## Boundary Value Problems

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

Boundary Value Problems20132013:44

DOI: 10.1186/1687-2770-2013-44

Accepted: 21 January 2013

Published: 4 March 2013

## 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 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 . 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 , 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 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])

Let with . If satisfies

then .

Theorem 1.2 ([9])

Let with . If and
is univalent in for , 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. [1114] 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 , , and . The class of admissible functions consists of those functions that satisfy the admissibility condition
whenever
and

for , , and .

Theorem 2.1 Let . If satisfies
(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 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 2.2 Let . If satisfies
(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 2.3 Let and q be univalent in with . Let for some where . If satisfies
then

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

Theorem 2.4 Let h and q be univalent in with and set and . Let satisfy one of the following conditions:
1. (1)

for some , or

2. (2)

there exists such that for all .

If satisfies (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 2.5 Let h be univalent in and let . Suppose that the differential equation
(2.9)
has a solution q with and satisfies one of the following conditions:
1. (1)

and ,

2. (2)

q is univalent in and for some , or

3. (3)

q is univalent in and there exists such that for all .

If satisfies (2.8) and
is analytic in , then

and q is 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 , , and . The class of admissible functions consists of those functions such that
(2.10)

whenever , , , and .

Corollary 2.6 Let . If satisfies
then

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

Corollary 2.7 Let . If satisfies
then
Corollary 2.8 Let , and let be an analytic function in with for . If satisfies
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 , with , and . The class of admissible functions consists of those functions that satisfy the admissibility condition
whenever
and

for , , and .

Theorem 3.1 Let . 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 3.2 Let , h be analytic in and . 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 3.3 Let h be analytic in and . Suppose that the differential equation
has a solution . If , , and
is univalent in , then
implies

and q is 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 3.4 Let and be analytic functions in , be a univalent function in , with and . If , and
is univalent in , then
implies

## Declarations

### 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).

## Authors’ Affiliations

(1)
Department of Applied Mathematics, Pukyong National University

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