Volume 2012, Issue 1 816529
Research Article
Open Access

Viscosity Approximation Method for System of Variational Inclusions Problems and Fixed-Point Problems of a Countable Family of Nonexpansive Mappings

Chaichana Jaiboon

Corresponding Author

Chaichana Jaiboon

Department of Mathematics, Faculty of Liberal Arts, Rajamangala University of Technology Rattanakosin (RMUTR), Bangkok 10100, Thailand rmutr.ac.th

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Poom Kumam

Corresponding Author

Poom Kumam

Department of Mathematics, Faculty of Science, King Mongkut’s University of Technology Thonburi (KMUTT), Bangkok 10140, Thailand kmutt.ac.th

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First published: 13 May 2012
Academic Editor: Yonghong Yao

Abstract

We propose new iterative schemes for finding the common element of the set of common fixed points of countable family of nonexpansive mappings, the set of solutions of the variational inequality problem for relaxed cocoercive and Lipschitz continuous, the set of solutions of system of variational inclusions problem, and the set of solutions of equilibrium problems in a real Hilbert space by using the viscosity approximation method. We prove strong convergence theorem under some parameters. The results in this paper unify and generalize some well-known results in the literature.

1. Introduction

Let C be a nonempty closed convex subset of a real Hilbert space H. A mapping S of C into itself is called nonexpansive if ∥SxSy∥≤∥xy∥ for all x, yC. We denote by F(S) the set of fixed points of S; that is, F(S) = {xC : Sx = x}. If CH is nonempty, closed and convex and let S : CC be a nonexpansive mapping, then F(S) is closed and convex and F(S) ≠ , when C is bounded; see, for example, [1, 2]. The metric projection, PC, onto a given nonempty, closed and convex subset C, satisfies the nonexpansive with F(PC) = C. A mapping B : CC is called monotone if 〈BxBy, xy〉≥0 for all x, yC. A mapping B : CC is called β-inverse-strongly monotone if there exists a constant β > 0 such that 〈BxBy, xy〉≥βxy2, for all x, yC. A mapping B : CC is called relaxed (ϕ, ω)-cocoercive if there exists ϕ, ω > 0 such that
()
A mapping B : CC is said to be ξ-Lipschitz continuous if there exists ξ ≥ 0 such that
()
Let B : HH be a single-valued nonlinear mapping and M : H → 2H a multivalued mapping. The variational inclusion problem is to find such that
()
where θ is the zero vector in H. The set of solutions of problem (1.3) is denoted by I(B, M). If M = ψC, where C is a nonempty closed convex subset of H and ψC : H → [0, +] is the indicator function of C; that is,
()
then, the variational inclusion problem (1.3) is equivalent to the variational inequality problems denoted by VI (C, B) which is to find such that
()
In 2003, Takahashi and Toyoda [3] to find x*F(S)∩VI (C, B) introduced the following iterative scheme:
()
where B is a β-inverse-strongly monotone mapping, {αn} is a sequence in (0, 1), and {ξn} is a sequence in (0,2β). They showed that if F(S)∩VI (C, B) is nonempty, then the sequence {xn} generated by (1.6) converges weakly to some x*F(S)∩VI (C, B).
In 2008, Zhang et al. [4] to find x*F(S)∩I(M, B). They introduced the following new iterative scheme:
()
where JM,λ = (I + λM) −1 is the resolvent operator associated with M and a positive number λ, {αn} is a sequence in the interval [0,1].
Let F be a bifunction of C × C into , where is the set of real numbers. The equilibrium problem for F : C × C is to find such that
()
The set of solutions of (1.8) is denoted by EP (F). Many problems in applied sciences, such as monotone inclusion problems, variational inequality problems, saddle point problems, Nash equilibria in noncooperative games, as well as certain fixed-point problems reduce to finding some element to EP (F) in Hilbert and Banach spaces (see [514]).
Given any r > 0. The operator Tr : HC defined by
()
is called the resolvent of F (see [5, 6]).
It is shown in [6] that, under suitable hypotheses on F (to be stated precisely in Section 2), Tr : HC is single valued and firmly nonexpansive and satisfies
()
Using this result, for finding an element of F(S)∩VI (C, B)∩EP (F), Su et al. [15] introduced the following iterative scheme by the viscosity approximation method in Hilbert spaces:
()
where f : CC is a contraction (i.e., ∥f(x) − f(y)∥ ≤ ψxy∥, for  all  x, yC  and  0 ≤ ψ < 1) and {αn}⊂(0,1), ξn ⊂ (0,2β), and rn ⊂ (0, ) satisfy some appropriate conditions. Furthermore, they prove {xn} converges strongly to the same point x*F(S)∩VI (C, B)∩EP (F), where x* = PF(S)∩VI(C,B)∩EP(F)f(x*).

In this paper, motivated and inspired by the above facts, we introduce a new iterative scheme for finding a common element of the set of solutions of the variational inequalities for μ-Lipschitz continuous and relaxed (ϕ, ω)-cocoercive mapping, the set of solutions to the variational inclusion for family of α-inverse strongly monotone mappings, the set of fixed points of a countable family of nonexpansive mappings, and the set of solutions of an equilibrium problem in a real Hilbert space by using the viscosity approximation method. Strong convergence results are derived under suitable conditions in a real Hilbert space.

2. Preliminaries

In this section, we will recall some basic notations and collect some conclusions that will be used in the next section.

Let H be a real Hilbert space whose inner product and norm are denoted by 〈·, ·〉 and ∥·∥, respectively. We denote strong convergence of {xn} to xH by xnx and weak convergence by xnx. Let C be nonempty closed convex subset of H. Recall that for all xH there exists a unique nearest point in C to x denoted PCx; that is, ∥xPCx∥≤∥xy∥,   for  all  yC. The mapping PC is nonexpansive; that is, ∥PCxPCy∥≤∥xy∥,   for  all  x, yH. The mapping PC is firmly nonexpansive; that is, ∥PCxPCy2 ≤ 〈PCxPCy, xy〉,   for  all  x, yH. It is well known that
()
A set-valued mapping M : H → 2H is called monotone if, for all x, yH, fMx and gMy imply 〈xy, fg〉≥0. A monotone mapping M : H → 2H is called maximal, if its graph of any Graph (M) : = {(x, f) ∈ H × H   |   fM(x)} of M is not properly contained in the graph of any other monotone mapping. It is well known that a monotone mapping M is maximal if and only if for all (x, f) ∈ H × H, 〈xy, fg〉 ≥ 0,   for  all  (y, g)∈ Graph (M) (the graph of mapping M) implies that fMx.

Definition 2.1. Let M : H → 2H be a multivalued maximal monotone mapping; then the set-valued mapping JM,λ : HH defined by

()
is called the resolvent operator associated with M, where λ is any positive number and I is the identity mapping.

Lemma 2.2 (see [16].)Let M : H → 2H be a maximal monotone mapping and let B : HH be a Lipschitz continuous mapping. Then the mapping M + B : H → 2H is a maximal monotone mapping.

Lemma 2.3 (see [16], [17].)

  • (1)

    The resolvent operator JM,λ is single valued and nonexpansive for all λ > 0; that is,

()
  • (2)

    The resolvent operator JM,λ is 1-inverse-strongly monotone; that is,

()

Lemma 2.4 (see [17].)

  • (1)

    Let is a solution of problem (1.3) if and only if for all λ > 0; that is,

()
  • (2)

    If λ ∈ [0,2β], then I(B, M) is a closed convex subset in H.

Lemma 2.5 (see [18].)Each Hilbert space H satisfies Opial’s condition; that is, for any sequence {xn} ⊂ H with xnx, the inequality

()
holds for each yH with yx.

Lemma 2.6 (see [19].)Let {xn} and {zn} be bounded sequences in a Banach space E, and let {βn} be a sequence in [0,1] with 0 < liminf nβn ≤ limsup nβn < 1. Suppose xn+1 = (1 − βn)zn + βnxn for all integers n ≥ 1 and limsup n(∥zn+1zn∥−∥xn+1xn∥) ≤ 0. Then, lim nznxn∥ = 0.

Lemma 2.7 (see [20].)Assume {an} is a sequence of nonnegative real numbers such that

()
where {bn} is a sequence in (0,1) and {δn} is a sequence in such that
  • (1)

    ,

  • (2)

    limsup nδn/bn ≤ 0 or .

Then lim nan = 0.

Lemma 2.8. Let H be a real Hilbert space. Then hold the following identities:

  • (i)

    tx+(1−t)y2 = tx2 + (1 − t)∥y2t(1 − t)∥xy2,   t ∈ [0,1],   x, yH,

  • (ii)

    x+y2 ≤ ∥x2 + 2〈y, x + y〉,   x, yH.

Lemma 2.9 (see [21].)Let C be a nonempty closed subset of a Banach space, and let {Sn} be a sequence of mappings of C into itself. Suppose that . Then, for each yC, {Sny} converges strongly to some point of C. Moreover, let S be a mapping of C into itself defined by

()
Then lim nsup {∥SzSnz∥:zC} = 0.

For solving the equilibrium problem for a 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,     x, yC;

  • (A3)

    for each x, y, zC,   lim t↓0F(tz + (1 − t)x, y) ≤ F(x, y);

  • (A4)

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

Lemma 2.10 (see [5].)Let C be a nonempty closed convex subset of H, and let F be a bifunction of C × C into satisfying (A1)–(A4). Let r > 0 and xH. Then, there exists zC such that

()

Lemma 2.11 (see [6].)Assume that F : C × C satisfies (A1)–(A4). For r > 0 and xH, define a mapping Tr : HC as follows:

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

    Tr is single valued;

  • (ii)

    Tr is firmly nonexpansive; that is, for any ;

  • (iii)

    F(Tr) = EP (F);

  • (iv)

    EP (F) is closed and convex.

Lemma 2.12 (see [22].)Let H be a Hilbert space and M a maximal monotone on H. Then, the following holds:

()
where JM,r = (I + rM) −1 and JM,s = (I + sM) −1.

3. Main Results

In this section, we will use the viscosity approximation method to prove a strong convergence theorem for finding a common element of the set of fixed points of a countable family of nonexpansive mappings, the set of solutions of the variational inequality problem for relaxed cocoercive and Lipschitz continuous mappings, the set of solutions of system of variational inclusions, and the set of solutions of equilibrium problem in a real Hilbert space.

Theorem 3.1. Let C be a nonempty closed convex subset of a real Hilbert space H, and let B : CH be relaxed (ϕ, ω)-cocoercive and μ-Lipschitz continuous with ω > ϕμ2, for some ϕ, ω, μ > 0. Let 𝒢 = {Gk : k = 1,2, 3, …, N} be a finite family of β-inverse strongly monotone mappings from C into H, and let F be a bifunction from C × C satisfying (A1)–(A4). Let f : CC be a contraction with coefficient ψ  (0 ≤ ψ < 1), and let {Sn} be a sequence of nonexpansive mappings of C into itself such that

()
Let the sequences {xn} and {yn} be generated by
()
where {αn}, {βn}, {γn}⊂(0,1) and {ξn}, {rn}⊂(0, ) satisfy the following conditions:
  • (C1)

    αn + βn + γn = 1,

  • (C2)

    ,

  • (C3)

    0 < liminf nβn ≤ limsup nβn < 1,

  • (C4)

    {ξn}⊂[a, b] for some a, b with 0 ≤ ab ≤ 2(ωϕμ2)/μ2 and lim n|ξn+1ξn| = 0,

  • (C5)

    and lim n|λk,n+1λk,n| = 0, for each k ∈ {1,2, …, N},

  • (C6)

    liminf nrn > 0 and lim n|rn+1rn| = 0.

Suppose that for any bounded subset K of C. Let S be a mapping of C into itself defined by Sy = lim nSny  for  all  yC and suppose that . Then, the sequences {xn} and {yn} converge strongly to the same point x* ∈ Ω, where x* = PΩf(x*).

Proof. First, we prove that the mapping PΩf : HC has a unique fixed point.

In fact, since f : CC is a contraction with ψ ∈ [0,1) and PΩf : H → Ω is also a contraction, we obtain

()
Therefore, there exists a unique element x*C such that x* = PΩf(x*), where
()
Now, we prove that (IξnB) is nonexpansive.

Indeed, for any x, yC, since B : CH is a μ-Lipschitz continuous and relaxed (ϕ, ω)-cocoercive mappings with ω > ϕμ2 and ξn ≤ 2(ωϕμ2)/μ2, we obtain

()
Setting
()
thus,
()
which implies that
()
Hence (IξnB) is nonexpansive.

We divide the proof of Theorem 3.1 into five steps.

Step 1. We show that the sequence {xn} is bounded.

Now, let and if is a sequence of mappings defined as in Lemma 2.11, then , and let . So, we have

()
For k ∈ {1,2, …, N} and for any positive integer number n, we define the operator as follows:
()
for all n, we get . On the other hand, since Gk : CH is β-inverse strongly monotone and λk,n ⊂ [c, d]⊂(0,2β), then is nonexpansive. Thus is nonexpansive. From Lemma 2.4(1), we have . It follows that
()
Setting vn = PC(ynξnByn) and IξnB is a nonexpansive mapping, we obtain
()
From (3.2) and (3.12), we deduce that
()
It follows from induction that
()
Therefore, {xn} is bounded and hence so are {vn}, {yn}, {un}, {Byn}, and {Snvn}.

Step 2. We claim that lim nxn+1xn∥ = 0.

By the definition of Tr, and , we get

()
()
Taking y = un+1 in (3.15) and y = un in (3.16), we have
()
and hence
()
So, from (A2) we have
()
and hence
()
Without loss of generality, let us assume that there exists a real number c such that rn > c > 0 for all n. Then, we have
()
and hence
()
where M1 = sup {∥unxn∥:n}.

Notice from Lemma 2.12 that

()
where M2 is an appropriate constant such that
()
Since IξnB is nonexpansive mappings, we have the following estimates:
()
Substituting (3.23) into (3.25), we obtain
()
Indeed, define xn+1 = (1 − βn)zn + βnxn for all n. It follows that
()
Thus, we have
()
Now, compute
()
Combining (3.28) and (3.29), we have
()
It follows that
()
This together with conditions (C1)–(C6) and lim nsup {∥Sn+1zSnz∥:z ∈ {vn}} = 0 implies that
()
Hence, by Lemma 2.6, we obtain ∥znxn∥→0 as n. It then follows that
()
By (3.26), we also have
()

Step 3. We claim that lim nSvnvn∥ = 0.

Since {Gk : k = 1,2, 3, …, N} is β-inverse strongly monotone mappings, by the choice of {λk,n} for given and k ∈ {0,1, 2, …, N − 1}, we also have

()
Form (3.13), we have
()
It follows that
()
By condition (C2), (3.33), and liminf nγn > 0, we obtain
()
From Lemma 2.3(2) and as Iλk+1,nGk+1 is nonexpansive, we have
()
which yields that
()
Substituting (3.40) into (3.36), we obtain
()
It follows that
()
By condition (C2), (3.33), (3.38), and liminf nγn > 0, we obtain
()
For , we obtain
()
On the other hand, we have
()
It follows that
()
It now follows from the last inequality, conditions (C2), (3.33), and liminf nγn > 0 that
()
Since PC is firmly nonexpansive, we have
()
which yields that
()
Substituting (3.49) into (3.45), we obtain
()
It follows that
()
By condition (C2), (3.33), (3.47), and liminf nγn > 0, we obtain
()
On the other hand, in the light of Lemma 2.11(ii) is firmly nonexpansive; so we have
()
which implies that
()
Form (3.45), we have
()
It follows that
()
By condition (C2), (3.33), and liminf nγn > 0, we obtain
()
Observe that
()
By condition (C2) and (3.33), we have
()
Since
()
from (3.57) and (3.59), we have
()
Form (3.55), we have
()
It follows that
()
By condition (C2), (3.33), and liminf nγn > 0, we obtain
()
Since
()
from (3.57) and (3.64), we have
()
Furthermore, by the triangular inequality we also have
()
From (3.52), (3.61), and (3.66), we have
()
Applying Lemma 2.9 and (3.68), we have
()

Step 4. We claim that limsup nf(x*) − x*, xnx*〉 ≤ 0.

Indeed, we choose a subsequence of {vn} such that

()
Without loss of generality, let . From ∥Svnvn∥→0, we obtain . Then, (3.70) reduces to
()
In order to show 〈f(x*) − x*, zx*〉 ≤ 0, it suffices to show that
()
Firstly, we will show .

Assume zF(S). By Opial’s theorem (Lemma 2.5) and ∥Svnvn∥→0, we have

()
This is a contradiction. Thus, we obtain zF(S).

Next, we will show that zVI (C, B).

Let

()
Since B is relaxed (ϕ, ω)-cocoercive, μ-Lipschitz continuous with ω > ϕμ2, we obtain
()
which yields that B is monotone. Then T is maximal monotone (see [23]). Let (w1, w2) ∈ G(T). Since w2Bw1NC(w1) and vnC, we have 〈w1vn, w2Bw1〉 ≥ 0. On the other hand, from vn = PC(ynξnByn), we have
()
that is,
()
Therefore, we obtain
()
Noting that and B is relaxed (ϕ, ω)-cocoercive and (3.78), we obtain
()
Since T is maximal monotone, we have zT−10, and hence zVI (C, B).

Now, we will show that .

For this purpose, let k ∈ {1,2, 3, …, N} and Gk is β-inverse strongly monotone, Gk is an 1/β-Lipschitz continuous monotone mapping. From Lemma 2.2, we know that Mk + Gk is maximal monotone. Let (v, g) ∈ G(Mk + Gk); that is, gGkvMk(v). On the other hand, since , we have

()
that is,
()
By virtue of the maximal monotonicity of Mk + Gk, we have
()
and so
()
From , we also obtain that and {Gk : k = 1,2, 3, …, N} are Lipschitz continuous; we have
()
Since Mk + Gk is maximal monotone, we have θ ∈ (Mk + Gk)(z); that is, .

Finally, we will show that zEP (F).

Since , we have

()
If follows from (A2) that
()
and hence
()
Since and , it follows by (A4) that F(y, z) ≤ 0 for all yH. For t with 0 < t ≤ 1 and yH, let yt = ty + (1 − t)z. Since yH and zH, we have ytH, and hence F(yt, z) ≤ 0. So, from (A1) and (A4) we have
()
and hence F(yt, y) ≥ 0. From (A3), we have F(z, y) ≥ 0 for all yH and hence zEP (F). Therefore, it follows that z ∈ Ω.

Since x* = PΩf(x*), we have

()
On the other hand, we have
()
Since ∥xn+1xn∥→0 as n and (3.91), we have
()

Step 5. We claim that lim nxnx*∥ = 0.

Indeed, from (3.2) and (3.12), we obtain

()
which implies that
()
where bn = αn(1 − ψ2) and δn = 2αnf(x*) − x*, xn+1x*〉. It is easy to see that bn → 0, , and limsup nδn/bn ≤ 0. Applying Lemma 2.7 to (3.93), we conclude that
()
Consequently, also {yn} converges strongly to x*. The proof is now complete.

As in [21, Theorem  4.1], we can generate a sequence {Sn} of nonexpansive mappings satisfying condition for any bounded subset K of C by using convex combination of general sequence {Tk} of nonexpansive mappings with a common fixed point.

Corollary 3.2. Let C be a nonempty closed convex subset of a real Hilbert space H, and let B : CH be relaxed (ϕ, ω)-cocoercive and μ-Lipschitz continuous with ω > ϕμ2, for some ϕ, ω, μ > 0. Let 𝒢 = {Gk : k = 1,2, 3, …, N} be a finite family of β-inverse strongly monotone mappings from C into H, and let F be a bifunction from C × C satisfying (A1)–(A4). Let f : CC be a contraction with coefficient ψ  (0 ≤ ψ < 1), and let be a family of nonnegative numbers with indices n, k with kn such that

()
Let the sequences {xn} and {yn} be generated by
()
where {αn}, {βn}, {γn}⊂(0,1) and {ξn}, {rn}⊂(0, ) satisfy the following conditions:
  • (C1)

    αn + βn + γn = 1,

  • (C2)

    ,

  • (C3)

    0 < liminf nβn ≤ limsup nβn < 1,

  • (C4)

    {ξn} ⊂ [a, b] for some a, b with 0 ≤ ab ≤ 2(ωϕμ2)/μ2 and lim n|ξn+1ξn| = 0,

  • (C5)

    and lim n|λk,n+1λk,n| = 0, for each k ∈ {1,2, …, N},

  • (C6)

    liminf nrn > 0 and lim n|rn+1rn| = 0,

  • (C7)

    , and .

Then, the sequences {xn} and {yn} converge strongly to the same point x* ∈ Ω, where x* = PΩf(x*).

In Theorem 3.1, taking N = 1 and Sn = S, then we have the following corollary.

Corollary 3.3. Let C be a nonempty closed convex subset of a real Hilbert space H, and let B : CH be relaxed (ϕ, ω)-cocoercive and μ-Lipschitz continuous with ω > ϕμ2, for some ϕ, ω, μ > 0. Let G be an β-inverse strongly monotone mappings from C into H, and let F be a bifunction from C × C satisfying (A1)–(A4). Let f : CC be a contraction with coefficient ψ  (0 ≤ ψ < 1), and let S be a nonexpansive mappings of C into itself such that

()
Let the sequences {xn} and {yn} be generated by
()
where {αn}, {βn},{γn} ⊂ (0,1) and {ξn}, {rn} ⊂ (0, ) satisfy the following conditions:
  • (C1)

    αn + βn + γn = 1,

  • (C2)

    ,

  • (C3)

    0 < liminf nβn ≤ limsup nβn < 1,

  • (C4)

    {ξn} ⊂ [a, b] for some a, b with 0 ≤ ab ≤ 2(ωϕμ2)/μ2 and lim n | ξn+1ξn | = 0,

  • (C5)

    {λn} ⊂ [c, d] ⊂ (0,2β) and lim n|λn+1λn| = 0,

  • (C6)

    liminf nrn > 0 and lim n|rn+1rn| = 0.

Then, the sequences {xn} and {yn} converge strongly to the same point x* ∈ Ω, where x* = PΩf(x*).

Acknowledgments

This research was supported by the Rajamangala University of Technology Rattanakosin Research and Development Institute, the Thailand Research Fund, and the Commission on Higher Education under Grant no. MRG5480206. The second author would like to thank the Higher Education Research Promotion and National Research University Project of Thailand, Office ofthe Higher Education Commission (under the Project NRU-CSEC no. 54000267) for financial support. The authors are very grateful to the referees for their careful reading, comments, and suggestions which improved the presentation of this article.

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