Log-convexity and the overpartition function

Abstract

Let $\overline{p}(n)$ denote the overpartition function. In this paper, we obtain an inequality for the sequence $\Delta^{2}\log \ \sqrt[n-1]{\overline{p}(n-1)/(n-1)^{\alpha}}$ which states that \begin{equation*} \log \biggl(1+\frac{3\pi}{4n^{5/2}}-\frac{11+5\alpha}{n^{11/4}}\biggr) < \Delta^{2} \log \ \sqrt[n-1]{\overline{p}(n-1)/(n-1)^{\alpha}} < \log \biggl(1+\frac{3\pi}{4n^{5/2}}\biggr) \ \ \text{for}\ n \geq N(\alpha), \end{equation*} where $\alpha$ is a non-negative real number, $N(\alpha)$ is a positive integer depending on $\alpha$ and $\Delta$ is the difference operator with respect to $n$. This inequality consequently implies $\log$-convexity of $\bigl\{\sqrt[n]{\overline{p}(n)/n}\bigr\}_{n \geq 19}$ and $\bigl\{\sqrt[n]{\overline{p}(n)}\bigr\}_{n \geq 4}$. Moreover, it also establishes the asymptotic growth of $\Delta^{2} \log \ \sqrt[n-1]{\overline{p}(n-1)/(n-1)^{\alpha}}$ by showing $\underset{n \rightarrow \infty}{\lim} \Delta^{2} \log \ \sqrt[n]{\overline{p}(n)/n^{\alpha}} = \dfrac{3 \pi}{4 n^{5/2}}.$