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Kyungpook Mathematical Journal 2021; 61(3): 513-522

Published online September 30, 2021

Copyright © Kyungpook Mathematical Journal.

On Coefficients of a Certain Subclass of Starlike and Bistarlike Functions

Hesam Mahzoon, Janusz SokóŁ*

Department of Mathematics, Islamic Azad University, West Tehran Branch, Tehran, Iran
e-mail : mahzoon_hesam@yahoo.com

College of Natural Sciences, University of Rzeszow, ul. Prof. Pigonia 1, 35-310 Rzeszów, Poland
e-mail : jsokol@ur.edu.pl

Received: June 6, 2019; Revised: June 22, 2020; Accepted: July 2, 2020

Abstract

In this paper we investigate a subclass M(α) of the class of starlike functions in the unit disk |z| < 1. M(α), π/2 ≤ α < π, is the set of all analytic functions f in the unit disk |z| < 1 with the normalization f(0) = f′(0) − 1 = 0 that satisfy the condition 1+απ2sinα<Re{zf'(z)f(z)}<1+α2sinα(zΔ) The class M(α) was introduced by Kargar et al. [Complex Anal. Oper. Theory 11: 1639–1649, 2017]. In this paper some basic geometric properties of the class M(α) are investigated. Among others things, coefficients estimates and bound are given for the Fekete-Szegö functional associated with the k–th root transform [f(zk)]1/k. Also a certain subclass of bi–starlike functions is introduced and the bounds for the initial coe?cients are obtained.

Keywords: analytic functions, starlike and bistarlike functions, subordination, Fekete-Szegö, inequality.