DoroninV. G., and LigunA. A. “Exact Constants in Inequalities of the Jackson Type for Quadrature Formulas”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 52, no. 1, Jan. 2000, pp. 46-51, https://umj.imath.kiev.ua/index.php/umj/article/view/4395.