Chen, Chao-Ping, and Feng Qi. “Completely Monotonic Function Associated With the Gamma Functions and Proof of Wallis’ Inequality”. Tamkang Journal of Mathematics 36, no. 4 (December 31, 2005): 303-307. Accessed May 23, 2022. https://journals.math.tku.edu.tw/index.php/TKJM/article/view/101.