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

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 $θ(x)=\sqrt[x]{2 ζ(x)Γ(x+1)}$ and $F(x)=\sqrt[x]{\frac{Γ(ax+b+1)}{Γ(c x+d+1)Γ(e x+f+1)}}$ is considered, where $ζ(x)$ and $Γ(x)$ are the Riemann zeta function and the Euler Gamma function, respectively. As consequences, the strict log-concavities of the function $θ(x)$ (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 Stănică. (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 $\binom{dn}{δn}$ is log-convex in $n$. (4) The asymptotically infinite log-monotonicity of derangement numbers is showed. (5)The logarithmically complete monotonicity of functions $1/\sqrt[x]{a ζ(x+b)Γ(x+c)}$ and $\sqrt[x]{ρ\prod_{i=1}^n\frac{Γ(x+a_i)}{Γ(x+b_i)}}$ is also obtained, which generalizes the results of Lee and Tepedelenlioǧlu, Qi and Li.

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