The general Brannan coefficient conjecture and Watson's lemma
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arXiv
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| Natura: | Preprint |
| Pubblicazione: |
2026
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| _version_ | 1866915805541695488 |
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| author | Dunster, T. M. |
| author_facet | Dunster, T. M. |
| contents | 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 $ϕ_0=0.061$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2602_15308 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | The general Brannan coefficient conjecture and Watson's lemma Dunster, T. M. Classical Analysis and ODEs Complex Variables 33C05, 30C45, 30C50, 41A60 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 $ϕ_0=0.061$. |
| title | The general Brannan coefficient conjecture and Watson's lemma |
| topic | Classical Analysis and ODEs Complex Variables 33C05, 30C45, 30C50, 41A60 |
| url | https://arxiv.org/abs/2602.15308 |