Volume 2012, Issue 1 282094
Research Article
Open Access

The System of Mixed Equilibrium Problems for Quasi-Nonexpansive Mappings in Hilbert Spaces

Rabian Wangkeeree

Corresponding Author

Rabian Wangkeeree

Department of Mathematics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand nu.ac.th

Centre of Excellence in Mathematics, CHE, Si Ayutthaya Road, Bangkok 10400, Thailand ncetm.org.uk

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Panatda Boonman

Panatda Boonman

Department of Mathematics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand nu.ac.th

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First published: 22 May 2012
Academic Editor: Rudong Chen

Abstract

We first introduce the iterative procedure to approximate a common element of the fixed-point set of two quasinonexpansive mappings and the solution set of the system of mixed equilibrium problem (SMEP) in a real Hilbert space. Next, we prove the weak convergence for the given iterative scheme under certain assumptions. Finally, we apply our results to approximate a common element of the set of common fixed points of asymptotic nonspreading mapping and asymptotic TJ mapping and the solution set of SMEP in a real Hilbert space.

1. Introduction

Let H be a real Hilbert space with inner product 〈·, ·〉 and norm ∥·∥, let C be a nonempty closed convex subset of H, and let T be a mapping of C into H, then T : CH is said to be nonexpansive if ∥TxTy∥≤∥xy∥ for all x, yC. A mapping T : CH is said to be quasinonexpansive if ∥Txy∥≤∥xy∥ for all xC and yF(T): = {xC : Tx = x}. It is well known that the set F(T) of fixed points of a quasi-nonexpansive mapping T is a closed and convex set [1]. A mapping T : CC is said to be firmly nonexpansive [2] if
()
for all x, yC, and it is an important example of nonexpansive mappings in a Hilbert space.
Let φ : C be a real-valued function, and let F : C × C be an equilibrium bifunction, that is, F(u, u) = 0 for each uC. The mixed equilibrium problem is to find xC such that
()
Denote the set of solution of (1.2) by MEP (F, φ). In particular, if φ = 0, this problem reduces to the equilibrium problem, which is to find xC such that
()
The set of solution of (1.3) is denoted by EP (F).

The problem (1.2) 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 [3] and Moudafi [4]. Numerous problems in physics, optimization, and economics reduce to find a solution of (1.3).

Let F1, F2 : C × C be two monotone bifunctions and μ > 0 is constant. In 2009, Moudafi [5] introduced an alternating algorithm for approximating a solution of the system of equilibrium problems, finding (x, y) ∈ C × C such that
()
For such mappings F1 and F2 and two given positive constants λ, μ > 0, Plubtieng and Sombut [6] considered the following system of mixed equilibrium problem, finding (x, y) ∈ C × C such that
()
In particular, if λ = μ and φ ≡ 0, then problem (SMEP) reduces to (SEP). Furthermore, Plubtieng and Sombut [6] introduced the following iterative procedure to approximate a common element of the fixed-point set of a quasi-nonexpansive mapping T and the solution set of (SMEP) in a Hilbert space H. Let {xn}, {yn}, and {un} be given by
()
where {αn}⊂[a, b] for some a, b ∈ (0,1) and satisfying appropriate conditions. The weak convergence theorems are obtained in a real Hilbert space.
On the other hand, in 1953, Mann [7] introduced the following iterative procedure to approximate a fixed point of a nonexpansive mapping T in a Hilbert space H:
()
where the initial point x0 is taken in C arbitrarily, and {αn} is a sequence in [0,1].
For two nonexpansive mappings T1, T2 of C into itself, Moudafi [4] studied weak convergence theorems in the following iterative process:
()
for all n, where {αn} and {βn} are appropriate sequences in [0,1] and F(T1)∩F(T2) ≠ . Recently, Iemoto and Takahashi [8] also considered this iterative procedure for T1 is a nonexpansive mapping and T2 : CC is a nonspreading mapping. Very recently, Kim [9] studied the weak and strong convergence for the Moudafi’s iterative scheme (1.8) of two quasi-nonexpansive mappings.

In this paper, inspired and motivated by Plubtieng and Sombut [6], Moudafi [4], Iemoto and Takahashi [8], and Kim [9], we first introduce the iterative procedure to approximate a common element of the common fixed point set of two quasi-nonexpansive mappings and the solution set of SMEP in a real Hilbert space. Next, we prove the weak convergence theorem for the given iterative scheme under certain assumptions. Finally, we apply our results to approximate a common element of the set of common fixed point of asymptotic nonspreading mapping and asymptotic TJ mapping and the solution set of SMEP in a real Hilbert space.

2. Preliminaries

Throughout this paper, let be the set of positive integers, and let be the set of real numbers. Let H be a real Hilbert space with inner product 〈·, ·〉 and norm ∥·∥, respectively, and let C be a closed convex subset of H. We denote the strong convergence and the weak convergence of {xn} to xH by xnx and xnx, respectively.

From [10], for each x, yH and λ ∈ [0,1], we have
()
For every point xH, there exists a unique nearest point in C, denoted by PCx, such that
()
PC is called the metric projection of H onto C. It is well know that PC is a nonexpansive mapping of H onto C and satisfies
()
Moreover, PCx is characterized by the following properties: PCxC,
()
Further, for all xH and yC,   y = PCx if and only if 〈xy, yz〉≥0, for all zC.

Lemma 2.1 (see [11].)Let C be a nonempty closed convex subset of a real Hilbert space H. Let P be the metric projection of H onto C, and let {xn} n be in H. If

()
for all uC and n. Then, {PCxn} converges strongly to an element of C.

Theorem 2.2 (Opial’s theorem, [10]). Let H be a real Hilbert space, and suppose that xnx, then

()
for all yH with xy.

All Hilbert space and lp  (1 < p < ) satisfy Opial’s condition, while Lp with 1 < p ≠ 2 < do not.

For solving the mixed equilibrium problem for an equilibrium bifunction F : C × C, let us assume that F satisfies the following conditions:
  • (A1)

    F(x, x) = 0 for all xC,

  • (A2)

    F is monotone, that is, F(x, y) + F(y, x) ≤ 0 for all x, yC,

  • (A3)

    for each yC,   xF(x, y) is weakly upper semicontinuous,

  • (A4)

    for each xC,   yF(x, y) is convex and semicontinuous.

The following lemma appears implicitly in [3, 12].

Lemma 2.3 (see [3].)Let C be a nonempty closed convex subset of H, and let F : C × C be a bifunction satisfying (A1)–(A4). Let r > 0 and xH, then there exists zC such that

()

The following lemma was also given in [12].

Lemma 2.4 (see [12].)Let C be a nonempty closed closed convex subset of H and let F : C × C be a bifunction satisfying (A1)–(A4), then, for any r > 0 and xH, define a mapping Trx : HC as follows:

()
for all  zH, r. Then the following hold:
  • (i)

    Tr is single valued,

  • (ii)

    Tr is firmly nonexpansive, that is,

    ()

  • (iii)

    F(Tr) = EP (F),

  • (iv)

    EP (F) is closed and convex.

We note that Lemma 2.4 is equivalent to the following lemma.

Lemma 2.5 (see [6].)Let C be a nonempty closed convex subset of a real Hilbert space H. Let F : C × C be an equilibrium bifunction satisfying (A1)–(A4) and let φ : C be a lower semicontinuous and convex functional. For each r > 0 and xH, define a mapping

()
Then, the following results hold:
  • (i)

    for each xC, Sr(x) ≠ ,

  • (ii)

    Sr is single valued,

  • (iii)

    Sr is firmly nonexpansive, that is, for any x, yH,

    ()

  • (iv)

    F(Sr) = MEP (F, φ),

  • (v)

    MEP (F, φ) is closed and convex.

3. Main Results

In this section, we prove the weak convergence for approximating a common element of the common fixed point set of two quasi-nonexpansive mappings and the solution set of the system of mixed equilibrium problems in a Hilbert space.

To begin with, let us state and proof the following characterizations of the solution set of GMEP.

Lemma 3.1. Let C be a closed convex subset of a real Hilbert space H. Let F1 and F2 be two mappings from C × C satisfying (A1)–(A4), and let S1,λ and S2,μ be defined as in Lemma 2.5 associated to F1 and F2, respectively. For given x, yC, (x, y) is a solution of problem (1.5) if and only if x is a fixed point of the mapping G : CC defined by

()
where y = S2,μx.

Proof. For given x, yC, we observe the following equivalency:

()
This completes the proof.

We note from Lemma 3.1 that the mapping G is nonexpansive. Moreover, if C is a closed bounded convex subset of H, then the solution of problem (1.5) always exists. Throughout this paper, we denote the set of fixed points of G by Ω.

Theorem 3.2. Let C be a nonempty closed convex subset of a real Hilbert space H. Let F1 and F2 be two bifunctions from C × C satisfying (A1)–(A4). Let r, λ > 0 and S1,r and S2,λ be defined as in Lemma 2.5 associated to F1 and F2, respectively. Let Ti : CC, i = 1,2, be two quasi-nonexpansive mappings such that ITi are demiclosed at zero, that is, if {wn} ⊂ C, wnw, and (ITi)wn → 0, then wF(Ti), with F(T1)∩F(T2)∩Ω ≠ . Let the sequences {xn}, {yn}, and {un}, be given by

()
where {an}, {bn}⊂[a, b] for some a, b ∈ (0,1), and satisfy
()
then and is a solution of problem (1.5), where .

Proof. Let x*F(T1)∩F(T2)∩Ω, then x* = T1x* = T2x* and x* = S1,r(S2,λx*).

Putting y* = S2,λx*,   yn = S1,run, and un = S2,λxn, we have

()
Next, we prove that
()
Since T1 and T2 are quasi-nonexpansive, we obtain that
()
which gives that
()
Hence, {∥xnx*∥} is a nonincreasing sequence, and hence, lim nxnx*∥ exists. This implies that {xn}, {yn}, {un}, {T1yn}, and {T2yn} are bounded. From (3.8), we have
()
Since liminf nan(1 − an) > 0, this implies that
()
Furthermore, since 0 < aanb < 1, we have
()
From (3.10), we conclude that
()
From (3.7), we have
()
where M is a constant satisfying M ≥ sup n≥1[∥xnx*∥+∥bnT1yn + (1 − bn)T2ynx*∥]. Again from (3.10), we conclude that
()
Using liminf nbn(1 − bn) > 0, we have
()
Now, we prove that
()
We observe that
()
which gives that
()
Since 0 < aanb < 1, we have
()
Using (3.12) and (3.15), we conclude that
()
which gives that
()
since liminf nan(1 − an) > 0. Similarly, we have
()
which implies that
()
Thus, we have
()
Hence, lim nan(1 − an)∥xn+1T2yn∥ = 0. Since liminf nan(1 − an) > 0, we have
()
Next, we prove that
()
Since S1,r and S2,λ are firmly nonexpansive, it follows that
()
which gives that
()
This implies that
()
By the convexity of ∥·∥2, we have
()
Thus,
()
Since 0 < aanb < 1, we have
()
Since lim nxnx*∥ exists, we have
()
Similarly, we have
()
which gives that
()
This implies that
()
By the convexity of ∥·∥2, we have
()
Thus,
()
Since 0 < aanb < 1, we have
()
Since lim nxnx*∥ exists, we have
()
Hence,
()
It follow from (3.10), (3.33), and (3.40) that
()
from which it follows that
()
that is,
()
Thus,
()
Similarly, we have ∥ynT2yn∥→0. Since {yn} is bounded sequence, there exists a subsequence of {yn} such that as i. Since T1 and T2 are demiclosed at 0, we conclude that . Let G be a mapping which is defined as in Lemma 3.1. Thus, we have
()
and hence,
()
This together with implies that , if is another subsequence of {yn} such that as i. Since T1 and T1 are demiclosed at 0, we conclude that . From and , we will show that . Assume that . Since lim nxnx*∥ exists for all x*F(T1)∩F(T2)∩Ω, by Opial’s Theorem 2.2, we have
()
This is a contradiction. Thus, we have . This implies that . Since ∥xnyn∥→0, we have . Put . Finally, we show that . Now from (2.4) and , we have
()
Since {∥xnx*∥} is nonnegative and nonincreasing for all x*F(T1)∩F(T2)∩Ω, it follows by Lemma 2.1 that {zn} converges strongly to some . By (3.49), we have
()
Therefore, .

Setting T : = T1 = T2 in Theorem 3.2, we have the following result.

Corollary 3.3 (see [6].)Let C be a closed convex subset of a real Hilbert space H. Let F1 and F2 be two bifunctions from C × C satisfying (A1)–(A4). Let λ, μ > 0, and let S1,λ and S2,μ be defined as in Lemma 2.5 associated to F1 and F2, respectively. Let T : CC be a quasi-nonexpansive mapping such that IT is demiclosed at zero and F(T)∩Ω ≠ . Suppose that x0 = xC and {xn}, {yn},   and  {zn} are given by

()
for all n, where {an}⊂[a, b] for some a, b ∈ (0,1), and satisfy liminf nan(1 − an) > 0, then {xn} converges weakly to and is a solution of problem (1.5), where .

Setting F1 = F2 ≡ 0, φ ≡ 0 in Theorem 3.2, we have the following result.

Corollary 3.4 (see [9].)Let H be a Hilbert space, let C be a nonempty, closed, and convex subset of H, and let T1, T2 be two quasi-nonexpansive mappings of C into itself such that IT1, IT2 are demiclosed at zero with F(T1)∩F(T2) ≠ . For any x1 in C, let {xn} be defined by

()
where {an} and {bn} are chosen so that
()
then xnpF(T1)∩F(T2).

4. Applications

In this section, we apply our results to approximate a common element of the set of common fixed points of an asymptotic nonspreading mapping and an asymptotic TJ mapping and the solution set of SMEP in a real Hilbert space. We recall the following definitions. A mapping T : CC is called nonspreading [13] if
()
Furthermore, Takahashi and Yao [14] also introduced two nonlinear mappings in Hilbert spaces. A mapping T : CC is called a TJ − 1 mapping [14] if
()
for all x, yC. A mapping T : CC is called a TJ − 2 [14] mapping if
()
for all x, yC. For these two nonlinear mappings, TJ − 1 and TJ − 2 mappings, Takahashi and Yao [14] studied the existence results of fixed points in Hilbert spaces. Very recently, Lin et al. [15] introduced the following definitions of new mappings.

Definition 4.1. Let C be a nonempty closed convex subset of a Hilbert space H. We say that T : CC is an asymptotic nonspreading mapping if there exist two functions α : C → [0,2) and β : C → [0, k], k < 2, such that

  • (A1)

    2∥TxTy2α(x)∥Txy2 + β(x)∥Tyx2 for all x, yC,

  • (A2)

    0 < α(x) + β(x) ≤ 2 for all xC.

Remark 4.2. The class of asymptotic nonspreading mappings contains the class of nonspreading mappings and the class of TJ − 2 mappings in a Hilbert space. Indeed, in Definition 4.1, we know that

  • (i)

    if α(x) = β(x) = 1 for all xC, then T is a nonspreading mapping,

  • (ii)

    if α(x) = 4/3 and β(x) = 2/3 for all xC, then T is a TJ − 2 mapping.

Definition 4.3. Let C be a nonempty closed convex subset of a Hilbert space H. We say T : CC is an asymptotic TJ mapping if there exists two functions α : C → [0,2] and β : C → [0, k], k < 2, such that

  • (B1)

    2∥TxTy2α(x)∥xy2 + β(x)∥Txy2 for all x, yC,

  • (B2)

    α(x) + β(x) ≤ 2 for all xC.

Remark 4.4. The class of asymptotic TJ mappings contains the class of TJ − 1 mappings and the class of nonexpansive mappings in a Hilbert space. Indeed, in Definition 4.3, we know that

  • (i)

    if α(x) = 2 and β(x) = 0 for each xC, then T is a nonexpansive mapping,

  • (ii)

    if α(x) = β(x) = 1 for each xC, then T is a TJ − 1 mapping.

It is well known that the set F(T) of fixed points of a quasi-nonexpansive mapping T is a closed and convex set [1]. Hence, if T : CC is an asymptotic nonspreading mapping (resp., asymptotic TJ mapping) with F(T) ≠ , then T is a quasi-nonexpansive mapping, and this implies that F(T) is a nonempty closed convex subset of C.

Theorem 4.5 (see [15].)Let C be a nonempty closed convex subset of a real Hilbert space H, and let T : CC be an asymptotic nonspreading mapping, then IT is demiclosed at 0.

Theorem 4.6 (see [15].)Let C be a nonempty closed convex subset of a real Hilbert space H, and let T : CC be an asymptotic TJ mapping, then IT is demiclosed at 0.

Applying the above results, we have the following theorem.

Theorem 4.7. Let C be a nonempty closed convex subset of a real Hilbert space H. Let F1 and F2 be two bifunctions from C × C satisfying (A1)–(A4). Let r, λ > 0 and S1,r and S2,λ be defined as in Lemma 2.5 associated to F1 and F2, respectively. Let Ti : CC, i = 1,2, be any one of asymptotic nonspreading mapping and asymptotic TJ mapping such that F(T1)∩F(T2)∩Ω ≠ . Let {xn}, {yn}, and {un} be given by

()
where {an}, {bn}⊂[a, b] for some a, b ∈ (0,1) and satisfying
()
then , and is a solution of problem (1.5), where .

Setting F1 = F2 : = F and φ ≡ 0 in the above theorem, we have the following result.

Corollary 4.8. Let C be a nonempty closed convex subset of a real Hilbert space H. Let F be the bifunctions from C × C satisfying (A1)–(A4). Let Ti : CC, i = 1,2, be any one of asymptotic nonspreading mapping and asymptotic TJ mapping such that : = F(T1)∩F(T2)∩EP (F) ≠ . For given uC and r > 0, let the sequences {xn} and {un} be defined by

()
where {an}, {bn} are two sequences in (0,1) satisfying
()
then xnw for some w.

Setting F1 = F2 ≡ 0 and φ ≡ 0 in Theorem 4.7, we have the following result.

Corollary 4.9 (see [15].)Let C be a nonempty closed convex subset of a real Hilbert space H, and let Ti : CC, i = 1,2, be any one of asymptotic nonspreading mapping and asymptotic TJ mapping. Let : = F(T1)∩F(T2) ≠ . Let {an} and {bn} be two sequences in (0,1). Let {xn} be defined by

()
Assume that liminf nan(1 − an) > 0 and liminf nbn(1 − bn) > 0, then xnw for some w.

Acknowledgment

This paper is supported by the Centre of Excellence in Mathematics under the Commission on Higher Education, Ministry of Education, Thailand.

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