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Starlike functions associated with some hyperbola. (English) Zbl 0884.30015

Let \(S\) denote the class of functions \(f\) analytic and univalent in the unit disc \(U\), with \(f(0)= f'(0)-1=0\). In the present paper the authors introduce the classes \(SH(\alpha)\), \(\alpha>0\), \(f\in S\) is said to be in \(SH(\alpha)\) if it satisfies \[ |zf'(z)/f(z)- 2\alpha(\sqrt{2}-1)|< \text{Re} \{\sqrt{2} zf'(z)/f(z)\}+ 2\alpha(\sqrt{2}-1), \qquad z\in U.\tag \(*\) \] Note that \(\Omega(\alpha)= \{zf'(z)/ f(z)\mid z\in U\), \(f\in SH(\alpha)\}\) is the interior of a hyperbola in the right half-plan which is symmetric about the real axis and has vertex at the origin and \(\bigcup_{\alpha>0} SH(\alpha)=S^*\). For the classes \(SH(\alpha)\) an extremal function is determined and some subordination results, sharp growth and distorsion theorems, and sharp estimations for coefficients are given.

MSC:

30C75 Extremal problems for conformal and quasiconformal mappings, other methods
30C45 Special classes of univalent and multivalent functions of one complex variable (starlike, convex, bounded rotation, etc.)
30C50 Coefficient problems for univalent and multivalent functions of one complex variable