Dyuzhenkova, O. Yu. 1995. “A Remark Concerning the Modulus of Smoothness Introduced by Ditzian and Totik”.
Ukrains’kyi Matematychnyi Zhurnal
47 (12): 1627–1638.
https://umj.imath.kiev.ua/index.php/umj/article/view/5556
.