Criteria for univalence, starlikeness and convexity

Tom 85 / 2005

S. Ponnusamy, P. Vasundhra Annales Polonici Mathematici 85 (2005), 121-133 MSC: 30A10, 30C45, 30C55, 30C80. DOI: 10.4064/ap85-2-2

Streszczenie

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 $0<\lambda\leq 3-2\sqrt{2}$.

Autorzy

  • S. PonnusamyDepartment of Mathematics
    Indian Institute of Technology
    IIT-Madras
    Chennai 600 036, India
    e-mail
  • P. VasundhraDepartment of Mathematics
    Indian Institute of Technology
    IIT-Madras
    Chennai- 600 036, India

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