Monotonicity and log-behavior of some functions related to the Euler Gamma function

Bao-Xuan Zhu

Published 2013-09-23, updated 2015-04-27Version 2

The aim of this paper is to develop analytic techniques to deal with certain monotonicity of combinatorial sequences. (1) A criterion for the monotonicity of the function $\sqrt[x]{f(x)}$ is given, which is a continuous analog for one result of Wang and Zhu. (2) The log-behavior of the functions $\theta(x)=\sqrt[x]{2 \zeta(x)\Gamma(x+1)}$ and $F(x)=\sqrt[x]{\frac{\Gamma(ax+b+1)}{\Gamma(c x+d+1)\Gamma(e x+f+1)}}$ is considered, where $\zeta(x)$ and $\Gamma(x)$ are the Riemann zeta function and the Euler Gamma function, respectively. As consequences, the strict log-concavities of the function $\theta(x)$ (a conjecture of Chen {\it et al.}) and $\{\sqrt[n]{z_n}\}$ for some combinatorial sequences (including the Bernoulli numbers, the Tangent numbers, the Catalan numbers, the Fuss-Catalan numbers and some Binomial coefficients) are demonstrated. In particular, this contains some results of Chen {\it et al.}, Luca and St\u{a}nic\u{a}. (3). By researching logarithmically complete monotonicity of some functions, the infinite log-monotonicity of the sequence $\{\frac{(n_{0}+ia)!}{(k_0+ib)!(\overline{k_0}+i\overline{b})!}\}_{i\geq0}$ is proved. This generalizes two results of Chen {\it et al.} that both the Catalan numbers $\frac{1}{n+1}\binom{2n}{n}$ and central binomial coefficients $\binom{2n}{n}$ are infinitely log-monotonic and strengths one result of Su and Wang that $\binom{dn}{\delta n}$ is log-convex in $n$. (4) The asymptotically infinite log-monotonicity of derangement numbers is showed. (5)The logarithmically complete monotonicity of functions $1/\sqrt[x]{a \zeta(x+b)\Gamma(x+c)}$ and $\sqrt[x]{\rho\prod_{i=1}^n\frac{\Gamma(x+a_i)}{\Gamma(x+b_i)}}$ is also obtained, which generalizes the results of Lee and Tepedelenlio\v{g}lu, Qi and Li.