Abstract and Applied Analysis

Volume 2012, Article ID 379130, 10 pages

http://dx.doi.org/10.1155/2012/379130

## On Certain Classes of Biharmonic Mappings Defined by Convolution

Department of Mathematics, Hunan Normal University, Changsha, Hunan 410081, China

Received 1 March 2012; Accepted 7 August 2012

Academic Editor: Saminathan Ponnusamy

Copyright © 2012 J. Chen and X. Wang. 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 a class of complex-valued biharmonic mappings, denoted by , together with its subclass , and then generalize the discussions in Ali et al. (2010) to the setting of and in a unified way.

#### 1. Introduction

A four times continuously differentiable complex-valued function in a domain is * biharmonic* if , the Laplacian of , is harmonic in . Note that is * harmonic* in if satisfies the biharmonic equation in , where represents the Laplacian operator

It is known that, when is simply connected, a mapping is biharmonic if and only if has the following representation: where are complex-valued harmonic mappings in for (cf. [1–6]). Also it is known that can be expressed as the form for , where all and are analytic in (cf. [7, 8]).

Biharmonic mappings arise in a lot of physical situations, particularly, in fluid dynamics and elasticity problems, and have many important applications in engineering and biology (cf. [9–11]). However, the investigation of biharmonic mappings in the context of geometric function theory is a recent one (cf. [1–6]).

In this paper, we consider the biharmonic mappings in . Let denote the set of all biharmonic mappings in with the following form: with , , , and .

In [12], Qiao and Wang proved that for each , if the coefficients of satisfy the following inequality: then is sense preserving, univalent, and starlike in (see [12, Theorems 3.1 and 3.2]).

Let denote the set of all univalent harmonic mappings in , where with . In particular, we use to denote the set of all mappings in with . Obviously, .

In 1984, Clunie and Sheil-Small [7] discussed the class and its geometric subclasses. Since then, there have been many related papers on and its subclasses (see [13, 14] and the references therein). In 1999, Jahangiri [15] studied the class consisting of all mappings such that and are of the form and satisfy the condition in , where .

For two analytic functions and , if
then the * convolution* of and is defined by

By using the convolution, in [16], Ali et al. introduced the class of harmonic mappings in the form of (1.6) such that and the class such that where and are constants, and is analytic in .

Now we consider a class of biharmonic mappings, denoted by , as follows: with the form (1.4) is said to be in if and only if where are analytic in for , is a constant, , , are constants with , , and . Here and in what follows, “” always stands for “”.

Obviously, if , and , then reduces to , and if , and , then reduces to .

Further, we use to denote the class consisting of all mappings in with the form where

The object of this paper is to generalize the discussions in [16] to the setting of and in a unified way. The organization of this paper is as follows. In Section 2, we get a convolution characterization for . As a corollary, we derive a sufficient coefficient condition for mappings in to belong to . The main results are Theorems 2.1 and 2.3. In Section 3, first, we get a coefficient characterization for , and then find the extreme points of . The corresponding results are Theorems 3.1 and 3.6.

#### 2. A Convolution Characterization

We begin with a convolution characterization for .

Theorem 2.1. *Let . Then if and only if
**
for all and all with . *

*Proof. * By definition, a necessary and sufficient condition for a mapping in to be in is given by (1.13). Let
Then , and so the condition (1.13) is equivalent to
for all and all with and . Obviously, (2.3) holds if and only if

Straightforward computations show that
from which we see that (2.3) is true if and only if so is (2.1). The proof is complete.

*Remark 2.2. *If and , then Theorem 2.1 coincides with Theorem in [16], and if , , and , then Theorem 2.1 coincides with Theorem in [16].

As an application of Theorem 2.1, we derive a sufficient condition for mappings in to be in in terms of their coefficients.

Theorem 2.3. *Let . Then if
**
here and in the following, , where , and are constants. *

*Proof. * For given by (1.4), we see that
If is the identity, obviously, . If is not the identity, then
Hence the assumption implies that for all and all with . It follows from Theorem 2.1 that .

*Remark 2.4. *If and , then Theorem 2.3 coincides with Theorem in [16], and if and , then Theorem 2.3 coincides with Theorem in [16].

#### 3. A Coefficient Characterization and Extreme Points

We start with a coefficient characterization for .

Theorem 3.1. *Let with , and let be of the form (1.15). Then if and only if
*

*Proof. * By similar arguments as in the proof of Theorem 2.3, we see that it suffices to prove the “only if” part. For , obviously, (1.13) is equivalent to
in , where
Letting through real values leads to the desired inequality. So the proof is complete.

*Remark 3.2. *If , , and , then Theorem 3.1 coincides with Theorem in [16].

It follows from Theorem 3.1 that we have the following.

Corollary 3.3. * Let with and . If , then for , one has
**
The result is sharp with equality for mappings
*

Theorem 3.1 and Corollary 3.3 imply the following

Corollary 3.4. * Under the hypotheses of Corollary 3.3, one has that is closed under the convex combination. *

*Definition 3.5. * Let be a topological vector space over the field of complex numbers, and let be a subset of . A point is called an extreme point of if it has no representation of the form () as a proper convex combination of two distinct points and in (cf. [17]).

We now determine the extreme points of .

Theorem 3.6. *Let *(1)*, *(2)*, *(3)* for and all , *(4)* for and all . ** Under the hypotheses of Corollary 3.3, one has that if and only if it can be expressed as
**
where , all other and are nonnegative, and .**In particular, the extreme points of are all mappings and listed in , , and above. *

*Proof. * It follows from the assumptions that
whence
and so Theorem 3.1 implies that .

Conversely, assume , and let
for and all . Then
The proof of the theorem is complete.

*Remark 3.7. *If , and , then Theorem 3.6 coincides with Theorem in [16].

#### Acknowledgment

The research was partly supported by NSFs of China (No. 11071063).

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