Kalchuk, I. V. 2007. “Approximation of ( ψ, β )-Differentiable Functions Defined on the Real Axis by Weierstrass Operators”.
Ukrains’kyi Matematychnyi Zhurnal
59 (9): 1201–1220.
https://umj.imath.kiev.ua/index.php/umj/article/view/3382
.