
doi: 10.4064/ap85-2-2
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 \(0<\lambda\leq 3-2\sqrt{2}\).
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
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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