Abstract and Applied Analysis

Volume 2013 (2013), Article ID 849747, 9 pages

http://dx.doi.org/10.1155/2013/849747

Research Article

New Subclasses of Multivalent Analytic Functions Associated with a Linear Operator

Ding-Gong Yang¹ and Jin-Lin Liu²

¹School of Mathematical Sciences, Soochow University, Suzhou, Jiangsu 215006, China

²Department of Mathematics, Yangzhou University, Yangzhou, Jiangsu 225002, China

Received 14 August 2013; Accepted 29 October 2013

Academic Editor: Ngai-Ching Wong

Copyright © 2013 Ding-Gong Yang and Jin-Lin Liu. 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.

Making use of a linear operator, which is defined here by means of the Hadamard product (or convolution), we consider two subclasses and of multivalent analytic functions with negative coefficients in the open unit disk. Some modified Hadamard products, integral transforms, and the partial sums of functions belonging to these classes are studied.

1. Introduction

Let denote the class of functions of the form

which are analytic in the open unit disk .

For functions and analytic in , we say that is subordinate to in , written , if there exists an analytic function in such that

Furthermore, if the function is univalent in , then

In terms of the Pochhammer symbol given by , we define the function by

Corresponding to , we consider here a linear operator on by the following usual Hadamard product (or convolution):

for given by (1). For , on was first defined by Carlson and Shaffer [1]. Its differential-integral representation can be found in [2]. We remark in passing that a much more general convolution operator than the operator was introduced by Dziok and Srivastava [3].

Let denote the subclass of consisting of functions of the form

We now consider the following two subclasses of the class .

Definition 1. A function is said to be in the class if and only if

where

Definition 2. A function is said to be in the class if and only if

For functions given by

we denote by the modified Hadamard (or quasi-Hadamard) product of and ; that is,

The class

with was introduced and studied earlier by Lee et al. [4] (and was further investigated by Aouf and Darwish [5], Aouf et al. [6], and Yaguchi et al. [7]). The class

with was studied by Aouf [8] and Aouf et al. [6]. Recently, Aouf [9] investigated the modified Hadamard products of several functions in the classes and for .

In the present paper, we prove a number of theorems involving the modified Hadamard products, integral transforms, and the partial sums of functions in the classes and . Some of our results are generalizations of the corresponding results in [4–9].

In proving our main results, we need the following lemmas.

Lemma 3 (see [10, 11]). A function defined by (6) is in the class if and only if

Lemma 4 (see [10, 11]). A function defined by (6) is in the class if and only if

Making use of Lemmas 3 and 4, we can show the following two results.

Corollary 5. Let

Then

Corollary 6. Let . Then if and only if , where ,   and , and

If we let

then Lemmas 3 and 4 reduce to the following result.

Corollary 7. Let be defined by (6). Then

(i)  is in the class if and only if

(ii)  is in the class if and only if

2. Modified Hadamard Products

Hereafter in this paper we assume that (8) is satisfied:

Theorem 8. Let and . Then

where and

The result is sharp; that is, cannot be decreased for each .

Proof. By (24) we have for . Let

Then Lemma 3 gives

and hence

Also, using Lemma 3,

if and only if

To prove the result of Theorem 8, it follows from (27) and (29) that we need to find the smallest such that

that is, that

Since

we see that the right-hand side of (31) is a decreasing function of . Consequently, taking in (31), we have , where is given by (24).

Furthermore, by considering the functions defined by:

we have and

Noting that

we conclude that cannot be decreased for each .

By using Lemma 4 instead of Lemma 3, the following theorem can be proved on the lines of the proof of Theorem 8. We omit the details involved.

Theorem 9. Let and . Then

where and

The result is sharp for the functions defined by

Theorem 10. Let with and . Then

where and

The result is sharp for the functions defined by (33).

Proof. Obviously, for ,  , and . By applying Lemma 4, we know that

if and only if

Proceeding as in the proof of Theorem 8, we need to find the smallest such that

Defining the function by

we see that

for and . Hence, the right-hand side of (43) is a decreasing function of . Thus, we arrive at , where is given by (40).

Sharpness can be verified easily.

By putting

Theorem 10 reduces to the following.

Corollary 11. Let . Then , where ,  , and

The result is sharp for the functions

Theorem 12. Let ,

and . Then one has

 (i)  , where

 provided that . (ii)  , where

Proof. It is clear that ,  ,  ,  ,

for . Let

Then Lemma 3 gives

if and only if

Also, by Lemma 3 and (52), we deduce that

and hence

To prove Theorem 12(i), it follows from (55) and (57) that we need to find the smallest such that

for . This leads to the assertion of Theorem 12(i).

Analogously, we can prove Theorem 12(ii).

In the special case when

Theorem 12 reduces to the following.

Corollary 13. Let . Then

 (i)  , where

 provided that . (ii)  , where

Replacing Lemma 3 by Lemma 4 in the proof of Theorem 12, one can prove the following.

Theorem 14. Let ,

and . Then

 (i)  , where

 and , and are given as in Theorem 12, provided that . (ii)  , where

As a special case of Theorem 14, one has the following.

Corollary 15. Let . Then

 (i)  , where

 provided that . (ii)  , where

3. Integral Operator

Theorem 16. Let

If , then the function defined by

belongs to , where and

The result is sharp; that is, the number cannot be decreased for each .

Proof. Note that . For

it follows from (68) that

To prove the result of Theorem 16, we need to find the smallest such that

where we have used Lemma 3. In view of (67), it is easy to know that the right-hand side of (72) is a decreasing function of . Therefore, we conclude that

where is given by (69).

Furthermore, it can easily be verified that the result is sharp, with the extremal function

With the aid of Lemma 4 (instead of Lemma 3) and using the same steps as in the proof of Theorem 16, we can prove the following.

Theorem 17. Let (67) in Theorem 16 be satisfied. If , then the function defined by (68) belongs to , where is the same as in Theorem 16. The result is sharp for the function

If we let

then Theorem 17 yields the following.

Corollary 18. Let . Then the function defined by (68) belongs to , where

The number cannot be increased for each .

4. Partial Sums

In this section, we let be given by (6) and define the partial sums and by

Also we make the notation simple by writing

Theorem 19. Let and . Then for , one has the following.

The results are sharp for each .

Proof. Let and be given by (79). Then for , and so it follows from Lemma 3 that

for .

If we put

then and

because of (82). Hence, we have for , which implies that (80) holds true for .

Similarly, by setting

it follows from (82) that

Therefore, we see that for , that is, that (81) holds for .

For , replacing (82) by

and proceeding as the above, we know that (80) and (81) are also true.

Furthermore, by taking the function

we find that ,

The proof of Theorem 19 is thus completed.

By virtue of Theorem 19 and Definition 2, we easily have the following.

Corollary 20. Let and . Then we have

The results are sharp for each .

Theorem 21. Let with and . Then one has

The results are sharp for each .

Proof. Let , and be given by (79). Then it is easy to verify that

and hence we deduce from Lemma 3 that

for .

Defining the function by

it follows from (94) that

This leads to the inequality (91) for .

Similarly, for the function defined by

we deduce from (94) that

This yields the inequality (92) for .

For , replacing (94) by

we know that (91) and (92) are also true.

Furthermore, the bounds in (91) and (92) are the best possible for the function defined by (88).

Finally, Theorem 21 yields the following.

Corollary 22. Let with and . Then

The results are sharp for each .

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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