1. H. Alzer, “On some inequalities for the gamma and psi functions,” Math. Comput. 66(217), 373–389 (1997).
2. M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions with Formulas, Graphs and Mathematical Tables (Dover Publications, New York, 1972), pp. 259–260.
3. P. Bracken, “Properties of certain sequences related to Stirling’s approximation for the gamma function,” Expo. Math. 21(2), 171–178 (2003).
4. N. Batir, “On some properties of the gamma function,” Expo. Math. 26(2), 187–196 (2008).
5. Y. Chu, X. Zhang, and X. Tang, “An elementary inequality for psi function,” Bull. Inst. Math. Acad. Sin. 3(3), 373–380 (2008).