An Alternative Regularization Method for Equilibrium Problems and Fixed Point of Nonexpansive Mappings
Abstract
We introduce a new regularization iterative algorithm for equilibrium and fixed point problems of nonexpansive mapping. Then, we prove a strong convergence theorem for nonexpansive mappings to solve a unique solution of the variational inequality and the unique sunny nonexpansive retraction. Our results extend beyond the results of S. Takahashi and W. Takahashi (2007), and many others.
1. Introduction
In 2000, Moudafi [11] proved the following strong convergence theorem.
Theorem 1.1 (Moudafi [11]). Let C be a nonempty closed convex subset of a Hilbert space H and let S be a nonexpansive mapping of C into itself such that F(S) is nonempty. Let f be a contraction of C into itself and let {xn} be a sequence defined as follows: x1 = x ∈ C and
Such a method for approximation of fixed points is called the viscosity approximation method.
In this paper, motivated and inspired by the above results, we introduce an iterative scheme by the general iterative method for finding a common element of the set of solutions of (1.1) and the set of fixed points of a nonexpansive mapping in Hilbert space.
2. Preliminaries
- (i)
Q is a retraction provided Qx = x for x ∈ D;
- (ii)
Q is a nonexpansive retraction provided Q is a retraction that is also nonexpansive;
- (iii)
Q is a sunny nonexpansive retraction provided Q is a nonexpansive retraction with the additional property: Q(x + t(x − Qx)) = Qx whenever x + t(x − Qx) ∈ C, where x ∈ C and t ≥ 0.
-
A1 ϕ(x, x) = 0, for all x ∈ C;
-
A2 ϕ is monotone, that is, ϕ(x, y) + ϕ(y, x) ≤ 0, for all x, y ∈ C;
-
A3 For each x, y, z ∈ C, lim t↓0ϕ(tz + (1 − t)x, y) ≤ ϕ(x, y);
-
A4 For each x ∈ C, the function y ↦ ϕ(x, y) is convex and lower semicontinuous.
The following lemma appeared implicitly in [14].
Lemma 2.1 (see [14].)Let C be a nonempty closed convex subset of H and let ϕ : C × C → ℝ be a bifunction satisfying (A1)–(A4). Let r > 0 and x ∈ H, then, there exists z ∈ C such that
Lemma 2.2 (see [6].)Assume that ϕ : C × C → ℝ satisfies (A1)–(A4). For r > 0 and x ∈ H, define a mapping Tr : H → C as follows:
-
(1) Tr is single-valued;
-
(2) Tr is firmly nonexpansive, that is, for any x, y ∈ H,
-
(3) F(Tr) = EP (ϕ);
-
(4) EP (ϕ) is closed and convex.
Lemma 2.3 (see [15].)Let {an}⊂[0, ∞), {bn}⊂[0, ∞) and {cn}⊂[0,1) be sequences of real numbers such that
Lemma 2.4 (see [9].)Suppose that X is a smooth Banach space. Then a retraction Q : C → D is sunny nonexpansive if and only if
Lemma 2.5. Let X be a uniformly smooth Banach space, C a nonempty closed convex subset of X, and T : C → C a nonexpansive mapping. Let zt be defined by (2.1). Then (zt) remains bounded as t → 0 if and only if Fix (T)≠. Moreover, if Fix (T)≠, then (zt) converges in norm, as t → 0+, to a fixed point of T; and if one sets
Lemma 2.6. In the Hilbert space, the following inequalities always hold
- (i)
∥x+y∥2 ≤ ∥x∥2 + 2〈y, x + y〉;
- (ii)
∥tx+(1−t)y∥2 ≤ t∥x∥2 + (1 − t)∥y∥2.
3. Main Results
Theorem 3.1. Let C be a nonempty closed convex subset of H, ϕ : C × C → ∞ be a bifunction satisfying (A1)–(A4) and T : C → C be a nonexpansive mapping of C into H such that F(T)∩EP (ϕ) ≠ ∅. Let f be a contraction of H into itself with α ∈ (0,1), initially give an arbitrary element x1 ∈ H and let {xn} and {un} be sequences generated by (1.8), where {αn}⊂[0,1] and {rn}⊂(0, ∞) satisfy the following conditions:
- (I)
- (II)
- (III)
- (IV)
lim n→∞(αn/βn) = 0.
Proof. Let Q = PF(S)∩EP (ϕ). Then Qf is a contraction of H into itself. In fact, there exists a ∈ [0,1) such that ∥f(x) − f(y)∥≤a∥x − y∥ for all x, y ∈ H. So, we have that
Step 1. {xn} and {un} are all bounded. Let p ∈ F(T)∩EP (ϕ), Then from , we have
Therefore ∥yn − p∥≤∥xn − p∥,
Step 2. ∥xn+1 − xn∥→0 as n → ∞,
On the other hand, from and , we have
So, put (3.8) and (3.16) into (3.7) we have
Using Lemma 2.3 and conditions (I), (II), (III) we have
Step 3. we show that
From xn+1 − z = (1 − βn)(Tyn − z) + βn(f(xn) − z), we have
If zt is definition as (2.1), then, from Lemma 2.5, we have ∥zt − q∥→0 as t → 0, and if we set Q(u): = lim t→0zt, then Q defines the unique sunny nonexpansive retraction from C onto Fix (T). So, if we replace t with αn, the corollary still holds. and it is that zn = T(αnu + (1 − αn)zn) is a fixed point sequence and ∥zn − q∥→0 as n → ∞, and if we set Q(u) : = lim n→∞zn, then Q defines the unique sunny nonexpansive retraction from C onto Fix (T). In the iterative algorithm of Theorem 3.1, we can take zn to replace Tyn in particular. Then, we have xn+1 = βnf(xn)+(1 − βn)zn, so ∥xn+1 − zn∥ = βn∥f(xn) − zn∥→0 as n → ∞. By the uniqueness of limit, we have z = q, that is, z = Q(u), where Q defines the unique sunny nonexpansive retraction from C onto Fix (T).
Remark 3. We notice that has not influence on xn, un → z = PF(T)∩EP (ϕ)f(z).
As direct consequences of Theorem 3.1, we obtain corollary.
Corollary 3.2. Let C be a nonempty closed convex subset of H, S : C → C be a nonexpansive mapping of C into H such that F(S) ≠ ∅. Let f be a contraction of H into itself and let {xn} and {un} be sequences generated initially by an arbitrary elements x1 ∈ H and then by
- (I)
- (II)
- (III)
lim n→∞αn/βn = 0.
4. Application for Zeros of Maximal Monotone Operators
A monotone operator A is said to be maximal monotone of the graph G(T) is not properly contained in the graph of any other monotone defined in H. See Brezis [16] for more details on maximal monotone operators.
In this section we always assume that A is maximal monotone and the set of zeros of A, N(A) = {x ∈ D(A) : 0 ∈ Ax}, is nonempty so that the metric projection PN(A) from H onto N(A) is well-defined.
Before stating the convergence theorem of the algorithm (4.7), we list some properties of maximal monotone operators.
Proposition 4.1. Let A be a maximal monotone operator in H and let denote the resolvent, where c > 0,
- (a)
is nonexpansive for all c > 0;
- (b)
N(A) = F(Jc) for all c > 0;
- (c)
For ;
- (d)
(The Resolvent Identity) For λ, μ > 0, there holds the identity:
Theorem 4.2. Let C be a nonempty closed convex subset of H, ϕ : C × C → ∞ be a bifunction satisfying (A1)–(A4) and A be a maximal monotone operator such that N(A)∩EP (ϕ) ≠ ∅. Let f be a contraction of H into itself and let {xn} and {un} be sequences generated initially by an arbitrary elements x0 ∈ H and then by
- (I)
lim n→∞αn = 0, and ;
- (II)
liminf nrn > 0 and ;
- (III)
lim n→∞(cn+1/cn) = 1;
- (IV)
lim n→∞βn = 0, , , and lim n→∞(αn/βn) = 0.
Proof. Below we write for simplicity. Setting
- (1)
{xn} is bounded;
- (2)
∥xn+1 − xn∥ → 0, as n → 0;
- (3)
, as n → 0;
- (4)
∥xn − un∥ → 0, as n → 0;
- (5)
limsup n→∞〈f(z) − z, xn − z〉 ≤ 0;
- (6)
xn, un → z, as n → z.
5. Application for Optimization Problem
In fact, z is the minimum norm point from η onto the B, furthermore, if η = 0, then z is the minimum norm point on the B.
Acknowledgment
This work was supported by the National Natural Science Foundation of China under Grants (11071270), and (10771050).