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, Vol. 55, no. 7, July 2003, pp. 983-90, https://umj.imath.kiev.ua/index.php/umj/article/view/3973.