Volume 2012, Issue 1 202860
Research Article
Open Access

An Alternative Regularization Method for Equilibrium Problems and Fixed Point of Nonexpansive Mappings

Shuo Sun

Corresponding Author

Shuo Sun

Department of Mathematics, Tianjin Polytechnic University, Tianjin 300160, China tjpu.edu.cn

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First published: 13 May 2012
Citations: 1
Academic Editor: Rudong Chen

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

Let H be a real Hilbert space with inner product 〈·, ·〉 and norm ∥·∥, respectively. let C be a nonempty closed convex subset of H. Let ϕ be a bifunction of C × C into , where is the set of real numbers. The equilibrium problem for ϕ : C × C is to find xC such that
()
The set of solutions of (1.1) is denoted by EP (ϕ). Given a mapping T : CH, let ϕ(x, y) = 〈Tx, yx〉 for all x, yC. Then, zEP (ϕ) if and only if 〈Tz, yz〉 ≥ 0 for all yC, that is, z is a solution of the variational inequality. Numerous problems in physics, optimization, and economics reduce to find a solution of (1.1). Some methods have been proposed to solve the equilibrium problem; see, for instance, [16].
A mapping S of C into H is said to be nonexpansive if
()
We denote by F(S) the set of fixed points of S. The fixed point equation Tx = x is ill-posed (it may fail to have a solution, nor uniqueness of solution) in general. Regularization therefore is needed. Contractions can be used to regularize nonexpansive mappings. In fact, the following regularization has widely been implemented ([79]). Fixing a point uC and for each t ∈ (0,1), one defines a contraction Tt : CC by
()
In this paper we provide an alternative regularization method. Our idea is to shrink x first and then apply T to the convex combination of the shrunk x and the anchor u (this idea appeared implicitly in [10] where iterative methods for finding zeros of maximal monotone operators were investigated). In other words, we fix an anchor uC and t ∈ (0,1) and define a contraction Tt : CC by
()
Compared with (1.1), (1.4) looks slightly more compact in the sense that the mapping T is more directly involved in the regularization and thus may be more convenient in manipulations since the nonexpansivity of T is utilized first.

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 = xC and

()
for all nN, where {εn}⊂(0,1) satisfies
()
Then, {xn} converges strongly to zF(S), where z = PF(S)f(z) and PF(S) is the metric projection of H onto F(S).

Such a method for approximation of fixed points is called the viscosity approximation method.

In 2007, S. Takahashi and W. Takahashi [5] introduced and considered the following iterative algorithm by the viscosity approximation method in the Hilbert space:
()
for all nN, where {αn} ⊂ [0,1] and {rn} ⊂ (0, ) satisfy some appropriate conditions. Furthermore, they proved that {xn} and {un} converge strongly to zF(S)∩EP (ϕ), where z = PF(S)∩EP (ϕ)f(z).

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.

Let S : CC be a nonexpansive mapping. Starting with an arbitrary x1, uH, define sequences {xn} and {un} by
()
We will prove in Section 3 that if the sequences {αn}, {βn}, and {γn} of parameters satisfy appropriate conditions, then the sequence {xn} and {un} generated by (1.8) converges strongly to the unique solution of the variational inequality
()
which is the optimality condition for the minimization problem
()
where h is a potential function for f and at the same time, the sequence {xn} and {un} generated by (1.8) converges in norm to Q(u), where Q : C → Fix (T) is the sunny nonexpansive retraction.

2. Preliminaries

Throughout this paper, we consider H as the Hilbert space with inner product 〈·, ·〉 and norm ∥·∥, respectively, C is a nonempty closed convex subset of H. Consider a subset D of C and a mapping Q : CD. Then we say that
  • (i)

    Q is a retraction provided Qx = x  for  xD;

  • (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(xQx)) = Qx whenever x + t(xQx) ∈ C, where xC and t ≥ 0.

Let now T : CC be a nonexpansive mapping. For a fixed anchor uC and each t ∈ (0,1) recall that ztC is the unique fixed point of the contraction CxT(tu + (1 − t)x). Namely, ztC is the unique solution in C to the fixed point equation
()
In the Hilbert space (either uniformly smooth or reflexive with a weakly continuous duality map), then zt is strongly convergent should it is bounded as t → 0+.
We also know that for any sequence {xn} ⊂ H with xnx, the inequality
()
holds for every yH with xy, (we usually call it Opial’s condition); see [12, 13] for more details.
For solving the equilibrium problem for a bifunction ϕ : C × C, let us assume that ϕ satisfies the following conditions:
  • A1 ϕ(x, x) = 0,   for  all  xC;

  • A2 ϕ is monotone, that is, ϕ(x, y) + ϕ(y, x) ≤ 0,   for  all  x, yC;

  • A3 For each x, y, zC, lim t↓0ϕ(tz + (1 − t)x, y) ≤ ϕ(x, y);

  • A4 For each xC, the function yϕ(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 be a bifunction satisfying (A1)–(A4). Let r > 0 and xH, then, there exists zC such that

()

Lemma 2.2 (see [6].)Assume that ϕ : C × C satisfies (A1)–(A4). For r > 0 and xH, define a mapping Tr : HC as follows:

()
for all zH. Then, the following hold:
  • (1) Tr is single-valued;

  • (2) Tr is firmly nonexpansive, that is, for any x, yH,

()
  • (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

()
then, lim nan = 0.

Lemma 2.4 (see [9].)Suppose that X is a smooth Banach space. Then a retraction Q : CD 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 : CC 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

()
then Q defines the unique sunny nonexpansive retraction from C onto Fix (T).

Lemma 2.6. In the Hilbert space, the following inequalities always hold

  • (i)

    x+y2 ≤ ∥x2 + 2〈y,   x + y〉;

  • (ii)

    tx+(1−t)y2tx2 + (1 − t)∥y2.

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 : CC 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 x1H 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.

Then, the sequences {xn} and {un} converge strongly to zF(T)∩EP (ϕ), where z = PF(T)∩EP (ϕ)f(z) and converge in norm to Q(u), where Q : C → Fix (T) is the sunny nonexpansive retraction.

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)∥≤axy∥ for all x, yH. So, we have that

()
for all x, yH. So, Qf is a contraction of H into itself. Since H is complete, there exists a unique element zH such that z = Qf(z), such a zH is an element of C. We divide the proof into serval steps.

Step 1. {xn} and {un} are all bounded. Let pF(T)∩EP (ϕ), Then from , we have

()
for all nN. Put yn = αnu + (1 − αn)un, so {xn+1} can be rewritten as
()
Therefore, from (3.2) we get
()
If ∥xnp∥≤∥up∥, then {xn} is bounded. So, we assume that ∥xnp∥≥∥up∥.

Therefore ∥ynp∥≤∥xnp∥,

()
So, by induction, we have
()
hence {xn} is bounded. we also obtain that {un}, {Tun}, {Txn}, {f(xn)} and {yn} are bounded.

Step 2. xn+1xn∥→0 as n,

()
()

On the other hand, from and , we have

()
()
Putting y = un+1 in (3.9) and y = un in (3.10), we have
()
So, from (A2) we have
()
and hence
()
Without loss of generality, let us assume that there exists a real number b such that rn > b > 0 for all nN. Then, we have
()
and hence
()
where L = sup {∥unxn∥ : nN}. Then we obtain
()

So, put (3.8) and (3.16) into (3.7) we have

()
where K1 : = sup  {∥uun∥,   n ≥ 1} is a constant; K2 : = sup  {∥f(xn−1)∥ + ∥Tyn−1∥,   n ≥ 1} is a constant.

Using Lemma 2.3 and conditions (I), (II), (III) we have

()
From (3.15) and |rn+1rn| → 0, we have
()
Since yn = αnu + (1 − αn)un,   xn = βnf(xn−1) + (1 − βn)Tyn−1, we have
()
()
For pF(T)∩EP (ϕ), we have
()
Therefore, we have
()
By the above of what we have and the condition of lim nβn = 0, we get lim nxnun∥ = 0. Since ∥Tunun∥ ≤ ∥Tunxn∥ + ∥xnun∥, it follows that ∥Tunun∥ → 0.

Step 3. we show that

()
where z = PF(S)∩EP (ϕ)f(z). To show this inequality, we choose a subsequence of {un} such that
()
Since is bounded, there exists a subsequence of which converges weakly to w. Without loss of generality, we can assume that . From ∥Tunun∥→0, we obtain . Let us show wEP (ϕ). By , we have
()
From (A2), we also have
()
and hence
()
Since and , from (A4) we have 0 ≥ ϕ(y, w) for all yC. For t with 0 < t < 1 and yC, let yt = ty + (1 − t)w. Since yC and wC, we have ytC and hence ϕ(yt, w) ≤ 0. So, from (A1) and A4 we have
()
and hence 0 ≤ ϕ(yt, y). From (A3), we have 0 ≤ ϕ(w, y) for all yC, and hence wEP (ϕ). We will show that wF(T). Assume that wF(T). Since and wTw, from Opial’s theorem we have
()
This is a contradiction. So, we get wF(T). Therefore, wF(T)∩EP (ϕ). Since z = PF(T)∩EP (ϕ)f(z), we have
()

From xn+1z = (1 − βn)(Tynz) + βn(f(xn) − z), we have

()
where ,   δn = 2(1 − α)βn/1 − αβn and ζn : = βn/2(1 − α)M + (1 − βn) 2αn/2(1 − α)βnuz2 + 1/1 − αf(z) − z,   xn+1z〉. It is easy to see that and limsup nζn/δn ≤ 0 by (3.31) and the conditions. Hence, by Lemma 2.3, the sequence {xn} converges strongly to z.

If zt is definition as (2.1), then, from Lemma 2.5, we have ∥ztq∥→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 ∥znq∥→0 as n, and if we set Q(u) : = lim nzn, 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+1zn∥ = βnf(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,   unz = 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 : CC 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 x1H and then by

()
for all nN, where {αn}⊂(0, ) satisfies the following conditions:
  • (I)

  • (II)

  • (III)

    lim nαn/βn = 0.

Then, the sequences {xn} converge strongly to zF(S), where z = PF(S)f(z).

Proof. Put ϕ(x, y) = 0, for all x, yC and rn = 1, for all n in Theorem 3.1.

Then we have un = PCxn. So, from Theorem 3.1, the sequence xn generated by x1H and

()
for all n converges strongly to zF(S), where z = PF(S)f(z).

4. Application for Zeros of Maximal Monotone Operators

We adapt in this section the iterative algorithm (3.1) to find zeros of maximal monotone operators and find EP (ϕ). Let us recall that an operator A with domain D(A) and range R(A) in a real Hilbert space H with inner product 〈·, ·〉 and norm ∥·∥ is said to be monotone if the graph of A,
()
is a monotone set. Namely,
()

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) = {xD(A) : 0 ∈ Ax}, is nonempty so that the metric projection PN(A) from H onto N(A) is well-defined.

One of the major problems in the theory of maximal operators is to find a point in the zero set N(A) because various problems arising from economics, convex programming, and other applied areas can be formulated as finding a zero of maximal monotone operators. The proximal point algorithm (PPA) of Rockafellar [17] is commonly recognized as the most powerful algorithm in finding a zero of maximal monotone operators. This (PPA) generates, starting with any initial guess x0H, a sequence {xn} according to the inclusion:
()
where {en} is a sequence of errors and {cn} is a sequence of positive regularization parameters. Equivalently, we can write
()
where for denotes the resolvent of A,
()
with I being the identity operator on the space H.
Rockafellar [17] proved the weak convergence of his algorithm (4.4) provided the regularization sequence {cn} remains bounded away from zero and the error sequence {en} satisfies the condition
()
The aim of this section is to combine algorithm (3.1) with algorithm (4.4). Our algorithm generates a sequence {xn} and {un} be sequences generated initially by an arbitrary elements x1H and then by
()
where αn and cn are sequences of positive real numbers. Furthermore, we prove that {xn} and {un} converge strongly to zN(A)∩EP (ϕ), where z = PN(A)∩EP (ϕ)f(z).

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 x0H and then by

()
for all nN, where {αn}⊂[0,1] and {rn}⊂(0, ) satisfy the following conditions:
  • (I)

    lim nαn = 0, and ;

  • (II)

    liminf nrn > 0 and ;

  • (III)

    lim n(cn+1/cn) = 1;

  • (IV)

    lim nβn = 0, , , and lim n(αn/βn) = 0.

Then, the sequences {xn} and {un} converge strongly to zN(A)∩EP (ϕ), where z = PN(A)∩EP (ϕ)f(z).

Proof. Below we write for simplicity. Setting

()
we rewrite xn+1 of (4.7) as
()
Because the proof is similar to Theorem 3.1, here we just give the main steps as follows:
  • (1)

    {xn} is bounded;

  • (2)

    xn+1xn∥ → 0, as n → 0;

  • (3)

    , as n → 0;

  • (4)

    xnun∥ → 0, as n → 0;

  • (5)

    limsup nf(z) − z, xnz〉 ≤ 0;

  • (6)

    xn, unz, as nz.

5. Application for Optimization Problem

In this section, we study a kind of optimization problem by using the result of this paper. That is, we will give an iterative algorithm of solution for the following optimization problem with nonempty set of solutions
()
where h(x) is a convex and lower semicontinuous functional defined on a closed convex subset C of a Hilbert space H. We denoted by B the set of solutions of (5.1). Let ϕ be a bifunction from C × C to R defined by ϕ(x, y) = h(y) − h(x). We consider the following equilibrium problem, that is, to find xC such that
()
It is obvious that EP (ϕ) = B, where EP (ϕ) denotes the set of solutions of equilibrium problem (5.2). In addition, it is easy to see that ϕ(x, y) satisfies the conditions (A1)–(A4) in the Section 2. Therefore, from Theorem 3.1, we know that the following iterative algorithm:
()
for any initial guess x1, converges strongly to a solution z = PBf(z) of optimization problem (5.1), where {αn}⊂[0,1], {βn}⊂[0,1], and {rn}⊂[0, ) satisfy
()
For a special case, we pick f(x) = η, for all ηH, and rn = 1,  βn = 1/2 and αn = 0 for all n ≥ 1, then xn+1 = (1/2)Tun + (1/2)η, from (5.3), we get the special iterative algorithm as follows:
()
Then {un} converges strongly to a solution z = PBη of optimization problem (5.1).

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).

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