Qi, Feng, and Shu-Hong Wang. “Complete Monotonicity, Completely Monotonic Degree, Integral Representations, and an Inequality Related to the Exponential, Trigamma, and Modified Bessel Functions”. Global Journal of Mathematical Analysis, vol. 2, no. 3, June 2014, pp. 91-97, https://doi.org/10.14419/gjma.v2i3.2919.