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# Subordination properties for a general class of integral operators involving meromorphically multivalent functions

Nak Eun Cho1* and Rekha Srivastava2

Author Affiliations

1 Department of Applied Mathematics, Pukyong National University, Busan, 608-737, Korea

2 Department of Mathematics and Statistics, University of Victoria, Victoria, BC, V8W 3R4, Canada

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Advances in Difference Equations 2013, 2013:93  doi:10.1186/1687-1847-2013-93

 Received: 14 January 2013 Accepted: 14 March 2013 Published: 5 April 2013

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

The purpose of the present paper is to investigate some subordination- and superordination-preserving properties of certain nonlinear integral operators defined on the space of meromorphically p-valent functions in the punctured open unit disk. The sandwich-type theorems associated with these integral operators are established. Relevant connections of the various results presented here with those involving relatively simpler nonlinear integral operators are also indicated.

MSC: 30C45, 30C80.

##### Keywords:
differential subordination; differential superordination; meromorphic functions; integral operators; convex functions; close-to-convex functions; subordination (or Löwner) chain

### 1 Introduction, definitions and preliminaries

Let denote the class of analytic functions in the open unit disk

For and , let

Let f and F be members of ℋ. The function f is said to be subordinate to F, or F is said to be superordinate to f, if there exists a function w analytic in , with

such that

In such a case, we write

Furthermore, if the function F is univalent in , then we have (cf.[1,2] and [3])

Definition 1 (Miller and Mocanu [1])

Let

and let h be univalent in . If is analytic in and satisfies the differential subordination

(1.1)

then is called a solution of the differential subordination. The univalent function q is called a dominant of the solutions of the differential subordination or, more simply, a dominant if

for all satisfying (1.1). A dominant that satisfies the following condition:

for all dominants q of (1.1) is said to be the best dominant.

Definition 2 (Miller and Mocanu [4])

Let

and let h be analytic in . If p and are univalent in and satisfy the differential superordination

(1.2)

then is called a solution of the differential superordination. An analytic function q is called a subordinant of the solutions of the differential superordination or, more simply, a subordinant if

for all satisfying (1.2). A univalent subordinant that satisfies the following condition:

for all subordinants q of (1.2) is said to be the best subordinant.

Definition 3 (Miller and Mocanu [4])

We denote by the class of functions f that are analytic and injective on , where

and are such that

We also denote the class by

Let denote the class of functions of the form

which are analytic in the punctured open unit disk . Let and be the subclasses of consisting of all functions which are, respectively, meromorphically starlike and meromorphically convex in (see, for details, [1,5]).

For a function , we introduce the following general integral operator defined by

(1.3)

Several members of the family of integral operators defined by (1) have been extensively studied by many authors (see, for example, [6-10]; see also [11] and [12]) with suitable restrictions on the parameters α, β, γ and δ, and for f belonging to some favored subclasses of meromorphic functions. In particular, Bajpai [6] showed that the integral operator belongs to the classes and , whenever f belongs to the classes and , respectively.

Making use of the principle of subordination between analytic functions, Miller et al.[13] and, more recently, Owa and Srivastava [14] obtained some interesting subordination-preserving properties for certain integral operators. Moreover, Miller and Mocanu [4] considered differential superordinations as the dual concept of differential subordinations (see also [15]). It should be remarked that in recent years several authors obtained many interesting results involving various integral operators associated with differential subordination and superordination (for example, see [5,16-18]). In the present paper, we obtain the subordination- and superordination-preserving properties of the general integral operator defined by (1) with the sandwich-type theorem.

The following lemmas will be required in our present investigation.

Lemma 1 (Miller and Mocanu [19])

Suppose that the function

satisfies the following condition:

for all realsand for alltwith

If the function

is analytic inand

then

Lemma 2 (Miller and Mocanu [20])

Let

If

then the solution of the differential equation

is analytic inand satisfies the following inequality:

Lemma 3 (Miller and Mocanu [1])

Let

and let

be analytic inwith

If the functionqis not subordinate to, then there exist points

for which

Let

If R is the univalent function defined in by

then the open door function (see [1]) is defined by

(1.4)

Remark 1 The function defined by (1.4) is univalent in , where , and is the complex plane with slits along the half-lines given by

Lemma 4 (Totoi [21])

Letwith

If, where

(1.5)

andis defined by (1.4) with, then

and

whereis the integral operator defined by (1).

A function defined on is the subordination chain (or Löwner chain) if is analytic and univalent in for all , is continuously differentiable on for all and

(1.6)

Lemma 5 (Miller and Mocanu [4])

Let

Also set

If

is a subordination chain and

then the following subordination condition:

(1.7)

implies that

Furthermore, if

has a univalent solution, thenqis the best subordinant.

Lemma 6 (Pommerenke [22])

The function

with

Suppose thatis analytic infor alland thatis continuously differentiable onfor all. If the functionsatisfies the following inequalities:

(1.8)

and

(1.9)

for some positive constantsand, thenis a subordination chain.

### 2 Main results and their corollaries and consequences

We begin by proving a general subordination property involving the integral operator defined by (1), which is contained in Theorem 1 below.

Theorem 1Let, whereis defined by (1.5). Suppose also that

(2.1)

where

(2.2)

Then the subordination relation

(2.3)

implies that

(2.4)

whereis the integral operator defined by (1). Moreover, the function

is the best dominant.

Proof Let us define the functions F and G, respectively, by

(2.5)

We first show that if the function q is defined by

(2.6)

then

From the definition of (1), we obtain

(2.7)

We also have

(2.8)

It follows from (2.7) and (2.8) that

(2.9)

Now, by a simple calculation with (2.9), we obtain the following relationship:

(2.10)

Thus, from (2.1), we have

and by using Lemma 2, we conclude that the differential equation (2.10) has a solution with

Put

(2.11)

where ρ is given by (2.2). From (2.1), (2.10) and (2.11), we obtain

We now proceed to show that

(2.12)

Indeed, from (2.11), we have

(2.13)

where

(2.14)

For ρ given by (2.2), we note that the coefficient of in the quadratic expression given by (2.14) is positive or equal to zero and also is a perfect square. Hence from (2.13), we see that (2.12) holds true. Thus, by using Lemma 1, we conclude that

That is, the function defined by (2.5) is convex in .

We next prove that the subordination condition (2.3) implies that

(2.15)

for the functions F and G defined by (2.5). Without loss of generality, we can assume that G is analytic and univalent on and that

We now consider the function defined by

We note that

and

Furthermore, since G is convex, by using the well-known growth and distortion sharp inequalities for convex functions (see [23]), we can prove that the second condition of Lemma 6 is satisfied. Therefore, by virtue of Lemma 6, is a subordination chain. We observe from the definition of a subordination chain that

and

This implies that

We now suppose that F is not subordinate to G. Then, in view of Lemma 3, there exist points and such that

Hence we have

by virtue of the subordination condition (2.3). This contradicts the above observation that

Therefore, the subordination condition (2.3) must imply the subordination given by (2.15). Considering , we see that the function G is the best dominant. This evidently completes the proof of Theorem 1. □

We next prove a solution to a dual problem of Theorem 1 in the sense that the subordinations are replaced by superordinations.

Theorem 2Let, whereis defined by (1.5). Suppose also that

whereρis given by (2.2) andis univalent in, and

whereis the integral operator defined by (1). Then the superordination relation

(2.16)

implies that

Moreover, the function

is the best subordinant.

Proof Let the functions F and G be given by (2.5). We first note from (2.7) and (2.8) that

(2.17)

After a simple calculation, equation (2.17) yields the following relationship:

where the function q is given in (2.6). Then, by using the same method as in the proof of Theorem 1, we can prove that

that is, G defined by (2.5) is convex (univalent) in .

We next prove that the superordination condition (2.16) implies that

(2.18)

For this purpose, we consider the function defined by

Since G is convex and , we can prove easily that is a subordination chain as in the proof of Theorem 1. Therefore, according to Lemma 5, we conclude that the superordination condition (2.16) must imply the superordination given by (2.18). Furthermore, since the differential equation (2.17) has the univalent solution G, it is the best subordinant of the given differential superordination. Hence we complete the proof of Theorem 2. □

If we combine Theorem 1 and Theorem 2, then we obtain the following sandwich-type theorem.

Theorem 3Let (), whereis defined by (1.5). Suppose also that

(2.19)

whereρis given by (2.2) and the functionis univalent in, and

whereis the integral operator defined by (1). Then the subordination relation

implies that

Moreover, the functions

are the best subordinant and the best dominant, respectively.

The assumption of Theorem 3, that is, the functions

need to be univalent in , will be replaced by another set of conditions in the following result.

Corollary 1Let (), whereis defined by (1.5). Suppose also that the condition (2.19) is satisfied and

(2.20)

whereρis given by (2.2). Then the subordination relation

implies that

whereis the integral operator defined by (1). Moreover, the functions

are the best subordinant and the best dominant, respectively.

Proof In order to prove Corollary 1, we have to show that the condition (2.20) implies the univalence of and

(2.21)

By noting that from (2.2), we obtain from the condition (2.20) that ψ is a close-to-convex function in (see [24]), and hence is univalent in . Furthermore, by using the same techniques as in the proof of Theorem 3, we can prove the convexity (univalence) of F and so the details may be omitted. Therefore, by applying Theorem 3, we obtain Corollary 1. □

By setting in Theorem 3, we have the following result.

Corollary 2Let (), whereis defined by (1.5) with. Suppose also that

and the functionis univalent in, and

(2.22)

whereis the integral operator defined by (1) with. Then the subordination relation

implies that

Moreover, the functions

are the best subordinant and the best dominant, respectively.

If we take in Theorem 3, then we are easily led to the following result.

Corollary 3Let (), whereis defined by (1.5) with. Suppose also that

(2.23)

and the functionis univalent in, and

(2.24)

whereis the integral operator defined by (1) with. Then the subordination relation

implies that

Moreover, the functions

are the best subordinant and the best dominant, respectively.

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

Both authors jointly worked on the results and they read and approved the final manuscript.

### Acknowledgements

Dedicated to Professor Hari M Srivastava.

This research was supported by the Basic Science Research Program through the National Research Foundation of the Republic of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012-0002619).

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