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Volume 2010, Article ID 836714, 29 pagesdoi:10.1155/2010/836714 Research Article Iterative Methods for Finding Common Solution of Generalized Equilibrium Problems and Variational Inequal

Volume 2010, Article ID 836714, 29 pages

doi:10.1155/2010/836714

Research Article

Iterative Methods for Finding Common

Solution of Generalized Equilibrium Problems

and Variational Inequality Problems

and Fixed Point Problems of a Finite Family

of Nonexpansive Mappings

Atid Kangtunyakarn

Department of Mathematics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand

Correspondence should be addressed to Atid Kangtunyakarn,beawrock@hotmail.com

Received 7 October 2010; Accepted 2 November 2010

Academic Editor: T D Benavides

Copyrightq 2010 Atid Kangtunyakarn This is an open access article distributed under theCreative Commons Attribution License, which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly cited

We introduce a new method for a system of generalized equilibrium problems, system of

variational inequality problems, and fixed point problems by using S-mapping generated by a

finite family of nonexpansive mappings and real numbers Then, we prove a strong convergencetheorem of the proposed iteration under some control condition By using our main result, weobtain strong convergence theorem for finding a common element of the set of solution of a system

of generalized equilibrium problems, system of variational inequality problems, and the set ofcommon fixed points of a finite family of strictly pseudocontractive mappings

1 Introduction

Let H be a real Hilbert space, and let C be a nonempty closed convex subset of H Let A :

C → H be a nonlinear mapping, and let F : C × C → R be a bifunction A mapping T of H into itself is called nonexpansive if Tx − Ty ≤ x − y for all x, y ∈ H We denote by FT the set of fixed points of T i.e., FT  {x ∈ H : Tx  x} Goebel and Kirk 1 showed that FT

is always closed convex, and also nonempty provided T has a bounded trajectory.

A bounded linear operator A on H is called strongly positive with coefficient γ if there

is a constant γ > 0 with the property

Ax, x ≥ γx2. 1.1

The equilibrium problem for F is to find x ∈ C such that

≥ 0, ∀y ∈ C. 1.2

The set of solutions of1.2 is denoted by EPF Many problems in physics, optimization,

and economics are seeking some elements of EPF, see 2,3 Several iterative methods havebeen proposed to solve the equilibrium problem, see, for instance,2 4 In 2005, Combettesand Hirstoaga3 introduced an iterative scheme of finding the best approximation to theinitial data when EPF is nonempty and proved a strong convergence theorem

The variational inequality problem is to find a point u ∈ C such that

In the case of A ≡ 0, EPF, A  EPF Numerous problems in physics, optimization,

variational inequalities, minimax problems, the Nash equilibrium problem in noncooperativegames reduce to find element of1.5

A mapping A of C into H is called inverse-strongly monotone, see5, if there exists a

positive real number α such that

see 6,7

The ploblem of finding a common element of EPF, A and the set of all commonfixed points of a family of nonexpansive mappings is of wide interdisciplinary interest andimportance Many iterative methods are purposed for finding a common element of thesolutions of the equilibrium problem and fixed point problem of nonexpansive mappings,see8 10

In 2008, S.Takahashi and W.Takahashi11 introduced a general iterative method forfinding a common element of EPF, A and FT They defined {xn} in the following way:

where A is an α-inverse strongly monotone mapping of C into H with positive real number α,

and{a n } ∈ 0, 1, {β n } ⊂ 0, 1, {λ n } ⊂ 0, 2α, and proved strong convergence of the scheme

1.7 to z ∈ N

i1F T i  ∩ EPF, A, where z  P N

i1F T i ∩EPF, A u in the framework of a Hilbert

space, under some suitable conditions on{a n }, {β n }, {λ n } and bifunction F.

Very recently, in 2010, Qin, et al.12 introduced a iterative scheme method for finding

a common element of EPF1, A , EPF2, B and common fixed point of infinite family ofnonexpansive mappings They defined{x n} in the following way:

where f : C → C is a contraction mapping and W n is W-mapping generated by infinite

family of nonexpansive mappings and infinite real number Under suitable conditions ofthese parameters they proved strong convergence of the scheme1.8 to z  PFf z, where

where f : C → C and S n is S-mapping generated by T0, , T n and α n , α n−1, , α0 Undersuitable conditions, we proved strong convergence of{x n } to z  PFf z, and z is solution of

Ax∗, x − x∗ ≥ 0,

Bx∗, x − x∗ ≥ 0. 1.10

2 Preliminaries

In this section, we collect and give some useful lemmas that will be used for our main result

in the next section

Let C be closed convex subset of a real Hilbert space H, and let P C be the metric

projection of H onto C, that is, for x ∈ H, P C x satisfies the property

x − P C x  min

y ∈Cx − y. 2.1

The following characterizes the projection P C

Lemma 2.1 see 13 Given x ∈ H and y ∈ C Then P C x  y if and only if there holds the

Let {λ n } be a sequence of positive numbers with Σ∞

n1λ n  1 Then a mapping S on C defined by

S x  Σ∞

n1λ n T n x 2.4

Lemma 2.4 see 16 Let E be a uniformly convex Banach space, C a nonempty closed convex

Lemma 2.5 see 17 Let {x n } and {z n } be bounded sequences in a Banach space X, and let {β n}

A1 Fx, x  0 for all x ∈ C;

A2 F is monotone, that is, Fx, y  Fy, x ≤ 0, ∀x, y ∈ C,

A3 for all x, y, z ∈ C,

limt→ 0F

tz  1 − tx, y≤ Fx, y

A4 for all x ∈ C, y → Fx, y is convex and lower semicontinuous.

The following lemma appears implicitly in2

Lemma 2.6 see 2 Let C be a nonempty closed convex subset of H, and let F be a bifunction of

C × C into R satisfying (A1)–(A4) Let r > 0 and x ∈ H Then, there exists z ∈ C such that

In 2009, Kangtunyakarn and Suantai18 defined a new mapping and proved theirlemma as follows.

This mapping is called S-mapping generated by T1, , T N and α1, α2, , α N

Lemma 2.9 Let C be a nonempty closed convex subset of strictly convex Let {T i}N

∞ for i  1, 3 and j  1, 2, 3, , N Moreover, for every n ∈ N, let S and S n be the S-mappings

Lemma 2.11 see 19 Let C be a nonempty closed convex subset of a Hilbert space H, and let

G x  P C x − λAx, ∀x ∈ C, 2.12

with ∀λ > 0 Then x∗∈ V IC, A if and only if x∗∈ FG.

3 Main Result

Theorem 3.1 Let C be a nonempty closed convex subset of a Hilbert space H Let F and G be two

family of nonexpansive mappings withF  N

i1F T i  ∩ EPF, A ∩ EPG, B ∩ FG1 ∩ FG2 / ∅,

0, 2β, λ n ∈ e, f ⊂ 0, 2α, η n ∈ g, h ⊂ 0, 2β Assume that

i limn→ ∞n  0 and Σ∞

x, y ∈ C Since A is α-strongly monotone and λ n < 2α for all n∈ N, we have

ThusI − λ n A  is nonexpansive By using the same proof, we obtain that I − η n B  I − r n AandI − s n B are nonexpansive.

We will divide our proof into 6 steps

F u n , u   Ax n , u − u n  1

r n u − u n , u n − x n  ≥ 0, ∀u ∈ C, 3.4then we have

By induction we can prove that{x n } is bounded and so are {u n }, {v n }, {y n }, {S n y n} Without

of generality, assume that there exists a bounded set K ⊂ C such that

By definition of S n , for k ∈ {2, 3, , N}, we have

By3.11, we obtain that for each n ∈ N,

This together with the conditioniv, we obtain

From definition of y n, we have

By nonexpansiveness of T r n , T s n , I − λ n A, I − η n B and3.23, we have

y n − z2≤ δ n M n − z2 1 − δ n N n − z2

≤ δ n u n − z2 1 − δ n v n − z2.

3.35

By nonexpansiveness of I − r n A, we have

u n − z2  T r n x n − r n Ax n  − T r n z − r n Az2

≤ x n − r n Ax n  − z − r n Az , u n − z

 12



x n − r n Ax n  − z − r n Az2 u n − z2

−x n − r n Ax n  − z − r n Az  − u n − z2

≤ 12



x n − z2 u n − z2− x n − u n  − r n Ax n − Az2

 12

Since{x n i } is bounded, there exists a subsequence {x n ij } of {x n i} which converges weakly

to q Without loss of generality, we can assume that x n i q Since C is closed convex, C is

weakly closed So, we have q ∈ C Let us show that q ∈ F  N

i1F T i  ∩ EPF, A ∩ EPG, B ∩

F G1 ∩ FG2 We first show that q ∈ EPF, A ∩ EPG, B ∩ FG1 ∩ FG2 From 3.42, we

have u n i q Since u n  T r n I − r n A x n , for any y ∈ C, we have

FromA2, we have

Sinceu n i − x n i  → 0, we have Au n i − Ax n i  → 0 Further, from monotonicity of A, we have

z t − u n i , Az t − Au n i ≥ 0 So, from A4, we have

q ∈ EPF, A. 3.68

From3.43, we have v ni q Since v n  T s n I − s n B x n , for any y ∈ C, we have

Sincev n i − x n i  → 0, we have Bv n i − Bx n i  → 0 Further, from monotonicity of B, we have

z t − v n i , Bz t − Bv n i ≥ 0 So, from A4, we have

This implies that

where M1  supn≥0{P C I − λ n A x n   P C I − η n B x n} From 3.17, 3.42, 3.43, 3.55,and conditioniii, we have limn → n x n − Qx n   0 Since x n i q, it follows from3.80 that,limi→ ∞x n i − Qx n i  0 ByLemma 2.4, we obtain that

From 3.68, 3.77 3.82, and 3.84, we have q ∈ F Since PFf is contraction with the

coefficient θ ∈ 0, 1, PF has a unique fixed point Let z be a fixed point of PFf, that is

z  PFf z Since x n i q and q∈ F, we have

ByStep 5,3.87, andLemma 2.2, we have limn→ ∞x n  z, where z  PFf z It easy to see

that sequences{y n }, {u n }, and {v n } converge strongly to z  PFf z.

4 Application

Using our main theorem Theorem 3.1, we obtain the following strong convergence

theorems involving finite family of κ-strict pseudocontractions.

To prove strong convergence theorem in this section, we need definition and lemma

as follows

if there exist κ ∈ 0, 1 such that

Tx − Ty2 ≤x − y2 κI − Tx − I − Ty2

, ∀ x, y ∈ C. 4.1

Lemma 4.2 see 20 Let C be a nonempty closed convex subset of a real Hilbert space H and

Theorem 4.3 Let C be a nonempty closed convex subset of a Hilbert space H Let F and G be two

i1

i1F T i  ∩ EPF, A ∩ EPG, B ∩ FG1 ∩

F G2 / ∅, where G1, G2 : C → C are defined by G1x  P C x − λ n Ax , G2x  P C x −

η n Bx , for all x ∈ C Define a mapping T κ i by T κ i  κ i x 1−κ i T i x, for all x ∈ C, i ∈ {1, 2, , N}.

1 , α n,j2 , α n,j3  ∈ I × I × I, I  0, 1,

0, 2β, λ n ∈ e, f ⊂ 0, 2α, η n ∈ g, h ⊂ 0, 2β Assume that

i limn→ ∞α n  0 and Σ∞

FromTheorem 3.1, we can concluded the desired conclusion

Theorem 4.4 Let C be a nonempty closed convex subset of a Hilbert space H Let F and G be two

N

i1F T i ∩EPF, A∩FG1 / ∅, where G1: C → C defined by G1x  P C x−λ n Ax , for all x ∈

putting F ≡ G, A ≡ B, s n  r n , λ n  η n , and u n  v n FromTheorem 3.1, we can conclude thedesired conclusion.

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