- Research Article
- Open Access

# Shrinking Projection Method of Common Solutions for Generalized Equilibrium Quasi- -Nonexpansive Mapping and Relatively Nonexpansive Mapping

- Min Liu
^{1}, - Shih-Sen Chang
^{1}Email author and - Ping Zuo
^{1}

**2010**:101690

https://doi.org/10.1155/2010/101690

© Min Liu et al. 2010

**Received:**18 September 2009**Accepted:**18 February 2010**Published:**3 March 2010

## Abstract

We prove a strong convergence theorem for finding a common element of the set of solutions for generalized equilibrium problems, the set of fixed points of a relatively nonexpansive mapping, and the set of fixed points of a quasi- -nonexpansive mapping in a Banach space by using the shrinking Projection method. Our results improve the main results in S. Takahashi and W. Takahashi (2008) and Takahashi and Zembayashi (2008). Moreover, the method of proof adopted in the paper is different from that of S. Takahashi and W. Zembayashi (2008).

## Keywords

- Banach Space
- Iterative Method
- Equilibrium Problem
- Generalize Equilibrium
- Nonexpansive Mapping

## 1. Introduction

Let be a Banach space and let be a closed convex subsets of . Let be an equilibrium bifunction from into and let be a nonlinear mapping. Then, we consider the following generalized equilibrium problem: find such that

The set of solutions of (1.1) is denoted by , that is,

In the case of , is denoted by . In the case of , is denoted by .

A mapping is said to be nonexpansive if

We denote the set of fixed points of by .

A mapping is said to be Quasi- -nonexpansive if

where is defined by (2.3).

Recently, in Hilbert space, Tada and Takahashi [1], and S. Takahashi and W. Takahashi [2] considered iterative methods for finding an element of . Very recently, S. Takahashi and W. Takahashi [3] introduced an iterative method for finding an element of , where is an inverse-strongly monotone mapping and then proved a strong convergence theorem. On the other hand, Takahashi and Zembayashi [4] prove a strong convergence theorem for finding a common element of the set of solutions of an equilibrium problem and the set of fixed points of a relatively nonexpansive mapping in a Banach space by using the shrinking Projection method which is different from S. Takahashi and W. Takahashi's hybrid method [3].

In this paper, motivated by Takahashi and Zembayashi [4], in Banach space, we prove a strong convergence theorem for finding an element of , where is a continuous and monotone operator, is a relatively nonexpansive mapping, and is quasi- -nonexpansive mapping. Moreover, the method of proof adopted in the paper is different from that of [3].

## 2. Preliminaries

Throughout this paper, all the Banach spaces are real. We denote by and the sets of positive integers and real numbers, respectively. Let be a Banach space and let be the topological dual of . For all and , we denote the value of at by . Then, the duality mapping on is defined by

for every . By the Hahn-Banach theorem, is nonempty; see [5] for more details. We denote the weak convergence and the strong convergence of a sequence to in by and , respectively. We also denote the convergence of a sequence to in by . A Banach space is said to be strictly convex if for with and . It is also said to be uniformly convex if for each , there exists such that for with and . A uniformly convex Banach space has the Kadec-Klee property, that is, and imply . The space is said to be smooth if the limit

exists for all . It is also said to be uniformly smooth if the limit exists uniformly in . We know that if is smooth, strictly convex, and reflexive, then the duality mapping is single valued, one to one, and onto; see [6] for more details.

Let be a smooth, strictly convex, and reflexive Banach space and let be a closed convex subset of . Throughout this paper, we denote by the function defined by

Following Alber [7], the generalized projection from onto is defined by , where is the solution to the following minimization problem:

The generalized projection from onto is well defined, single valued and satisfies

If is a Hilbert space, then and is the metric projection of onto . It is well know that the following conclusions for generalized projections hold.

Lemma 2.1 (Alber [7] and Kamimura and Takahashi [8]).

Lemma 2.2.

Let be a nonempty closed convex subset of a smooth, strictly convex and reflexive Banach space , and let be a mapping from into itself. We denoted by the set of fixed points of . A point is said to be an asymptotic fixed point of [9, 10] if there exists in which converges weakly to and . We denote the set of all asymptotic fixed point of by . Following Matsushita and Takahashi [11], a mapping is said to be relatively nonexpansive if the following conditions are satisfied:

(1) is nonempty;

(2) for all ;

(3) .

The following lemma is due to Matsushita and Takahashi [11].

Lemma 2.3 (Matsushita and Takahashi [11]).

Let be a nonempty closed convex subset of a smooth, strictly convex and reflexive Banach space , and let be a relatively nonexpansive mapping from into itself. Then is closed and convex.

We also know the following lemmas.

Lemma 2.4 (see [8]).

Let be a smooth and uniformly convex Banach space and let and be sequences in such that either or is bounded. If , then .

Lemma 2.5 (see [12]).

for all and .

For solving the equilibrium problem for bifunction , let us assume that satisfies the following conditions:

for all ;

is monotone, that is, for all ;

for each ,

for each is a convex and lower semicontinuous.

If an equilibrium bifunction satisfies conditions ( )–( ), then we have the following two important results.

Lemma 2.6 (see [13]).

Lemma 2.7 (see [4]).

for all . Then, the following holds:

(1) is single-valued;

(3) ;

(4) is a closed and convex set.

Lemma 2.8 (see [4]).

## 3. The Main Results

In this section, we prove a strong convergence theorem which is the main result in the paper.

Theorem 3.1.

for every , where is the duality mapping on , , and for some . If the following conditions are satisfied

,

,

then converges strongly to where is the generalized projection of onto .

Proof.

Next, we prove that the bifunction satisfies conditions ( )–( ).

for all .

Since for all .

is monotone, that is, for all .

For each ,

For each is a convex and lower semicontinuous.

So, is convex.

Similarly, we can prove that is lower semicontinuous.

Therefore, the generalized equilibrium problem (1.1) is equivalent to the following equilibrium problem: find such that

Since the bifunction satisfies conditions ( )–( ), from Lemma 2.7, for a given and , we can define a mapping as follows:

Moreover, satisfies the conclusions in Lemma 2.7.

Putting for all , we have from Lemmas 2.7 and 2.8 that are relatively nonexpansive.

We divide the proof of Theorem 3.1 into six steps.

Step 1.

Step 2.

So, is well defined.

Step 3.

for all . Then is bounded. Therefore, , , and are bounded.

Step 4.

Step 5.

where .

(a)We prove that

In fact, for any given , there exists a subsequence of such that . Since and is relatively nonexpansive, we have , that is, .

(b)We prove that .

In fact, from , (3.12) and Lemma 2.8, we have that

For any given , there exists a subsequence such that . Since , we have .

Since is uniformly norm-to-norm continuous on bounded sets, from (3.38), we have

This implies that . Hence from condition , we have for all , and hence .

(c)Now we prove that .

In fact, for any given , there exists a subsequence such that . Since , we have Since is a closed convex subset of . we have , that is, . From (3.14) and (3.16), we have

that is, , then, . Since is uniformly convex Banach space, has a Kadec-Klee property, we have . From (3.34) and being closed, we have , that is, .

Step 6.

Since has the Kadec-Klee property, we have that . Therefore, converges strongly to .

This completes the proof of Theorem 3.1.

Theorem 3.2.

for every , where is the duality mapping on , satisfies and for some . Then converges strongly to , where is the generalized projection of onto .

Proof.

In Theorem 3.1, take , we get Therefore, the conclusion of Theorem 3.2 can be obtained from Theorem 3.1.

Remark 3.3.

Theorem 3.1 in [3] and Theorem 3.1 in [4] are special cases of Theorem 3.2.

## Declarations

### Acknowledgment

The authors would like to express their thanks to the referees for their helpful suggestions and comments.

## Authors’ Affiliations

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