Iof periodic functions by Fourier sums (small smoothness). I. Ukrains’kyi Matematychnyi Zhurnal, v. 50, n. 2, p. 274-291, 25 Feb. 1998.