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ISRN Mathematical Analysis

Volume 2013 (2013), Article ID 387178, 5 pages

http://dx.doi.org/10.1155/2013/387178

## New Subclasses of Biunivalent Functions Involving Dziok-Srivastava Operator

^{1}Department of Mathematics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt^{2}Department of Mathematics, Faculty of Science, Damietta University, New Damietta 34517, Egypt^{3}Department of Mathematics, Faculty of Science, Fayoum University, Fayoum 63514, Egypt

Received 23 June 2013; Accepted 15 July 2013

Academic Editors: R. Avery, D. Bahuguna, and Y. Han

Copyright © 2013 M. K. Aouf et al. 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

We introduce two new subclasses of biunivalent functions which are defined by using the Dziok-Srivastava operator. Furthermore, we find estimates on the coefficients and for functions in these new subclasses.

#### 1. Introduction

Let denote the class of all functions of the form which are analytic in the open unit disc . Also let denote the class of all functions in which are univalent in .

Some of the important and well-investigated subclasses of the univalent function class include, for example, the class of starlike functions of order in and the class of convex functions of order in . By definition, we have

Ding et al. [1] introduced the following class of analytic functions defined as follows:

It is easy to see that for . Thus, for , , and hence is univalent class (see [2–4]).

It is well known that every function has an inverse , defined by where

A function is said to be bi-univalent in if both and are univalent in . Let denote the class of bi-univalent functions in given by (1). For a brief history and interesting examples in the class see [5].

Brannan and Taha [6] (see also [7]) introduced certain subclasses of the bi-univalent function class similar to the familiar subclasses and of starlike and convex functions of order , respectively (see [8]). Thus, following Brannan and Taha [6] (see also [7]), a function is in the class of strongly bi-starlike functions of order if each of the following conditions is satisfied: where is the extension of to . The classes and of bi-starlike functions of order and biconvex functions of order , corresponding, respectively, to the function classes and , were also introduced analogously. For each of the function classes and , they found nonsharp estimates on the first two Taylor-Maclaurin coefficients and (for details, see [6, 7]).

For function given by (1) and given by the Hadamard product (or convolution) of and is defined by

For complex parameters and , the generalized hypergeometric function is defined by the following infinite series: where is the Pochhammer symbol (or shift factorial) defined, in terms of the Gamma function , by Correspondingly a function is defined by Dziok and Srivastava [9] (see also [10]) considered a linear operator defined by the following Hadamard product: If is given by (1), then we have where To make the notation simple, we write It easily follows from (14) that The linear operator is a generalization of many other linear operators considered earlier.

The object of the present paper is to introduce two new subclasses of the bi-univalent functions which are defined by using the Dziok-Srivastava operator and find estimates on the coefficients and for functions in these new subclasses of the function class employing the techniques used earlier by Srivastava et al. [5] (see also [11]).

In order to derive our main results, we have to recall here the following lemma [12].

Lemma 1. * If , then for each , where is the family of all functions analytic in for which Re for .*

Unless otherwise mentioned, we assume throughout this paper that , , ; , , , , is given by (15) and all powers are understood as principle values.

#### 2. Coefficient Bounds of the Function Class

*Definition 2. * One says that a function given by (1) is said to be in the class if it satisfies the following condition:

where the function is given by

*Remark 3. *(i) For , , and , we have , where the class was introduced and studied by Frasin and Aouf [11].

(ii) For , , and , we have , where the class was introduced and studied by Srivastava et al. [5].

Theorem 4. * Letting given by (1) be in the class , then
*

*Proof. * It follows from (18) that
where and in have the forms
Now, equating the coefficients in (22), we get
From (25) and (27), we get
Now from (26), (28), and (30), we obtain
Therefore, we have
Applying Lemma 1 for the coefficients and , we immediately have
This gives the bound on as asserted in (20).

Next, in order to find the bound on , by subtracting (28) from (26) and using (29), we get
It follows from (30) and (34) that
And, then,
Applying Lemma 1 once again for the coefficients , , , and , we readily get

This completes the proof of Theorem 4.

*Remark 5. * (i) Taking , , and , in Theorem 4, we obtain the result obtained by Frasin and Aouf [11, Theorem 2.2].

(ii) Taking , , and , in Theorem 4, we obtain the result obtained by Srivastava et al. [5, Theorem 1].

#### 3. Coefficient Bounds of the Function Class

*Definition 6. *One says that a function given by (1) is said to be in the class if it satisfies the following condition:
where the function is defined by (19).

*Remark 7. *(i) For , , and , we have , where the class was introduced and studied by Frasin and Aouf [11].

(ii) For , , and , we have , where the class was introduced and studied by Srivastava et al. [5].

Theorem 8. * Letting given by (1) be in the class , and , then
*

* Proof. * It follows from (38) that
where and have the forms (23) and (24), respectively.

As in the proof of Theorem 4, by suitably comparing coefficients in (41), we get
From (42) and (44), we get
Also, from (43) and (45), we find that
Therefore, we have
Applying Lemma 1 for the coefficients and , we immediately have
This gives the bound on as asserted in (39).

Next, in order to find the bound on , by subtracting (45) from (43), we get
or, equivalently,
and, then from (47), we find that
Applying Lemma 1 once again for the coefficients , , , and , we readily get
This completes the proof of Theorem 8.

*Remark 9. *(i) Taking , , and , in Theorem 8, we obtain the result obtained by Frasin and Aouf [11, Theorem 3.2].

(ii) Taking , , and , in Theorem 8, we obtain the result obtained by Srivastava et al. [5, Theorem 2].

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