Open Access

Strong convergence theorems based on the viscosity approximation method for a countable family of nonexpansive mappings

Journal of Inequalities and Applications20142014:513

https://doi.org/10.1186/1029-242X-2014-513

Received: 2 April 2014

Accepted: 8 December 2014

Published: 18 December 2014

Abstract

In a real Hilbert space, an iterative scheme is considered to obtain a common fixed point for a countable family of nonexpansive mappings. In addition, strong convergence to the common fixed point of this sequence is investigated. As an application, an equilibrium problem is solved. We also state more applications of this procedure to obtain a common fixed point of W-mappings.

MSC:47H09, 47H10, 47J20.

Keywords

viscosity approximation method nonexpansive mapping equilibrium problem W-mapping Hilbert space

1 Introduction

Let H be a real Hilbert space, C be a nonempty closed convex subset of H, and I be an identity mapping on H. The strong (weak) convergence of { x n } to x is written by x n x ( x n x ) as n .

It is well known that H satisfies Opial’s condition [1]; for any sequence { x n } with x n x , the inequality
lim inf n x n x < lim inf n x n y

holds for every y H with x y .

A metric (nearest point) projection P C from a Hilbert space H to a closed convex subset C of H is defined as follows.

For any point x H , there exists a unique P C x C such that
x P C x x y ,
for all y C . It is well known that P C is a nonexpansive mapping from H onto C and satisfies the following:
x y , P C x P C y P C x P C y 2 ,
for all x , y H . Furthermore, P C x is characterized by the following properties: P C x C and
x P C x , P C x y 0 , x y 2 x P C x 2 + y P C x 2 ,
(1.1)

for all y C .

Let A be a mapping of C into H. The variational inequality problem is to find an x C such that
A x , y x 0 , y C .
(1.2)
We shall denote the set of solutions of the variational inequality problem (1.2) by VI ( C , A ) . Then we have
x VI ( C , A ) x = P C ( x λ A x ) , λ > 0 .
(1.3)

A mapping S from C into itself is called nonexpansive if S x S y x y , for all x , y C . Fix ( S ) : = { x C : S x = x } is the set of fixed point of S. Note that Fix ( S ) is closed and convex if S is nonexpansive. A mapping f from C into C is said to be contraction, if there exists a constant k [ 0 , 1 ) such that f ( x ) f ( y ) k x y , for all x , y C .

In 2000, Moudafi [2] introduced the following viscosity approximation methods: x 1 C and
x n + 1 = α n f ( x n ) + ( 1 α n ) S x n , n N ,
where f is a contraction on closed convex subset of a real Hilbert space. It was shown in [2] (also see Xu [3]) that such a sequence converges strongly to the unique solution of the variational inequality problem. In 2007, Chen et al. [4] suggested the following iterative scheme:
x n + 1 = α n f ( x n ) + ( 1 α n ) S P C ( I λ n A ) x n , n N ,
where x 1 C , S is a nonexpansive self-mapping and A an α-inverse strongly monotone mapping. They proved that the sequence { x n } converges strongly to a common fixed point of a nonexpansive mapping which solves the corresponding variational inequality. Recently, Kumam and Plubtieng [5] used the following viscosity iterative method for a countable family of nonexpansive mappings: x 1 C and
x n + 1 = α n f ( x n ) + ( 1 α n ) S n P C ( I λ n A ) x n , n N .

They proved the generated sequence { x n } converges strongly to a common element of the set of common fixed points of a countable family of nonexpansive mappings and the set of solutions of the variational inequality.

On the other hand, in 2009, Yao et al. [6] considered a new sequence that is generated by x 1 C and
x n + 1 = α n x n + ( 1 α n ) S P C ( 1 λ n ) x n , n N ,

to find a fixed point of a nonexpansive mapping.

It is worth pointing out that many authors have extended the results in Hilbert space to the more general uniformly convex and uniformly smooth Banach space (see, for instance, [3, 711]).

In this work, motivated and inspired by the above results, an iterative scheme based on the viscosity approximation method is utilized to find a common element of the set of common fixed points of a countable family of nonexpansive mappings. Moreover, a strong convergence theorem with different conditions on the parameters is studied. As an application, an equilibrium problem is solved. In addition, a common fixed point for W-mappings is obtained.

The following lemmas will be useful in the sequel.

Lemma 1.1 ([12])

Let { x n } and { y n } be bounded sequences in a Banach space X and { β n } be a sequence in [ 0 , 1 ] with 0 < lim inf n β n lim sup n β n < 1 . Suppose x n + 1 = ( 1 β n ) y n + β n x n for all integers n 1 and lim sup n ( y n + 1 y n x n + 1 x n ) 0 . Then lim n y n x n = 0 .

Lemma 1.2 ([13])

Let { a n } be a sequence of nonnegative real numbers satisfying the following relation:
a n + 1 ( 1 α n ) a n + α n σ n + γ n , n 0 ,
where
  1. (1)

    { α n } [ 0 , 1 ] , n = 1 α n = ;

     
  2. (2)

    lim sup n σ n 0 ;

     
  3. (3)

    γ n 0 , for all n N and n = 1 γ n < .

     

Then lim n a n = 0 .

Lemma 1.3 ([14])

Let C be a nonempty closed subset of a Banach space and { S n } be a sequence of nonexpansive mappings from C into itself. Suppose n = 1 sup { S n + 1 x S n x : x C } < . Then, for each x C , { S n x } converges strongly to some point of C. If S is a mapping from C into itself which is defined by S x : = lim n S n x , for all x C , then lim n sup { S n x S x : x C } = 0 .

2 Strong convergence theorem

In this section, we use the viscosity approximation method to find a common element of the set of common fixed points of a countable family of nonexpansive mappings.

Theorem 2.1 Let C be a nonempty closed convex subset of a real Hilbert space H. Assume that { S n } is a sequence of nonexpansive mappings from C into itself such that n = 1 Fix ( S n ) , and f is a contraction from H into C with constant k 1 2 . Suppose that { α n } , { β n } , { γ n } , and { λ n } are real sequences in ( 0 , 1 ) . Set x 1 C and let { x n } be the iterative sequence defined by
{ y n : = P C ( 1 λ n ) x n , x n + 1 : = α n x n + β n f ( x n ) + γ n S n y n , n N ,
satisfying the following conditions:
  1. (1)

    α n + β n + γ n = 1 ,

     
  2. (2)

    lim n β n = 0 , n = 1 β n = ,

     
  3. (3)

    0 < lim inf n γ n lim sup n γ n < 1 ,

     
  4. (4)

    lim n λ n = 0 , n = 1 λ n = , n = 1 | λ n + 1 λ n | < ,

     
  5. (5)

    n = 1 sup { S n + 1 x S n x : x B } < , for any bounded subset B of C.

     

Let S be a mapping from C into itself defined by S x : = lim n S n x for all x C and Fix ( S ) : = n = 1 Fix ( S n ) . Then { x n } converges strongly to an element ω Fix ( S ) , where ω = P Fix ( S ) f ( ω ) .

Proof Fix ( S ) is a closed convex set, then P Fix ( S ) is well defined and P Fix ( S ) is nonexpansive. In addition,
P Fix ( S ) f ( x ) P Fix ( S ) f ( y ) f ( x ) f ( y ) k x y ,

for all x , y H . This shows that P Fix ( S ) f is a contraction from H into C. Since H is complete, there exists a unique element of ω Fix ( S ) H such that ω = P Fix ( S ) f ( ω ) .

Let x Fix ( S ) , we note that
x n + 1 x α n x n x + β n f ( x n ) x + γ n S n y n x α n x n x + k β n x n x + β n f ( x ) x + γ n y n x α n x n x + k β n x n x + β n f ( x ) x + γ n ( 1 λ n ) x n x α n x n x + k β n x n x + β n f ( x ) x + γ n ( 1 λ n ) x n x + γ n λ n x = ( α n + k β n + γ n γ n λ n ) x n x + β n f ( x ) x + γ n λ n x ( 1 β n + k β n γ n λ n ) x n x + ( 1 k ) β n f ( x ) x 1 k + γ n λ n x max { x n x , x , f ( x ) x 1 k } max { x 1 x , x , f ( x ) x 1 k } .
Therefore { x n } is bounded. Hence, { f ( x n ) } , { y n } , and { S n y n } are bounded. Also
S n + 1 y n + 1 S n y n S n + 1 y n + 1 S n + 1 y n + S n + 1 y n S n y n y n + 1 y n + sup { S n + 1 x S n x : x { y n } } ( 1 λ n + 1 ) x n + 1 ( 1 λ n ) x n + sup { S n + 1 x S n x : x { y n } } x n + 1 x n + λ n + 1 x n + 1 x n + | λ n + 1 λ n | x n + sup { S n + 1 x S n x : x { y n } } .
(2.1)
Now, we define x n + 1 = α n x n + ( 1 α n ) w n , for all n N . One can observe that
w n + 1 w n = β n + 1 f ( x n + 1 ) + γ n + 1 S n + 1 y n + 1 1 α n + 1 β n f ( x n ) + γ n S n y n 1 α n = β n + 1 1 α n + 1 f ( x n + 1 ) + 1 α n + 1 β n + 1 1 α n + 1 S n + 1 y n + 1 β n 1 α n f ( x n ) 1 α n β n 1 α n S n y n = β n + 1 1 α n + 1 ( f ( x n + 1 ) S n + 1 y n + 1 ) + β n 1 α n ( S n y n f ( x n ) ) + S n + 1 y n + 1 S n y n .
(2.2)
Substituting (2.1) into (2.2), it follows that
w n + 1 w n x n + 1 x n β n + 1 1 α n + 1 f ( x n + 1 ) S n + 1 y n + 1 + β n 1 α n S n y n f ( x n ) + λ n + 1 x n + 1 x n + | λ n + 1 λ n | x n + sup { S n + 1 x S n x : x { y n } } .
Therefore,
lim sup n w n + 1 w n x n + 1 x n 0 .
In view of Lemma 1.1, we obtain lim n w n x n = 0 , which implies that
lim n x n + 1 x n = lim n ( 1 α n ) w n x n = 0 .
On the other hand, one has
x n + 1 x n = β n ( f ( x n ) S n y n ) + ( 1 α n ) ( S n y n x n ) .
It follows that
( 1 α n ) S n y n x n x n + 1 x n + β n S n y n f ( x n ) .
Hence, lim n x n S n y n = 0 . Also, from y n S n y n x n S n y n + λ n x n , we obtain
lim n y n S n y n = 0 .
Now, we prove
lim sup n f ( ω ) ω , S n y n ω 0 ,
where ω = P Fix ( S ) f ( ω ) . Indeed, since { S n y n } is bounded, one can find a subsequence { S n i y n i } of { S n y n } such that
lim sup n f ( ω ) ω , S n y n ω = lim i f ( ω ) ω , S n i y n i ω .

{ y n i } is bounded, there exists a subsequence { y n i j } of { y n i } which converges weakly to z. Without loss of generality, assume that y n i z . y n i is a sequence in C and C is closed and convex, so z C . Now, using the fact that S n y n y n 0 , we obtain S n i y n i z . Next we show z Fix ( S ) .

Assume that z Fix ( S ) . From Opial’s condition and Lemma 1.3, we have
lim inf i y n i z < lim inf i y n i S z = lim inf i y n i S n i y n i + S n i y n i S y n i + S y n i S z lim inf i S y n i S z lim inf i y n i z .

This is a contradiction. Thus, z Fix ( S ) .

Also, we note that ω = P Fix ( S ) f ( ω ) and so, by (1.1), we have
lim sup n f ( ω ) ω , S n y n ω = lim i f ( ω ) ω , S n i y n i ω = f ( ω ) ω , z ω 0 .
To complete the proof, we show { x n } converges strongly to ω F ( S ) . For this, by convexity of 2 , we have
S n y n ω 2 y n ω 2 ( 1 λ n ) x n ω 2 ( 1 λ n ) x n ω 2 + λ n ω 2 .
Hence,
x n + 1 ω 2 = α n ( x n ω ) + β n ( f ( x n ) ω ) + γ n ( S n y n ω ) 2 α n ( x n ω ) + β n ( f ( x n ) ω ) 2 + γ n 2 S n y n ω 2 + 2 γ n α n ( x n ω ) + β n ( f ( x n ) ω ) , S n y n ω = ( α n x n ω + β n f ( x n ) ω ) 2 + γ n 2 S n y n ω 2 + 2 α n γ n x n ω , S n y n ω + 2 β n γ n f ( x n ) ω , S n y n ω α n 2 x n ω 2 + β n 2 f ( x n ) ω 2 + γ n 2 S n y n ω 2 + 2 α n β n x n ω f ( x n ) ω + 2 α n γ n x n ω S n y n ω + 2 β n γ n f ( x n ) f ( ω ) , S n y n ω + 2 β n γ n f ( ω ) ω , S n y n ω α n 2 x n ω 2 + β n 2 f ( x n ) ω 2 + γ n 2 S n y n ω 2 + α n β n ( x n ω 2 + f ( x n ) ω 2 ) + α n γ n ( x n ω 2 + S n y n ω 2 ) + 2 k β n γ n x n ω S n y n ω + 2 β n γ n f ( ω ) ω , S n y n ω ( α n 2 + α n β n + α n γ n ) x n ω 2 + ( β n 2 + α n β n ) f ( x n ) ω 2 + ( γ n 2 + α n γ n ) S n y n ω 2 + k β n γ n ( x n ω 2 + S n y n ω 2 ) + 2 β n γ n f ( ω ) ω , S n y n ω ( α n + k β n γ n ) x n ω 2 + β n ( 1 γ n ) f ( x n ) ω 2 + ( γ n 2 + α n γ n + k β n γ n ) S n y n ω 2 + 2 β n γ n f ( ω ) ω , S n y n ω ( α n + k β n γ n ) x n ω 2 + β n ( 1 γ n ) f ( x n ) ω 2 + ( γ n 2 + α n γ n + k β n γ n ) [ ( 1 λ n ) x n ω 2 + λ n ω 2 ] + 2 β n γ n f ( ω ) ω , S n y n ω ( α n + k β n γ n + ( γ n 2 + α n γ n + k β n γ n ) ( 1 λ n ) ) x n ω 2 + β n ( 1 γ n ) f ( x n ) ω 2 + ( γ n 2 + α n γ n + β n γ n ) λ n ω 2 + 2 β n γ n f ( ω ) ω , S n y n ω .
Now, suppose L = sup { x n ω , f ( x n ) ω , ω } . Then
x n + 1 ω 2 ( 1 β n γ n ) x n ω 2 + β n γ n [ 2 f ( ω ) ω , S n y n ω + λ n β n ω 2 + 1 γ n γ n f ( x n ) ω 2 + α n + k β n γ n + ( γ n 2 + α n γ n + k β n γ n ) ( 1 λ n ) + β n γ n 1 β n γ n x n ω 2 ] ( 1 β n γ n ) x n ω 2 + β n γ n [ 2 f ( ω ) ω , S n y n ω + [ λ n γ n + β n β n γ n + α n + k β n γ n + ( γ n ( 1 β n ) + k β n γ n ) ( 1 λ n ) β n γ n + β n γ n 1 β n γ n ] L 2 ] = ( 1 δ n ) x n ω 2 + δ n σ n ,
where
δ n = β n γ n , σ n = 2 f ( ω ) ω , S n y n ω σ n = + [ λ n γ n + β n β n γ n + α n + k β n γ n + ( γ n ( 1 β n ) + k β n γ n ) ( 1 λ n ) β n γ n σ n = + β n γ n 1 β n γ n ] L 2 σ n = 2 f ( ω ) ω , S n y n ω σ n = + [ λ n γ n + k β n γ n γ n + γ n ( 1 β n ) ( 1 λ n ) + k β n γ n ( 1 λ n ) β n γ n ] L 2 σ n = 2 f ( ω ) ω , S n y n ω σ n = + [ λ n + k β n 1 + ( 1 β n ) ( 1 λ n ) + k β n ( 1 λ n ) β n ] L 2 σ n 2 f ( ω ) ω , S n y n ω + [ ( 2 k 1 ) + ( 1 k ) λ n ] L 2 .

It is easy to see that { δ n } [ 0 , 1 ] , n = 1 δ n = and lim sup n σ n 0 . Hence, by Lemma 1.2, we find that { x n } strongly converges to ω Fix ( S ) , where ω = P Fix ( S ) f ( ω ) . This completes the proof of this theorem. □

The following example shows that this theorem is not a special case of [[5], Theorem 3.1].

Example 2.2 Let C = [ 1 , 1 ] H = R with α n = n 1 10 n 9 , β n = 1 n , and λ n = 9 10 n . Set f ( x ) = x 10 and S n ( x ) = x n . Then f is a 1 10 -contraction and S n is a sequence of nonexpansive mappings. It readily follows that the sequence { x n } generated by
{ y n : = P C ( 1 λ n ) x n = 10 n 9 10 n x n , x n + 1 : = α n x n + β n f ( x n ) + γ n S n y n = 10 n 2 9 100 n 2 90 n x n + 9 n 2 18 n + 9 10 n 3 9 n 2 y n ,

with initial value x 1 C , converges strongly to an element (zero) of Fix ( S ) = n = 1 Fix ( S n ) and P Fix ( S ) f ( 0 ) = 0 .

3 Applications

In this section, we consider the equilibrium problems and W-mappings.

3.1 Equilibrium problems

Equilibrium theory plays a central role in various applied sciences such as physics, mechanics, chemistry, and biology. In addition, it represents an important area of the mathematical sciences such as optimization, operations research, game theory, and financial mathematics. Equilibrium problems include fixed point problems, optimization problems, variational inequalities, Nash equilibria problems, and complementary problems as special cases.

Let φ : C R be a real-valued function and A : C H a nonlinear mapping. Also suppose F : C × C R is a bifunction. The generalized mixed equilibrium problem is to find x C (see [15]) such that
F ( x , y ) + φ ( y ) φ ( x ) + A x , y x 0 ,
(3.1)

for all y C .

We shall denote the set of solutions of this generalized mixed equilibrium problem by GMEP; that is
GMEP : = { x C : F ( x , y ) + φ ( y ) φ ( x ) + A x , y x 0 , y C } .
We now discuss several special cases of GMEP as follows:
  1. 1.
    If φ = 0 , then the problem (3.1) is reduced to generalized equilibrium problem, i.e., finding x C such that
    F ( x , y ) + A x , y x 0 ,
     
for all y C .
  1. 2.
    If A = 0 , then the problem (3.1) is reduced to the mixed equilibrium problem, that is, to find x C such that
    F ( x , y ) + φ ( y ) φ ( x ) 0 ,
     
for all y C . We shall write the set of solutions of the mixed equilibrium problem by MEP.
  1. 3.
    If φ = 0 , A = 0 , then the problem (3.1) is reduced to the equilibrium problem, which is to find x C such that
    F ( x , y ) 0 ,
     
for all y C .
  1. 4.

    If φ = 0 , F = 0 , then the problem (3.1) is reduced to the variational inequality problem (1.2).

     

Now let φ : C R be a real-valued function. To solve the generalized mixed equilibrium problem for a bifunction F : C × C R , let us assume that F, φ, and C satisfy the following conditions:

(A1) F ( x , x ) = 0 for all x C ;

(A2) F is monotone, i.e., F ( x , y ) + F ( y , x ) 0 for all x , y C ;

(A3) for each x , y , z C , lim t 0 + F ( t z + ( 1 t ) x , y ) F ( x , y ) ;

(A4) for each x C , y F ( x , y ) is convex and lower semicontinuous;

(B1) for each x H and r > 0 , there exist a bounded subset D x C and y x C such that for each z C D x ,
F ( z , y x ) + φ ( y x ) φ ( z ) + 1 r y x z , z x < 0 ;

(B2) C is a bounded set.

In what follows we state some lemmas which are useful to prove our convergence results.

Lemma 3.1 ([16])

Assume that F : C × C R satisfies (A1)-(A4), and let φ : C R be a lower semicontinuous and convex function. Assume that either (B1) or (B2) holds. For r > 0 and x H , define a mapping T r ( F , φ ) : H C as follows:
T r ( F , φ ) ( x ) : = { z C : F ( z , y ) + φ ( y ) φ ( z ) + 1 r y z , z x 0 , y C } ,
for all x H . Then the following assertions hold:
  1. (1)

    For each x H , T r ( F , φ ) ;

     
  2. (2)

    T r ( F , φ ) is single-valued;

     
  3. (3)
    T r ( F , φ ) is firmly nonexpansive, i.e., for any x , y H ,
    T r ( F , φ ) x T r ( F , φ ) y 2 T r ( F , φ ) x T r ( F , φ ) y , x y ;
     
  4. (4)

    Fix ( T r ( F , φ ) ) = MEP ;

     
  5. (5)

    MEP is closed and convex.

     

Lemma 3.2 ([17])

Let C be a nonempty closed convex subset of a real Hilbert space H. Assume that S 1 is a nonexpansive mapping from C into H and S 2 a firmly nonexpansive mapping from H into C such that Fix ( S 1 ) Fix ( S 2 ) . Then S 1 S 2 is a nonexpansive mapping from H into itself and Fix ( S 1 S 2 ) = Fix ( S 1 ) Fix ( S 2 ) .

Lemma 3.3 ([18, 19])

Let C be a nonempty closed convex subset of a real Hilbert space H. Let F be a bifunction from C × C into satisfying (A1)-(A4) and φ : C R be a lower semicontinuous and convex function. Assume that either (B1) or (B2) holds. Let { r n } be a sequence in ( 0 , ) , such that inf { r n : n N } > 0 , n = 1 | r n + 1 r n | < and T r n ( F , φ ) be a mapping defined as in Lemma  3.1. Then
  1. (1)

    n = 1 sup { T r n + 1 ( F , φ ) x T r n ( F , φ ) x : x B } < , for any bounded subset B of C;

     
  2. (2)

    Fix ( T r ( F , φ ) ) = n = 1 Fix ( T r n ( F , φ ) ) , where T r ( F , φ ) is a mapping defined by T r ( F , φ ) x : = lim n T r n ( F , φ ) x , for all x C . Moreover, lim n T r n ( F , φ ) x T r ( F , φ ) x = 0 .

     
Now let C be a nonempty closed convex subset of a real Hilbert space H. A mapping A : C H is called monotone if A x A y , x y 0 for all x , y C . It is called α-inverse strongly monotone if there exists a positive real number α such that A x A y , x y α A x A y 2 , for all x , y C . An α-inverse strongly monotone mapping is sometimes called α-cocoercive. A mapping A is said to be relaxed α-cocoercive if there exists α > 0 such that
A x A y , x y α A x A y 2 ,
for all x , y C . The mapping A is said to be relaxed ( α , λ ) -cocoercive if there exist α , λ > 0 such that
A x A y , x y α A x A y 2 + λ x y 2 ,
for all x , y C . A mapping A : H H is said to be μ-Lipschitzian if there exists μ 0 such that
A x A y μ x y ,

for all x , y H . It is clear that each α-inverse strongly monotone mapping is monotone and 1 α -Lipschitzian and that each μ-Lipschitzian, relaxed ( α , λ ) -cocoercive mapping with α μ 2 λ is monotone. Also, if A is an α-inverse strongly monotone, then I λ A is a nonexpansive mapping from C to H, provided that λ 2 α (see [20]).

Now we have the following theorem.

Theorem 3.4 Let C be a nonempty closed convex subset of a real Hilbert space H. Let F be a bifunction from C × C into satisfying (A1)-(A4) and φ : C R a lower semicontinuous and convex function. Assume that either (B1) or (B2) holds. Let A be a μ-Lipschitzian, relaxed ( α , λ ) -cocoercive mapping from C into H, B a β-inverse strongly monotone mapping from C into H and f a contraction from H into C with constant k 1 2 . Suppose that S n is a sequence of nonexpansive mappings from H into C such that n = 1 Fix ( S n ) VI ( C , A ) GMEP . Let { α n } , { β n } , { γ n } , and { λ n } be real sequences in ( 0 , 1 ) and { r n } , { s n } ( 0 , ) . Let { y n } , { u n } , and { x n } be generated by x 1 C ,
{ y n : = P C ( 1 λ n ) x n , u n : = T r n ( F , φ ) ( y n r n B y n ) , x n + 1 : = α n x n + β n f ( x n ) + γ n S n P C ( u n s n A u n ) , n N .
Suppose that the following conditions are satisfied:
  1. (1)

    α n + β n + γ n = 1 ,

     
  2. (2)

    lim n β n = 0 , n = 1 β n = ,

     
  3. (3)

    0 < lim inf n γ n lim sup n γ n < 1 ,

     
  4. (4)

    lim n λ n = 0 , n = 1 λ n = , n = 1 | λ n + 1 λ n | < ,

     
  5. (5)

    0 < a r n 2 β , n = 1 | r n + 1 r n | < ,

     
  6. (6)

    0 < s n 2 ( λ α μ 2 ) μ 2 , n = 1 | s n + 1 s n | < ,

     
  7. (7)

    n = 1 sup { S n + 1 x S n x : x E } < , for any bounded subset E of C.

     

Then { x n } converges strongly to an element ω n = 1 Fix ( S n ) VI ( C , A ) GMEP , where ω = P n = 1 Fix ( S ) VI ( C , A ) GMEP f ( ω ) .

Proof For all x , y C and s n [ 0 , 2 ( λ α μ 2 ) μ 2 ] , we obtain
( I s n A ) x ( I s n A ) y 2 = x y s n ( A x A y ) 2 = x y 2 2 s n x y , A x A y + s n 2 A x A y 2 x y 2 2 s n [ α A x A y 2 + λ x y 2 ] + s n 2 A x A y 2 x y 2 + 2 s n μ 2 α x y 2 2 s n λ x y 2 + μ 2 s n 2 x y 2 = ( 1 + 2 s n μ 2 α 2 s n λ + μ 2 s n 2 ) x y 2 x y 2 .
This shows that I s n A is nonexpansive for each n N . By Lemma 3.3, it implies that
n = 1 sup { T r n + 1 ( F , φ ) x T r n ( F , φ ) x : x E } < ,

for any bounded subset E of C. In addition, the mapping T r ( F , φ ) , defined by T r ( F , φ ) x : = lim n T r n ( F , φ ) x for all x C , satisfies Fix ( T r ( F , φ ) ) = n = 1 Fix ( T r n ( F , φ ) ) = MEP .

Put T n : = S n P C ( I s n A ) T r n ( F , φ ) ( I r n B ) = S n P C ( I s n A ) U n . Then, by Lemmas 3.2, 3.1, and (1.3), we find that T n is a nonexpansive mapping from C into itself and Fix ( T n ) = Fix ( S n ) VI ( C , A ) Fix ( T r n F , φ ( I r n B ) ) = Fix ( S n ) VI ( C , A ) GMEP , for all n N , and so
n = 1 Fix ( T n ) = n = 1 Fix ( S n ) VI ( C , A ) GMEP .
Also we note that
T n + 1 x T n x = S n + 1 P C ( I s n + 1 A ) U n + 1 x S n P C ( I s n A ) U n x S n + 1 P C ( I s n + 1 A ) U n + 1 x S n P C ( I s n + 1 A ) U n + 1 x + S n P C ( I s n + 1 A ) U n + 1 x S n P C ( I s n A ) U n x S n + 1 v n S n v n + ( I s n + 1 A ) U n + 1 x ( I s n A ) U n x S n + 1 v n S n v n + ( I s n + 1 A ) U n + 1 x ( I s n + 1 A ) U n x + ( I s n + 1 A ) U n x ( I s n A ) U n x S n + 1 v n S n v n + T r n + 1 ( F , φ ) ( I r n + 1 B ) x T r n ( F , φ ) ( I r n B ) x + | s n + 1 s n | A U n x S n + 1 v n S n v n + T r n + 1 ( F , φ ) ( I r n + 1 B ) x T r n ( F , φ ) ( I r n + 1 B ) x + T r n ( F , φ ) ( I r n + 1 B ) x T r n ( F , φ ) ( I r n B ) x + | s n + 1 s n | A U n x S n + 1 v n S n v n + T r n + 1 ( F , φ ) w n T r n ( F , φ ) w n + | r n + 1 r n | B x + | s n + 1 s n | A U n x ,
where v n = P C ( I s n + 1 A ) U n + 1 x and w n = ( I r n + 1 B ) x . Moreover, for any bounded subset E of C, F = { P C ( I s n + 1 A ) U n + 1 x : x E , n N } and G = { ( I r n + 1 B ) x : x E , n N } are bounded and
n = 1 sup { T n + 1 x T n x : x E } n = 1 sup { S n + 1 y S n y : y F } + n = 1 sup { T r n + 1 ( F , φ ) z T r n ( F , φ ) z : z G } + n = 1 | r n + 1 r n | sup { B x : x E } + n = 1 | s n + 1 s n | sup { A U n x : x E } < .

Therefore, by Theorem 2.1, { x n } converges strongly to an element ω n = 1 Fix ( S n ) VI ( C , A ) GMEP , where ω = P n = 1 Fix ( S n ) VI ( C , A ) GMEP f ( ω ) . This completes the proof. □

3.2 W-Mappings

The concept of W-mappings was introduced in [21, 22]. It is now one of the main tools in studying convergence of iterative methods to approach a common fixed point of nonlinear mapping; more recent progress can be found in [23] and the references cited therein.

Let { S n } be a countable family of nonexpansive mappings S n : H H and δ 1 , δ 2 , be real numbers such that 0 δ n 1 for every n N . We consider the mapping W n defined by
U n , n + 1 : = I , U n , n : = δ n S n U n , n + 1 + ( 1 δ n ) I , U n , n 1 : = δ n 1 S n 1 U n , n + ( 1 δ n 1 ) I , U n , k : = δ k S k U n , k + 1 + ( 1 δ k ) I , U n , k 1 : = δ k 1 S k 1 U n , k + ( 1 δ k 1 ) I , U n , 2 : = δ 2 S 2 U n , 3 + ( 1 δ 2 ) I , W n : = U n , 1 = δ 1 S 1 U n , 2 + ( 1 δ 1 ) I .
(3.2)

One can find the proof of the following lemma in [24].

Lemma 3.5 Let H be a real Hilbert space. Let { S n } be a sequence of nonexpansive mappings from H into itself such that n = 1 Fix ( S n ) . Let δ 1 , δ 2 , be real numbers such that 0 < δ n b < 1 for all n N . Then
  1. (1)

    W n is nonexpansive and Fix ( W n ) = i = 1 n Fix ( S i ) for all n N ;

     
  2. (2)

    lim n U n , k x exists, for all x H and k N ;

     
  3. (3)

    the mapping W : C C defined by W x : = lim n W n x = lim n U n , 1 x , for all x C is a nonexpansive mapping satisfying Fix ( W ) = n = 1 Fix ( S n ) ; and it is called W-mapping generated by S 1 , S 2 , , S n , and δ 1 , δ 2 , , δ n .

     
Theorem 3.6 Let C be a nonempty closed convex subset of a real Hilbert space H. Assume that { S n } is a sequence of nonexpansive mappings from C into itself such that n = 1 Fix ( S n ) , and f a contraction from H into C with constant k 1 2 . Let { α n } , { β n } , { γ n } and { λ n } be real sequences in ( 0 , 1 ) . Also, suppose W n are the W-mappings from C into itself generated by S 1 , S 2 , , S n , and δ 1 , δ 2 , , δ n such that 0 < δ n b < 1 for every n N . Set x 1 C and let { x n } be the iterative sequence defined by
{ y n : = P C ( 1 λ n ) x n , x n + 1 : = α n x n + β n f ( x n ) + γ n W n y n , n N ,
satisfying the following conditions:
  1. (1)

    α n + β n + γ n = 1 ,

     
  2. (2)

    lim n β n = 0 , n = 1 β n = ,

     
  3. (3)

    0 < lim inf n γ n lim sup n γ n < 1 ,

     
  4. (4)

    lim n λ n = 0 , n = 1 λ n = , n = 1 | λ n + 1 λ n | < .

     

Let W be a mapping from C into itself defined by W x : = lim n W n x for all x C . Then { x n } converges strongly to an element ω n = 1 Fix ( S n ) , where ω = P n = 1 Fix ( S n ) f ( ω ) .

Proof Since S i and U n , i are nonexpansive, by (3.2), we deduce that, for each n N ,
W n + 1 x W n x = δ 1 S 1 U n + 1 , 2 x δ 1 S 1 U n , 2 x δ 1 U n + 1 , 2 x U n , 2 x = δ 1 δ 2 S 2 U n + 1 , 3 x δ 2 S 2 U n , 3 x δ 1 δ 2 U n + 1 , 3 x U n , 3 x δ 1 δ 2 δ n U n + 1 , n + 1 x U n , n + 1 x M i = 1 n δ i ,
where M > 0 is a constant such that sup { U n + 1 , n + 1 x U n , n + 1 x : x B } M , for any bounded subset B of C. Then
n = 1 sup { W n + 1 x W n x : x B } < .

Now, by setting S n : = W n in Theorem 2.1 and using Lemma 3.5, we obtain the result. □

Applying Lemma 3.5 and Theorem 3.4, we obtain the following result.

Corollary 3.7 Let C be a nonempty closed convex subset of a real Hilbert space H. Let F be a bifunction from C × C into satisfying (A1)-(A4) and φ : C R a lower semicontinuous and convex function. Assume that either (B1) or (B2) holds. Let A be a μ-Lipschitzian, relaxed ( α , λ ) -cocoercive mapping from C into H, B a β-inverse strongly monotone mapping from C into H and f a contraction from H into C with constant k 1 2 . Suppose that S n is a sequence of nonexpansive mappings from H into C such that n = 1 Fix ( S n ) VI ( C , A ) GMEP . Let { y n } , { u n } , and { x n } be generated by x 1 C ,
{ y n : = P C ( 1 λ n ) x n , u n : = T r n ( F , φ ) ( y n r n B y n ) , x n + 1 : = α n x n + β n f ( x n ) + γ n W n P C ( u n s n A u n ) , n N ,
where { α n } , { β n } , { γ n } , and { λ n } are real sequences in ( 0 , 1 ) and { r n } , { s n } ( 0 , ) satisfying the following conditions:
  1. (1)

    α n + β n + γ n = 1 ,

     
  2. (2)

    lim n β n = 0 , n = 1 β n = ,

     
  3. (3)

    0 < lim inf n γ n lim sup n γ n < 1 ,

     
  4. (4)

    lim n λ n = 0 , n = 1 λ n = , n = 1 | λ n + 1 λ n | < ,

     
  5. (5)

    0 < a r n 2 β , n = 1 | r n + 1 r n | < ,

     
  6. (6)

    0 < s n 2 ( λ α μ 2 ) μ 2 , n = 1 | s n + 1 s n | < .

     

Then { x n } converges strongly to an element ω n = 1 Fix ( S n ) VI ( C , A ) GMEP , where ω = P n = 1 Fix ( S n ) VI ( C , A ) GMEP f ( ω ) .

Declarations

Acknowledgements

The authors express their gratitude to the referees for reading this paper carefully.

Authors’ Affiliations

(1)
Department of Mathematics, College of Basic Sciences, Karaj Branch, Islamic Azad University
(2)
Department of Mathematics, Faculty of Science, Imam Khomeini International University
(3)
Research Institute for Natural Sciences, Hanyang University

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© Bagherboum et al.; licensee Springer. 2014

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