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Geometry

Volume 2013 (2013), Article ID 348251, 4 pages

http://dx.doi.org/10.1155/2013/348251

## Hankel Determinant for -Valent Alpha-Convex Functions

Department of Mathematics, M.S.K. Girls College, Bharowal (Tarn-Taran), Punjab 143401, India

Received 14 June 2013; Accepted 29 August 2013

Academic Editor: Reza Saadati

Copyright © 2013 Gagandeep Singh and B. S. Mehrok. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The objective of the present paper is to obtain the sharp upper bound of for *p*-valent *α*-convex functions of the form in the unit disc .

#### 1. Introduction

Let be the class of analytic functions of the form in the unit disc with . Let be the subclass of , consisting of univalent functions.

is the class consisting of functions of the form (1) and satisfying the condition

The functions of the class are called *p*-valent starlike functions. In particular, , the class of starlike functions.

is the class of functions of the form (1), satisfying the condition

The functions of the class are known as *p*-valent convex functions. Particularly, , the class of convex functions.

Obviously if and only if .

Let be the class of functions of the form (1), satisfying the condition

Functions in the class are known as -valent alpha-convex functions. For , the class reduces to the class of alpha-convex functions introduced by Mocanu [1]. Also and .

In 1976, Noonan and Thomas [2] stated the th Hankel determinant for and as

This determinant has also been considered by several authors. For example, Noor [3] determined the rate of growth of as for functions given by (1) with bounded boundary. Ehrenborg [4] studied the Hankel determinant of exponential polynomials. Also Hankel determinant was studied by various authors including Hayman [5] and Pommerenke [6]. In [7], Janteng et al. studied the Hankel determinant for the classes of starlike and convex functions. Again Janteng et al. discussed the Hankel determinant problem for the classes of starlike functions with respect to symmetric points and convex functions with respect to symmetric points in [8] and for the functions whose derivative has a positive real part in [9]. Also Hankel determinant for various subclasses of -valent functions was investigated by various authors including Krishna and Ramreddy [10] and Hayami and Owa [11].

Easily, one can observe that the Fekete and Szegö functional is . Fekete and Szegö [12] then further generalised the estimate , where is real and . For our discussion in this paper, we consider the Hankel determinant in the case of and :

In this paper, we seek sharp upper bound of the functional for functions belonging to the class . The results due to Janteng et al. [7] follow as special cases.

#### 2. Preliminary Results

Let be the family of all functions analytic in for which and for .

Lemma 1 (see [6]). *If , then *

Lemma 2 (see [13, 14]). * If , then
**
for some and satisfying and .*

#### 3. Main Result

Theorem 3. *If , then
**
where
*

*Proof . *Since , so from (4)
On expanding and equating the coefficients of , and in (11), we get
Equation (12) yields:
where .

Using Lemmas 1 and 2 in (13), we obtain
Assume that and ; using triangular inequality and , we have
It is easy to verify that is an increasing function and so .

Consequently
where
So
where is defined in (10).

Now
gives
is negative at .

So
Hence from (15), we obtain (9).

The result is sharp for , , and .

For , Theorem 3 gives the following result.

Corollary 4. *If , then
*

For , Theorem 3 yields.

Corollary 5. *If , then
*

Putting and in Theorem 3, we obtain the following result due to Janteng et al. [7].

Corollary 6. *If , then
*

Putting and in Theorem 3, we obtain the following result due to Janteng et al. [7].

Corollary 7. *If , then
*

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