Pečarić, J. E., et al. “Generalizations of Steffensen’s Inequality by Lidstone’s Polynomials”. Ukrains’kyi Matematychnyi Zhurnal, vol. 67, no. 11, Nov. 2015, pp. 1525-39, https://umj.imath.kiev.ua/index.php/umj/article/view/2087.