Research

# Strong differential subordination properties for analytic functions involving the Komatu integral operator

Nak Eun Cho

Author Affiliations

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

Boundary Value Problems 2013, 2013:44  doi:10.1186/1687-2770-2013-44

 Received: 27 November 2012 Accepted: 21 January 2013 Published: 4 March 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 strong differential subordination and superordination implications involving the Komatu integral operator which are obtained by considering suitable classes of admissible functions. The sandwich-type theorems for these operators are also considered.

MSC: 30C80, 30C45, 30A20.

##### Keywords:
strong differential subordination; strong differential superordination; univalent function; convex function; Komatu integral operator

### 1 Introduction

Let ℋ denote the class of analytic functions in the open unit disk . For a positive integer n and , let

and let . We also denote by the subclass of with the usual normalization .

Let and be members of ℋ. The function is said to be subordinate to , or is said to be superordinate to , if there exists a function analytic in , with and , and such that . In such a case, we write or . If the function F is univalent in , then if and only if and (cf.[1]).

Following Komatu [2], we introduce the integral operator defined by

(1.1)

where the symbol Γ stands for the gamma function. We also note that the operator defined by (1.1) can be expressed by the series expansion as follows:

(1.2)

Obviously, we have, for ,

Moreover, from (1.2), it follows that

(1.3)

In particular, the operator is closely related to the multiplier transformation studied earlier by Flett [3]. Various interesting properties of the operator have been studied by Jung et al.[4] and Liu [5].

To prove our results, we need the following definitions and theorems considered by Antonimo [6,7] and Oros [8,9].

Definition 1.1 ([6], cf.[7,8])

Let be analytic in and let be analytic and univalent in . Then the function is said to be strongly subordinate to , or is said to be strongly superordinate to , written as , if for , as the function of z is subordinate to . We note that if and only if and .

Definition 1.2 ([8], cf.[1])

Let and let be univalent in . If is analytic in and satisfies the (second-order) differential subordination

(1.4)

then is called a solution of the strong differential subordination. The univalent function is called a dominant of the solutions of the strong differential subordination, or more simply a dominant, if for all satisfying (1.4). A dominant that satisfies for all dominants of (1.4) is said to be the best dominant.

Recently, Oros [9] introduced the following strong differential superordinations as the dual concept of strong differential subordinations.

Definition 1.3 ([9], cf.[10])

Let and let be analytic in . If and are univalent in for and satisfy the (second-order) strong differential superordination

(1.5)

then is called a solution of the strong differential superordination. An analytic function is called a subordinant of the solutions of the strong differential superordination, or more simply a subordinant, if for all satisfying (1.5). A univalent subordinant that satisfies for all subordinants of (1.5) is said to be the best subordinant.

Denote by the class of functions q that are analytic and injective on , where

and are such that for . Further, let the subclass of for which be denoted by and .

Definition 1.4 ([8])

Let Ω be a set in ℂ, and n be a positive integer. The class of admissible functions consists of those functions that satisfy the admissibility condition

whenever , and

for , , and . We write as .

Definition 1.5 ([9])

Let Ω be a set in ℂ and with . The class of admissible functions consists of those functions that satisfy the admissibility condition

whenever , for and

for , , and . We write as .

For the above two classes of admissible functions, Oros and Oros proved the following theorems.

Theorem 1.1 ([8])

Letwith. Ifsatisfies

then.

Theorem 1.2 ([9])

Letwith. Ifand

is univalent infor, then

implies.

In the present paper, making use of the differential subordination and superordination results of Oros and Oros [8,9], we determine certain classes of admissible functions and obtain some subordination and superordination implications of multivalent functions associated with the Komatu integral operator defined by (1.1). Additionally, new differential sandwich-type theorems are obtained. We remark in passing that some interesting developments on differential subordination and superordination for various operators in connection with the Komatu integral operator were obtained by Ali et al.[11-14] and Cho et al.[15].

### 2 Subordination results

Firstly, we begin by proving the subordination theorem involving the integral operator defined by (1.1). For this purpose, we need the following class of admissible functions.

Definition 2.1 Let Ω be a set in ℂ, , and . The class of admissible functions consists of those functions that satisfy the admissibility condition

whenever

and

for , , and .

Theorem 2.1Let. Ifsatisfies

(2.1)

then

Proof Define the function in by

(2.2)

From (2.2) with the relation (1.3), we get

(2.3)

Further computations show that

(2.4)

Define the transformation from to ℂ by

(2.5)

Let

(2.6)

Using equations (2.2), (2.3) and (2.4), from (2.6), we obtain

(2.7)

Hence, (2.1) becomes

Note that

and so the admissibility condition for is equivalent to the admissibility condition for . Therefore, by Theorem 1.1, or

which evidently completes the proof of Theorem 2.1. □

If is a simply connected domain, then for some conformal mapping h of onto Ω. In this case, the class is written as . The following result is an immediate consequence of Theorem 2.1.

Theorem 2.2Let. Ifsatisfies

(2.8)

then

Our next result is an extension of Theorem 2.1 to the case where the behavior of q on is not known.

Corollary 2.3Letandqbe univalent inwith. Letfor somewhere. Ifsatisfies

then

Proof Theorem 2.1 yields . The result is now deduced from . □

Theorem 2.4Lethandqbe univalent inwithand setand. Letsatisfy one of the following conditions:

(1) for some, or

(2) there existssuch thatfor all.

Ifsatisfies (2.8), then

Proof The proof is similar to that of [[1], Theorem 2.3d] and so is omitted. □

The next theorem yields the best dominant of the differential subordination (2.7).

Theorem 2.5Lethbe univalent inand let. Suppose that the differential equation

(2.9)

has a solutionqwithand satisfies one of the following conditions:

(1) and,

(2) qis univalent inandfor some, or

(3) qis univalent inand there existssuch thatfor all.

Ifsatisfies (2.8) and

is analytic in, then

andqis the best dominant.

Proof Following the same arguments as in [[1], Theorem 2.3e], we deduce that q is a dominant from Theorem 2.2 and Theorem 2.4. Since q satisfies (2.9), it is also a solution of (2.8) and therefore q will be dominated by all dominants. Hence, q is the best dominant. □

In the particular case , , and in view of Definition 2.1, the class of admissible functions , denoted by , is described below.

Definition 2.2 Let Ω be a set in ℂ, , and . The class of admissible functions consists of those functions such that

(2.10)

whenever , , , and .

Corollary 2.6Let. Ifsatisfies

then

In the special case , the class is simply denoted by .

Corollary 2.7Let. Ifsatisfies

then

Corollary 2.8Let, and letbe an analytic function inwithfor. Ifsatisfies

then

Proof This follows from Corollary 2.6 by taking and , where . To use Corollary 2.6, we need to show that , that is, the admissible condition (2.10) is satisfied. This follows since

for , , , and . Hence, by Corollary 2.6, we deduce the required results. □

### 3 Superordination and sandwich-type results

The dual problem of differential subordination, that is, differential superordination of the Komatu integral operator defined by (1.1), is investigated in this section. For this purpose, the class of admissible functions is given in the following definition.

Definition 3.1 Let Ω be a set in ℂ, with , and . The class of admissible functions consists of those functions that satisfy the admissibility condition

whenever

and

for , , and .

Theorem 3.1Let. If, and

is univalent in, then

(3.1)

implies

Proof From (2.7) and (3.1), we have

From (2.5), we see that the admissibility condition for is equivalent to the admissibility condition for ψ as given in Definition 1.2. Hence, , and by Theorem 1.2, or

which evidently completes the proof of Theorem 3.1. □

If is a simply connected domain, then for some conformal mapping h of onto Ω. In this case, the class is written as . Proceeding similarly as in the previous section, the following result is an immediate consequence of Theorem 3.1.

Theorem 3.2Let, hbe analytic inand. If, and

is univalent in, then

(3.2)

implies

Theorem 3.1 and Theorem 3.2 can only be used to obtain subordinants of differential superordination of the form (3.1) or (3.2). The following theorem proves the existence of the best subordinant of (3.2) for certain ϕ.

Theorem 3.3Lethbe analytic inand. Suppose that the differential equation

has a solution. If, , and

is univalent in, then

implies

andqis the best subordinant.

Proof The proof is similar to that of Theorem 2.5 and so is omitted. □

Combining Theorem 2.2 and Theorem 3.2, we obtain the following sandwich-type theorem.

Theorem 3.4Letandbe analytic functions in, be a univalent function in, withand. If, and

is univalent in, then

implies

### Competing interests

The author declares that they have no competing interests.

### Authors’ contributions

The author worked on the results and he 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 Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012-0002619).

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