The general Brannan coefficient conjecture and Watson's lemma

Abstract

The coefficients $A_n(α,β,ω)$ in the Maclaurin expansion $(1+ωz)^α(1-z)^{-β}= \sum_{n=0}^{\infty} A_n(α,β,ω)z^n$ are studied, where $ω,z \in \mathbb{C}$ with $|z| < |ω|=1$, and $α,β\in (0,1]$. In 1973 Brannan conjectured that $|A_n(α,β,ω)|\le A_n(α,β,1)$ for each positive odd integer $n$, and showed it is true for $n=3$. This has recently been proven for all odd integers $n\ge5$ by a number of authors in aggregate for the special case $β=1$. In this paper hypergeometric integral representations and Watson-type approximations are utilised, from which the general problem is reduced to numerically evaluating the minima of certain simple, explicit, slowly-varying functions over compact domains. From the positivity of these constants it is shown that the conjecture holds for $α, β\in (0,1]$, $0 \le |\arg(ω)| \le π-φ_0$ and $n=5,7,9,\ldots$, where $φ_0=0.061$.

Figures (5)

• Figure 1: Graph of $L_0(\alpha,\beta,\phi_0;s_0)$ for $\alpha,\beta \in [0,1]$.
• Figure 1: Graph of $P_5(\alpha,\beta,\phi_0)$ for $\alpha,\beta \in [0,1]$.
• Figure 1: Graph of $F(\alpha,\beta,\phi_0)$ for $\alpha,\beta \in [0,1]$.
• Figure 2: Graph of $L_\infty(\alpha,\beta,\phi_0;s_\infty)$ for $\alpha,\beta \in [0,1]$.
• Figure 2: Graph of $P_5(\alpha,\beta,\phi_0)$ for $\alpha \in [0,0.1]$ and $\beta \in [0.9,1]$.

Theorems & Definitions (22)

• Remark 1
• Lemma 2.1
• Proof 1
• Lemma 2.2
• Remark 2
• Proof 2
• Lemma 3.1
• Remark 3
• Proof 3
• Theorem 3.2