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On a conjecture of S.P. Robinson

Authors: Ruscheweyh, S; Salinas, L;

On a conjecture of S.P. Robinson

Abstract

Let \(H(D)\) denote the set of analytic functions in \(D= \{z\in\mathbb{C}:|z|< 1\}\) and \(H_0(D)\) the subset of \(H(D)\) whose members are normalized \(f(0)= 0\), \(f'(0)= 1\). For \(F(z)= \sum_{k=0} a_k z^k\), \(G(z)= \sum_{k=0} b_k z^k\) belonging to \(H(D)\) we define the Hadamard product as \((F* G)(z)= \sum^\infty_{k=0} a_k b_k z^k\). We say that \(f\in H_0(D)\) is prestarlike of order \(\alpha\) if \(f*{z\over (1-z)^{2(1-\alpha)}}\) is an \(\alpha\)-starlike function. We write then \(f\in R_\alpha\). S. P. Robinson in his thesis ``Approximate Identities for Certain Dual Classes'', University of York, 1996, introduced the polynomials \[ p^\beta_n(z)= 1+ \sum^n_{k=1} \prod^{k-1}_{j=0} {n-j\over \beta+ n+ j} z^k,\quad n\in\mathbb{N},\quad \beta\geq 1 \] and made the conjecture that \(zp^\beta_n(z)\in R_{{3-\beta\over 2}}\). In this paper the authors proved this conjecture for some generalized functions which are the special hypergeometric functions \[ p^\beta_\lambda(z):= {_2F_1}(1, -\lambda,\lambda+ \beta,-1),\quad \lambda\geq-1,\quad \beta\geq 1. \] Theorem. For \(\beta\geq \max\{1,-2\lambda\}\) we have \(zp^\beta_\lambda(z)\in R_{{3-\beta\over 2}}\), and for \(0\leq \lambda_1\leq \lambda_2\), \(1\leq \beta_2\leq \beta_1\) we have \(p^{\beta_1}_{\lambda_1}\prec p^{\beta_2}_{\lambda_2}\), where ``\(\prec\)'' stands for the subordination. We can easily verify that \(p^\beta_\lambda(z)\) agrees with Robinson's definition for \(\lambda= n\in\mathbb{N}\).

Keywords

Convex univalent, Applied Mathematics, Subordination, subordination, convex univalent, Cesàro means, Special classes of univalent and multivalent functions of one complex variable (starlike, convex, bounded rotation, etc.), prestarlike, Extremal problems for conformal and quasiconformal mappings, other methods, Polynomials and rational functions of one complex variable, Cesáro means, Prestarlike, de la Vallée Poussin means, Analysis

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
1
Average
Average
Average
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