Zastavnyi, V. P. (2003). 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, 55(7), 983-990. https://umj.imath.kiev.ua/index.php/umj/article/view/3973