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Abstract and Applied Analysis
Volume 2012 (2012), Article ID 161897, 22 pages
http://dx.doi.org/10.1155/2012/161897
Research Article

Iterative Algorithms with Perturbations for Solving the Systems of Generalized Equilibrium Problems and the Fixed Point Problems of Two Quasi-Nonexpansive Mappings

Department of Mathematics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand

Received 3 September 2012; Accepted 1 November 2012

Academic Editor: Xiaolong Qin

Copyright © 2012 Rabian Wangkeeree and Uraiwan Boonkong. 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 new iterative algorithms with perturbations for finding a common element of the set of solutions of the system of generalized equilibrium problems and the set of common fixed points of two quasi-nonexpansive mappings in a Hilbert space. Under suitable conditions, strong convergence theorems are obtained. Furthermore, we also consider the iterative algorithms with perturbations for finding a common element of the solution set of the systems of generalized equilibrium problems and the common fixed point set of the super hybrid mappings in Hilbert spaces.

1. Introduction

Let be a real Hilbert space with inner product and norm and a nonempty closed convex subset of and let be a mapping of into . Then, is said to be nonexpansive if for all . A mapping is said to be quasi-nonexpansive if for all and . It is well known that the set of fixed points of a quasi-nonexpansive mapping is closed and convex; see Itoh and Takahashi [1]. A mapping is called nonspreading [2] if for all . We remark that nonlinear every nonspreading mappings are quasi-nonexpansive mappings if the set of fixed points is nonempty.

Recall that a mapping is said to be -inverse strongly monotone if there exists a positive real number such that If is a -inverse strongly monotone mapping of into , then it is obvious that is -Lipschitz continuous.

Let be a bifuction and be -inverse strongly monotone mapping. The generalized equilibrium problem (for short, ) for and is to find such that The problem (1.3) was studied by Moudafi [3]. The set of solutions for problem (1.3) is denoted by , that is, If in (1.3), then reduces to the classical equilibrium problem and is denoted by , that is, If in (1.3), then reduces to the classical variational inequality and is denoted by , that is, The problem (1.3) is very general in the sense that it includes, as special cases, optimization problems, variational inequalities, Min–Max problems, the Nash equilibrium problems in noncooperative games, and others; see, for example, Blum and Oettli [4] and Moudafi [3].

In 2005, Combettes and Hirstoaga [5] introduced an iterative algorithm of finding the best approximation to the initial data and proved a strong convergence theorem. In 2007, by using the viscosity approximation method, S. Takahashi and W. Takahashi [6] introduced another iterative scheme for finding a common element of the set of solutions of the equilibrium problem and the set of fixed points of a nonexpansive mapping. Subsequently, algorithms constructed for solving the equilibrium problems and fixed point problems have further developed by some authors. In particular, Ceng and Yao [7] introduced an iterative scheme for finding a common element of the set of solutions of the mixed equilibrium problem and the set of common fixed points of finitely many nonexpansive mappings. Mainge and Moudafi [8] introduced an iterative algorithm for equilibrium problems and fixed point problems. Wangkeeree [9] introduced a new iterative scheme for finding the common element of the set of common fixed points of nonexpansive mappings, the set of solutions of an equilibrium problem, and the set of solutions of the variational inequality. Wangkeeree and Kamraksa [10] introduced an iterative algorithm for finding a common element of the set of solutions of a mixed equilibrium problem, the set of fixed points of an infinite family of nonexpansive mappings and the set of solutions of a general system of variational inequalities for a cocoercive mapping in a real Hilbert space. Their results extend and improve many results in the literature.

In 1967, Wittmann [11] (see also [12]) proved the strong convergence theorem of Halpern’s type [13] defined by, for any , where satisfies , and . In [14], Kurokawa and Takahashi also studied the following Halpern’s type for nonspreading mappings in a Hilbert space; see also Hojo and Takahashi [15]. Let be a nonspreading mapping of into itself. Let and define two sequences and in as follows: and for all , where , and . If is nonempty, they proved that and converge strongly to , where is the metric projection of onto . Recently, Yao and Shahzad [16] gave the following iteration process for nonexpansive mappings with perturbation: and where and are sequences in , and the sequence is a small perturbation for the -step iteration satisfying as . In fact, there are perturbations always occurring in the iterative processes because the manipulations are inaccurate.

On the other hand, very recently, Chuang et al. [17] considered the following iteration process for finding a common element of the set of solutions of the equilibrium problem and the set of common fixed points for a quasi-nonexpansive mapping with perturbation where is a nonempty closed convex subset of , is a function, and are real sequences in , and is a convergent sequence and for some . They obtained a strong convergence theorem for such iterations.

In this paper, inspired and motivated by Yao and Shahzad [16], S. Takahashi and W. Takahashi [18] and Chuang et al. [17], we introduce a new iterative algorithms with perturbations for finding a common element of the set of solutions of the system of generalized equilibrium problems and the set of common fixed points of two quasi-nonexpansive mappings in a Hilbert space. Under suitable conditions, strong convergence theorems are obtained. Furthermore, we also consider the iterative algorithms with perturbations for finding a common element of the solution set of the system of generalized equilibrium problems and the common fixed point set of the super hybrid mappings in a Hilbert space.

2. Preliminaries

Let be a real Hilbert space with inner product and norm . We denote the strongly convergence and the weak convergence of to by and , respectively. In a Hilbert space, it is known that for all and ; see [19]. Furthermore, we have that for any Let be a nonempty closed convex subset of and . We know that there exists a unique nearest point such that . We denote such a correspondence by . The mapping is called the metric projection of onto . It is known that is nonexpansive and for all and ; see [19, 20] for more details.

Let be a nonempty, closed and convex subset of and let be a bifunction. For solving the generalized equilibrium problem, let us assume that the bifunction satisfies the following conditions:(A1) for all ; (A2) is monotone, that is, for any ;(A3) for each (A4) for each is convex and lower semicontinuous.We know the following lemma which appears implicitly in Blum and Oettli [4].

Lemma 2.1 (see [4]). Let be a nonempty closed convex subset of and let be a bifunction of into satisfying . Let and . Then, there exists a unique such that

The following lemma was also given in Combettes and Hirstoaga [5].

Lemma 2.2 (see [5]). Let be a nonempty closed convex subset of a real Hilbert space . Let be a bifunction which satisfies conditions . For and , define a mapping as follows: for all . Then the following hold:(i) is single-valued;(ii) is firmly nonexpansive, that is, for any , (iii) is a closed convex subset of ;(iv).

Remark 2.3. For any and , by Lemma 2.2 (i), there exists such that Replacing with in (2.8), we have where is an inverse strongly monotone mapping.

Lemma 2.4 (see [21]). Let be a sequence of real numbers that does not decrease at infinity in the sense that there exists a subsequence of which satisfies for all . Define the sequence of integers as follows: where such that . Then, the following hold:(i) and ;(ii) and .

Lemma 2.5 (see [22]). Let be a sequence of nonnegative real numbers, a sequence of real numbers in with a sequence of nonnegative real numbers with a sequence of real numbers with . Suppose that Then .

3. Main Results

Let be a nonempty closed convex subset of a Hilbert space . For each , let be a bifunction satisfying and a -inverse strongly monotone mapping. For each , let be two mappings. Let be a sequence generated in the following manner: where are sequences in (0, 1) and is a sequence and for some and for all . Under certain appropriate assumptions imposed on the sequences , the strong convergence theorem of defined by (3.1) is studied in the following theorem.

Theorem 3.1. Let be a nonempty closed convex subset of a Hilbert space . For each , let be a bifunction satisfying and a -inverse strongly monotone mapping. For each , let be two quasi-nonexpansive mappings such that are demiclosed at zero with . Let the sequences ,  , and be defined by (3.1), where , and satisfy the following conditions:(C1) and ;(C2);(C3); (C4) for some .Then converges strongly to , where .

Proof. We first have that for all , is a nonexpansive mapping. Indeed, for all , we obtain Thus is nonexpansive for each . Now, let be arbitrary. By (C4), is a bounded sequence, there exists such that For each and , we have from that which gives also that Since is quasi-nonexpansive, we have So, we have from (3.5) and (3.6) and the quasi-nonexpansiveness of that By Induction, we have that Thus we obtain that is bounded, so also , and are bounded. Since is closed and convex, we can take . It follows that From (3.9), we have Hence we have from (3.5), (3.9), and (3.10) that We also have that Furthemore, we have from that On the other hand, since , we have We also have that Moreover, for any , we have from that It follows that This implies that and hence Furthermore, we have from Lemma 2.2 that for any , we have This implies that Then we have from (3.22) that Hence we have from (3.23) that It follows that Next, we will consider the following two cases.
Case A. Put for all . Suppose that for all . In this case exists and then . By (C1), (C3), and (3.12), we have Similarly by (C1), (C2), and (3.13), we also have So, we have from (3.14), (3.26), and (3.27) that Since exists, we have from (3.11) and (3.26) We also have from (C1), (3.16), (3.26), and (3.27) that Since exists we have from (C1) and (3.20) that This together with (3.25) and the existence of implies that which gives that So, from (3.30), . Furthermore, we have from (3.33) that that is Now, since is a bounded sequence, there exists a subsequence of such that Without loss of generality, we may assume that . Since is demiclosed at zero and by (3.26), we conclude that . Similarly, since is demiclosed at zero and by (3.28), we have . Therefore, we get that Next, we show that . For each , since , we have From , we also have Replacing by , we have Put for all and . Since , then and Since as , we obtain that as . Furthermore, by the monotonicity of , we obtain that Taking in (3.41), we have from (A4) that Now, from (A1), (A4), and (3.43), we also have which yields that Taking , we have, for each This shows , for all . Then, . Hence we have . So, we have from (3.36) that By (C1), (C4), (3.15), (3.30), (3.47), and Lemma 2.5, we obtain that . Hence we have from (3.29) that converges to , where .
Case B. Assume that there exists a subsequence of such that for all . In this case, it follows from Lemma 2.4 that there exists a subsequence of such that , where is defined by So, from (3.12), that Since , we have By (C1) and (C3), we have By (3.15), we have Now, in view of , we see that Furthermore, we also have from (3.13) that Applying (C1) and (C2) to the last inequality, we get that By (C1), (3.16), (3.51), and (3.55), we have By (3.33), we have It follows from (3.56) and (3.57) that Since is a bounded sequence, there exists a subsequence such that Following the same argument as the proof of Case A for , we have that Using (C4), (3.53), (3.56), and (3.60), we have that By (3.58) and (3.61), we have that By Lemma 2.4 (ii), we get ; that is . We observe that Applying (C1), (C4), and , we have immediately that is, converges strongly to , where . This completes the proof.

Setting for all in Theorem 3.1, we obtain the following result.

Corollary 3.2. Let be a nonempty closed convex subset of a Hilbert space . For each , let be a bifunction satisfying. For each , let be two quasi-nonexpansive mappings such that are demiclosed at zero with . Let the sequences ,   , and be defined by where satisfy the following conditions.(C1) and ;(C2);(C3); (C4) for some .Then converges strongly to , where .

In the next results, using Theorem 3.1, we have new strong convergence theorems for two nonexpansive mappings in a Hilbert space.

Corollary 3.3. Let be a nonempty closed convex subset of a Hilbert space . For each , let be a bifunction satisfying (A1)–(A4) and a -inverse strongly monotone mapping. For each , let be two nonexpansive mappings such that . Let the sequences ,  , and be defined by (3.1), where satisfy the following conditions.(C1) and ;(C2);(C3); (C4) for some .Then converges strongly to , where .

4. Applications

In this section, we present some convergence theorems deduced from the results in the previous section. Recall that a mapping is said to be nonspreading [2] if for all . Further, a mapping is said to be hybrid [23] if for all . These mappings are deduced from a firmly nonexpansive mapping in a Hilbert space.

A mapping is said to be firmly nonexpansive if for all ; see, for instance, Browder [24] and Goebel and Kirk [25]. We also know that a firmly nonexpansive mapping can be deduced from an equilibrium problem in a Hilbert space.

Recently, Kocourek et al. [26] introduced a more broad class of nonlinear mappings call generalized hybrid if there are such that for all . Very recently, they defined a more broad class of mappings than the class of generalized hybrid mappings in a Hilbert space. A mapping is called super hybrid if there are such that for all . We call such a mapping an -super hybrid mapping. We notice that an -super hybrid mapping is -generalized hybrid. So, the class of super hybrid mappings contains the class of generalized hybrid mappings. A super hybrid mapping is not quasi-nonexpansive generally. For more details, see [27]. Before proving, we need the following lemmas.

Lemma 4.1 (see [27]). Let be a nonempty subset of a Hilbert space and let and be real numbers with . Let and be mappings of into such that . Then, is -super hybrid if and only if is -generalized hybrid. In this case, .

Lemma 4.2 (see [27]). Let be a Hilbert space and let be a nonempty closed convex subset of . Let be a generalized hybrid mapping. Then is demiclosed on .
Setting in Theorem 3.1, where is a super hybrid mapping and is a real number, we obtain the following result.

Theorem 4.3. Let be a nonempty closed convex subset of a Hilbert space . For each , let be a bifunction satisfying (A1)–(A4) and a -inverse strongly monotone mapping. For each , let be -super hybrid mappings such that . Let the sequences ,  , and be defined by where are sequences in and is a sequence and for some and for all . Suppose the following conditions are satisfied.(C1) and ;(C2);(C3); (C4) for some .Then converges strongly to , where .

Proof. For each , setting we have from Lemma 4.1 that each is a generalized hybrid mapping and . Since , we have that each is quasi-nonexpansive. Following the proof of Theorem 3.1 and applying Lemma 4.2, we have the desired result. This completes the proof.

Setting in Theorem 4.3, we obtains the following result.

Corollary 4.4. Let be a nonempty closed convex subset of a Hilbert space . For each , let be a bifunction satisfying . For each , let be <