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Annales Polonici Mathematici
Article . 2005 . Peer-reviewed
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Criteria for univalence, starlikeness and convexity

Authors: Ponnusamy, S.; Vasundhra, P.;

Criteria for univalence, starlikeness and convexity

Abstract

Let \(A\) denote the class of all normalized analytic functions \(f\) \((f(0)=0=f^{\prime}(0)-1)\) in the open unit disc \(\Delta\). For \(0<\lambda\leq 1\), define \[ U(\lambda )=\{ f\in A\;:\;\left| \left( \frac{z}{f(z)}\right) ^2 f^{\prime}(z)-1\right| <\lambda\;,\;z\in \Delta \} \] and \[ P(2\lambda )=\{ f\in A\;:\;\left| \left( \frac{z}{f(z)}\right) ^{\prime\prime} \right| <2\lambda\;,\;z\in \Delta \}\;. \] Recently, the problem of finding the starlikeness of these classes has been considered by Obradovic and Ponnusamy, and later by Obradovic {et al}. In this paper, the authors consider the problem of finding the order of starlikeness and convexity of \(\;U(\lambda )\) and \(P(2\lambda )\), respectively. Theorem 1.2 If \(f\in U(\lambda )\) and \(a=| f^{\prime\prime}(0)| /2 \leq 1\), then \(f\in S^{*}(\delta )\) whenever \(0<\lambda \leq \lambda (\delta )\), where \[ \lambda (\delta )={\frac{\sqrt{(1-2\delta )(2-a^2-2\delta )}-a(1-2\delta )}{2(1-\delta )}}\;\;\;\text{ if }\;0\leq \delta <\frac{1+a}{3+a} \] and \[ \lambda (\delta )={\frac{1-\delta (1+a)}{1+\delta }} \;\;\;\text{ if }\;\frac{1+a}{3+a}\leq\delta <\frac{1}{1+a}\;. \] Theorem 1.5 Let \(f\in A\) with \(f^{\prime\prime}(0)=0\) and suppose that \[ {\left| \left( \frac{z}{f(z)}\right)^2 f^{\prime}(z)\left( 1+\frac{zf^{\prime\prime}(z)}{2f^{\prime}(z)} -\frac{zf^{\prime}(z)}{f(z)}\right) \right| <\lambda\;,\;z\in\Delta} \] or equivalently \[ { \left| z^2 \left( \frac{z}{f(z)}\right) ^{\prime\prime} \right| <2\lambda\;,\;z\in\Delta}\;, \] for some \(0<\lambda\leq \frac{1}{\sqrt{2}}\). Then \(f\in K(\beta )\), where \[ \beta =\beta (\lambda )= { \frac{1+\lambda ^2-6\lambda}{1-\lambda ^2}}\;\;\;\text{ for }\;0<\lambda \leq\frac{1}{2} \] and \[ \beta =\beta (\lambda )= { \frac{-\lambda (2+3\lambda )}{1-\lambda ^2}\;\;\;} \text{ for }\;\frac{1}{2}<\lambda \leq\frac{1}{\sqrt{2}}\;. \] In particular, \(P(2\lambda )\subset K\) if \(0<\lambda\leq 3-2\sqrt{2}\;.\) In addition to these results, the authors provide a coefficient condition for a function to be in \(K(\beta )\) and propose a conjecture that each function \(f\in U(\lambda )\) with \(f^{\prime\prime} (0)=0\) is convex at least when \(0<\lambda\leq 3-2\sqrt{2}\).

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Keywords

Special classes of univalent and multivalent functions of one complex variable (starlike, convex, bounded rotation, etc.), starlike functions, convex functions, univalent functions, Maximum principle, Schwarz's lemma, Lindelöf principle, analogues and generalizations; subordination, General theory of univalent and multivalent functions of one complex variable, Inequalities in the complex plane

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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!
9
Average
Top 10%
Average
bronze