Some improvements and generalizations of Steffensen's integral inequality

In this paper, the authors present several interesting results on improvements of Steffensen's integral inequality. Moreover, an error in a generalization of Steffensen's inequality due to Mercer [P.R. Mercer, Extensions of Steffensen's inequality, J. Math. Anal. Appl. 246 (2000) 325-329] is corrected.