ZastavnyiV. P. Extension of a Function from the Exterior of an Interval to a Positive-Definite Function on the Entire Axis and an Approximation Characteristic of the Class $W_M^{r, β}$. Ukrains’kyi Matematychnyi Zhurnal, v. 55, n. 7, p. 983-990, 25 Jul. 2003.