
handle: 10533/176440
Let \(H(D)\) denote the set of analytic functions in \(D= \{z\in\mathbb{C}:|z|< 1\}\) and \(H_0(D)\) the subset of \(H(D)\) whose members are normalized \(f(0)= 0\), \(f'(0)= 1\). For \(F(z)= \sum_{k=0} a_k z^k\), \(G(z)= \sum_{k=0} b_k z^k\) belonging to \(H(D)\) we define the Hadamard product as \((F* G)(z)= \sum^\infty_{k=0} a_k b_k z^k\). We say that \(f\in H_0(D)\) is prestarlike of order \(\alpha\) if \(f*{z\over (1-z)^{2(1-\alpha)}}\) is an \(\alpha\)-starlike function. We write then \(f\in R_\alpha\). S. P. Robinson in his thesis ``Approximate Identities for Certain Dual Classes'', University of York, 1996, introduced the polynomials \[ p^\beta_n(z)= 1+ \sum^n_{k=1} \prod^{k-1}_{j=0} {n-j\over \beta+ n+ j} z^k,\quad n\in\mathbb{N},\quad \beta\geq 1 \] and made the conjecture that \(zp^\beta_n(z)\in R_{{3-\beta\over 2}}\). In this paper the authors proved this conjecture for some generalized functions which are the special hypergeometric functions \[ p^\beta_\lambda(z):= {_2F_1}(1, -\lambda,\lambda+ \beta,-1),\quad \lambda\geq-1,\quad \beta\geq 1. \] Theorem. For \(\beta\geq \max\{1,-2\lambda\}\) we have \(zp^\beta_\lambda(z)\in R_{{3-\beta\over 2}}\), and for \(0\leq \lambda_1\leq \lambda_2\), \(1\leq \beta_2\leq \beta_1\) we have \(p^{\beta_1}_{\lambda_1}\prec p^{\beta_2}_{\lambda_2}\), where ``\(\prec\)'' stands for the subordination. We can easily verify that \(p^\beta_\lambda(z)\) agrees with Robinson's definition for \(\lambda= n\in\mathbb{N}\).
Convex univalent, Applied Mathematics, Subordination, subordination, convex univalent, Cesàro means, Special classes of univalent and multivalent functions of one complex variable (starlike, convex, bounded rotation, etc.), prestarlike, Extremal problems for conformal and quasiconformal mappings, other methods, Polynomials and rational functions of one complex variable, Cesáro means, Prestarlike, de la Vallée Poussin means, Analysis
Convex univalent, Applied Mathematics, Subordination, subordination, convex univalent, Cesàro means, Special classes of univalent and multivalent functions of one complex variable (starlike, convex, bounded rotation, etc.), prestarlike, Extremal problems for conformal and quasiconformal mappings, other methods, Polynomials and rational functions of one complex variable, Cesáro means, Prestarlike, de la Vallée Poussin means, Analysis
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