Zeta Functions and the Log-behavior of Combinatorial Sequences

In this paper, we use the Riemann zeta function $ζ(x)$ and the Bessel zeta function $ζ_μ(x)$ to study the log-behavior of combinatorial sequences. We prove that $ζ(x)$ is log-convex for $x>1$. As a consequence, we deduce that the sequence $\{|B_{2n}|/(2n)!\}_{n\geq 1}$ is log-convex, where $B_n$ is the $n$-th Bernoulli number. We introduce the function $θ(x)=(2ζ(x)Γ(x+1))^{\frac{1}{x}}$, where $Γ(x)$ is the gamma function, and we show that $\log θ(x)$ is strictly increasing for $x\geq 6$. This confirms a conjecture of Sun stating that the sequence $\{\sqrt[n] {|B_{2n}}|\}_{n\geq 1}$ is strictly increasing. Amdeberhan, Moll and Vignat defined the numbers $a_n(μ)=2^{2n+1}(n+1)!(μ+1)_nζ_μ(2n)$ and conjectured that the sequence $\{a_n(μ)\}_{n\geq 1}$ is log-convex for $μ=0$ and $μ=1$. By proving that $ζ_μ(x)$ is log-convex for $x>1$ and $μ>-1$, we show that the sequence $\{a_n(μ)\}_{n\geq 1}$ is log-convex for any $μ>-1$. We introduce another function $θ_μ(x)$ involving $ζ_μ(x)$ and the gamma function $Γ(x)$ and we show that $\log θ_μ(x)$ is strictly increasing for $x>8e(μ+2)^2$. This implies that $\sqrt[n]{a_n(μ)}<\sqrt[n+1]{a_{n+1}(μ)}$ for $n> 4e(μ+2)^2$. Based on Dobinski's formula, we prove that $\sqrt[n]{B_n}<\sqrt[n+1]{B_{n+1}}$ for $n\geq 1$, where $B_n$ is the $n$-th Bell number. This confirms another conjecture of Sun. We also establish a connection between the increasing property of $\{\sqrt[n]{B_n}\}_{n\geq 1}$ and Hölder's inequality in probability theory.

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