Alomari, M. W. “A Sharp Companion of Ostrowski’s Inequality for the Riemann–Stieltjes Integral and Applications”. Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica, vol. 15, Dec. 2016, pp. 69-78, https://studmath.uken.krakow.pl/article/view/13143.