Acu, A.-M., H. Gonska, and Ra¸saI. “Grüss-Type and Ostrowski-Type Inequalities in Approximation Theory”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 6, June 2011, pp. 723-40, https://umj.imath.kiev.ua/index.php/umj/article/view/2758.