Asymptotics and inequalities for the broken $k$-diamond partition function

Abstract

Many papers have studied inequalities for Andrews and Paule's broken $k$-diamond partition function $Δ_{k}(n)$ when $k=1$ or $2$. In this paper, we derive an exact formula for $Δ_{k}(n)$ when $k\geq 1$. Building on this result, we also derive an asymptotic formula for $Δ_{k}(n)$ with an explicit error bound. Using this formula, we prove that for $k\geq 1$ and sufficiently large $n$, $Δ_{k}(n)$ satisfies the Turán and Laguerre inequalities of any order and exhibits asymptotic complete monotonicity. Define $n_k:=\max\left\{\left\lceil8k^{3}+\frac{k+1}{12}\right\rceil,526\right\}$. Furthermore, we show that $Δ_{k}(n)$ is log-concave for $k\ge3$ and $n\ge n_k$. Consequently, it follows that $Δ_{k}(a)Δ_{k}(b)\geΔ_{k}(a+b)$ for $k\ge3$ and $a,b \ge n_k$.