The System of Mixed Equilibrium Problems for Quasi-Nonexpansive Mappings in Hilbert Spaces
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
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).
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 ℕ be the set of positive integers, and let ℝ 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 x ∈ H by xn → x and xn⇀x, respectively.
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∈ℕ be in H. If
Theorem 2.2 (Opial’s theorem, [10]). Let H be a real Hilbert space, and suppose that xn⇀x, then
All Hilbert space and lp (1 < p < ∞) satisfy Opial’s condition, while Lp with 1 < p ≠ 2 < ∞ do not.
- (A1)
F(x, x) = 0 for all x ∈ C,
- (A2)
F is monotone, that is, F(x, y) + F(y, x) ≤ 0 for all x, y ∈ C,
- (A3)
for each y ∈ C, x ↦ F(x, y) is weakly upper semicontinuous,
- (A4)
for each x ∈ C, y ↦ F(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 → ℝ be a bifunction satisfying (A1)–(A4). Let r > 0 and x ∈ H, then there exists z ∈ C 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 → ℝ be a bifunction satisfying (A1)–(A4), then, for any r > 0 and x ∈ H, define a mapping Trx : H → C as follows:
- (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 → ℝ be an equilibrium bifunction satisfying (A1)–(A4) and let φ : C → ℝ be a lower semicontinuous and convex functional. For each r > 0 and x ∈ H, define a mapping
- (i)
for each x ∈ C, Sr(x) ≠ ∅,
- (ii)
Sr is single valued,
- (iii)
Sr is firmly nonexpansive, that is, for any x, y ∈ H,
() - (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 → ℝ 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′, y′ ∈ C, (x′, y′) is a solution of problem (1.5) if and only if x′ is a fixed point of the mapping G : C → C defined by
Proof. For given x′, y′ ∈ C, we observe the following equivalency:
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 → ℝ 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 : C → C, i = 1,2, be two quasi-nonexpansive mappings such that I − Ti are demiclosed at zero, that is, if {wn} ⊂ C, wn⇀w, and (I − Ti)wn → 0, then w ∈ F(Ti), with F(T1)∩F(T2)∩Ω ≠ ∅. Let the sequences {xn}, {yn}, and {un}, be given by
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
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 → ℝ 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 : C → C be a quasi-nonexpansive mapping such that I − T is demiclosed at zero and F(T)∩Ω ≠ ∅. Suppose that x0 = x ∈ C and {xn}, {yn}, and {zn} are given by
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 I − T1, I − T2 are demiclosed at zero with F(T1)∩F(T2) ≠ ∅. For any x1 in C, let {xn} be defined by
4. Applications
Definition 4.1. Let C be a nonempty closed convex subset of a Hilbert space H. We say that T : C → C 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∥Tx − Ty∥2 ≤ α(x)∥Tx − y∥2 + β(x)∥Ty − x∥2 for all x, y ∈ C,
- (A2)
0 < α(x) + β(x) ≤ 2 for all x ∈ C.
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 x ∈ C, then T is a nonspreading mapping,
- (ii)
if α(x) = 4/3 and β(x) = 2/3 for all x ∈ C, 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 : C → C 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∥Tx − Ty∥2 ≤ α(x)∥x − y∥2 + β(x)∥Tx − y∥2 for all x, y ∈ C,
- (B2)
α(x) + β(x) ≤ 2 for all x ∈ C.
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 x ∈ C, then T is a nonexpansive mapping,
- (ii)
if α(x) = β(x) = 1 for each x ∈ C, 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 : C → C 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 : C → C be an asymptotic nonspreading mapping, then I − T 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 : C → C be an asymptotic TJ mapping, then I − T 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 → ℝ 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 : C → C, 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
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 → ℝ satisfying (A1)–(A4). Let Ti : C → C, i = 1,2, be any one of asymptotic nonspreading mapping and asymptotic TJ mapping such that ℱ : = F(T1)∩F(T2)∩EP (F) ≠ ∅. For given u ∈ C and r > 0, let the sequences {xn} and {un} be defined by
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 : C → C, i = 1,2, be any one of asymptotic nonspreading mapping and asymptotic TJ mapping. Let ℱ : = F(T1)∩F(T2) ≠ ∅. Let {an} and {bn} be two sequences in (0,1). Let {xn} be defined by
Acknowledgment
This paper is supported by the Centre of Excellence in Mathematics under the Commission on Higher Education, Ministry of Education, Thailand.