Criteria for univalence, starlikeness and convexity

S. Ponnusamy¹ ; P. Vasundhra²

¹ Department of Mathematics Indian Institute of Technology IIT-Madras Chennai 600 036, India
² Department of Mathematics Indian Institute of Technology IIT-Madras Chennai- 600 036, India

Annales Polonici Mathematici, Tome 85 (2005) no. 2, pp. 121-133.

Résumé

Let ${\mathcal A}$ denote the class of all normalized analytic functions $f$ ($f(0)=0= f'(0)-1$) in the open unit disc $\mit\Delta$. For $0\lambda\leq 1$, define $${\mathcal U}(\lambda )=\bigg \{f\in {\mathcal A}: \bigg|\bigg(\frac{z}{f(z)}\bigg)^{2}f'(z)-1\bigg|\lambda, \, z\in {\mit\Delta} \bigg \} $$ and $$ {\mathcal P}(2\lambda )=\bigg \{f\in {\mathcal A}: \bigg|\bigg(\frac{z}{f(z)}\bigg)' '\bigg|2\lambda, \, z\in {\mit\Delta}\bigg \}. $$ Recently, the problem of finding the starlikeness of these classes has been considered by Obradović and Ponnusamy, and later by Obradović et al. In this paper, the authors consider the problem of finding the order of starlikeness and of convexity of ${\mathcal U}(\lambda )$ and ${\mathcal P}(2\lambda )$, respectively. In particular, for $f\in {\mathcal A}$ with $f' '(0)=0$, we find conditions on $\lambda$, $\beta^* (\lambda )$ and $\beta (\lambda )$ so that ${\mathcal U}(\lambda ) \subsetneq {\mathcal S}^*(\beta^* (\lambda ))$ and ${\mathcal P}(2\lambda )\subsetneq {\mathcal K}(\beta (\lambda ))$. Here, ${\mathcal S}^*(\beta)$ and ${\mathcal K}(\beta)$ ($\beta 1$) denote the classes of functions in ${\mathcal A}$ that are starlike of order $\beta$ and convex of order $\beta$, respectively. In addition to these results, we also provide a coefficient condition for functions to be in ${\mathcal K}(\beta)$. Finally, we propose a conjecture that each function $f\in {\mathcal U}(\lambda )$ with $f' '(0)=0$ is convex at least when $0\lambda\leq 3-2\sqrt{2}$.

Détail

DOI : 10.4064/ap85-2-2

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