Viscosity Approximation Method for System of Variational Inclusions Problems and Fixed-Point Problems of a Countable Family of Nonexpansive Mappings
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
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.
Definition 2.1. Let M : H → 2H be a multivalued maximal monotone mapping; then the set-valued mapping JM,λ : H → H defined by
Lemma 2.2 (see [16].)Let M : H → 2H be a maximal monotone mapping and let B : H → H be a Lipschitz continuous mapping. Then the mapping M + B : H → 2H is a maximal monotone mapping.
- (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 xn⇀x, the inequality
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 n→∞βn ≤ limsup n→∞βn < 1. Suppose xn+1 = (1 − βn)zn + βnxn for all integers n ≥ 1 and limsup n→∞(∥zn+1 − zn∥−∥xn+1 − xn∥) ≤ 0. Then, lim n→∞∥zn − xn∥ = 0.
Lemma 2.7 (see [20].)Assume {an} is a sequence of nonnegative real numbers such that
- (1)
,
- (2)
limsup n→∞δn/bn ≤ 0 or .
Lemma 2.8. Let H be a real Hilbert space. Then hold the following identities:
- (i)
∥tx+(1−t)y∥2 = t∥x∥2 + (1 − t)∥y∥2 − t(1 − t)∥x−y∥2, ∀ t ∈ [0,1], ∀ x, y ∈ H,
- (ii)
∥x+y∥2 ≤ ∥x∥2 + 2〈y, x + y〉, ∀ x, y ∈ H.
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 y ∈ C, {Sny} converges strongly to some point of C. Moreover, let S be a mapping of C into itself defined by
- (A1)
F(x, x) = 0 for all x ∈ C;
- (A2)
F is monotone, that is, F(x, y) + F(y, x) ≤ 0, ∀ x, y ∈ C;
- (A3)
for each x, y, z ∈ C, lim t↓0F(tz + (1 − t)x, y) ≤ F(x, y);
- (A4)
for each x ∈ C, y ↦ F(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 ℝ satisfying (A1)–(A4). Let r > 0 and x ∈ H. Then, there exists z ∈ C such that
Lemma 2.11 (see [6].)Assume that F : C × C → ℝ satisfies (A1)–(A4). For r > 0 and x ∈ H, define a mapping Tr : H → C as follows:
- (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:
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 : C → H 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 → ℝ satisfying (A1)–(A4). Let f : C → C be a contraction with coefficient ψ (0 ≤ ψ < 1), and let {Sn} be a sequence of nonexpansive mappings of C into itself such that
- (C1)
αn + βn + γn = 1,
- (C2)
,
- (C3)
0 < liminf n→∞βn ≤ limsup n→∞βn < 1,
- (C4)
{ξn}⊂[a, b] for some a, b with 0 ≤ a ≤ b ≤ 2(ω − ϕμ2)/μ2 and lim n→∞|ξn+1 − ξn| = 0,
- (C5)
and lim n→∞|λk,n+1 − λk,n| = 0, for each k ∈ {1,2, …, N},
- (C6)
liminf n→∞rn > 0 and lim n→∞|rn+1 − rn| = 0.
Proof. First, we prove that the mapping PΩf : H → C has a unique fixed point.
In fact, since f : C → C is a contraction with ψ ∈ [0,1) and PΩf : H → Ω is also a contraction, we obtain
Indeed, for any x, y ∈ C, since B : C → H is a μ-Lipschitz continuous and relaxed (ϕ, ω)-cocoercive mappings with ω > ϕμ2 and ξn ≤ 2(ω − ϕμ2)/μ2, we obtain
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
Step 2. We claim that lim n→∞∥xn+1 − xn∥ = 0.
By the definition of Tr, and , we get
Notice from Lemma 2.12 that
Step 3. We claim that lim n→∞∥Svn − vn∥ = 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
Step 4. We claim that limsup n→∞〈f(x*) − x*, xn − x*〉 ≤ 0.
Indeed, we choose a subsequence of {vn} such that
Assume z ∉ F(S). By Opial’s theorem (Lemma 2.5) and ∥Svn − vn∥→0, we have
Next, we will show that z ∈ VI (C, B).
Let
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, g − Gkv ∈ Mk(v). On the other hand, since , we have
Finally, we will show that z ∈ EP (F).
Since , we have
Since x* = PΩf(x*), we have
Step 5. We claim that lim n→∞∥xn − x*∥ = 0.
Indeed, from (3.2) and (3.12), we obtain
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 : C → H 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 → ℝ satisfying (A1)–(A4). Let f : C → C be a contraction with coefficient ψ (0 ≤ ψ < 1), and let be a family of nonnegative numbers with indices n, k ∈ ℕ with k ≤ n such that
- (C1)
αn + βn + γn = 1,
- (C2)
,
- (C3)
0 < liminf n→∞βn ≤ limsup n→∞βn < 1,
- (C4)
{ξn} ⊂ [a, b] for some a, b with 0 ≤ a ≤ b ≤ 2(ω − ϕμ2)/μ2 and lim n→∞|ξn+1 − ξn| = 0,
- (C5)
and lim n→∞|λk,n+1 − λk,n| = 0, for each k ∈ {1,2, …, N},
- (C6)
liminf n→∞rn > 0 and lim n→∞|rn+1 − rn| = 0,
- (C7)
, and .
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 : C → H 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 → ℝ satisfying (A1)–(A4). Let f : C → C be a contraction with coefficient ψ (0 ≤ ψ < 1), and let S be a nonexpansive mappings of C into itself such that
- (C1)
αn + βn + γn = 1,
- (C2)
,
- (C3)
0 < liminf n→∞βn ≤ limsup n→∞βn < 1,
- (C4)
{ξn} ⊂ [a, b] for some a, b with 0 ≤ a ≤ b ≤ 2(ω − ϕμ2)/μ2 and lim n→∞ | ξn+1 − ξn | = 0,
- (C5)
{λn} ⊂ [c, d] ⊂ (0,2β) and lim n→∞|λn+1 − λn| = 0,
- (C6)
liminf n→∞rn > 0 and lim n→∞|rn+1 − rn| = 0.
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.