A mention of the references Mortici (2011b, 2013b) was removed because
the results contained therein are weaker than (5.6.1).
Addition (effective with 1.0.10):
The reference to Mortici (2013b) has been added at the end of this subsection.
Addition (effective with 1.0.7):
The reference to Mortici (2011b) has been added at the end of this subsection.
Addition (effective with 1.0.5):
References to Alzer (2008), Qi (2008), and Koumandos and Lamprecht (2010)
have been added at the end of this subsection. Also,
(5.6.5) has been identified as Kershaw’s Inequality.