Acu, A.-M., H. Gonska, and I. Ra¸sa. 2011. “Grüss-Type and Ostrowski-Type Inequalities in Approximation Theory”. Ukrains’kyi Matematychnyi Zhurnal 63 (6): 723-40. https://umj.imath.kiev.ua/index.php/umj/article/view/2758.