N. S. Barnett Sever Silvestru Dragomir The Beesack-Darst-Pollard inequalities and approximations of the Riemann-Stieltjes integral. 58-63 2009 22 Appl. Math. Lett. 1 http://dx.doi.org/10.1016/j.aml.2008.02.005 db/journals/appml/appml22.html#BarnettD09