- Research Article
- Open Access

# Fixed Point Iterations of a Pair of Hemirelatively Nonexpansive Mappings

- Yan Hao
^{1, 2}Email author and - SunYoung Cho
^{3}

**2010**:270150

https://doi.org/10.1155/2010/270150

© Y. Hao and S. Y. Cho. 2010

**Received:**27 September 2009**Accepted:**22 March 2010**Published:**31 March 2010

## Abstract

We introduce an iterative method for a pair of hemirelatively nonexpansive mappings. Strong convergence of the purposed iterative method is obtained in a Banach space.

## Keywords

- Banach Space
- Iterative Method
- Convex Subset
- Nonexpansive Mapping
- Strong Convergence

## 1. Introduction and Preliminaries

Let be a Banach space with the dual . We denote by the normalized duality mapping from to defined by

where denotes the generalized duality pairing. A Banach space is said to be strictly convex if for all with and It is said to be uniformly convex if for any two sequences in such that and . Let be the unit sphere of . Then the Banach space is said to be smooth provided that

exists for each It is also said to be uniformly smooth if the limit (1.2) is attained uniformly for . It is well known that if is uniformly smooth, then is uniformly norm-to-norm continuous on each bounded subset of . It is also well known that is uniformly smooth if and only if is uniformly convex.

Recall that a Banach space has the Kadec-Klee property if for any sequences and with and then as ; for more details on Kadec-Klee property, the readers is referred to [1, 2] and the references therein. It is well known that if is a uniformly convex Banach space, then enjoys the Kadec-Klee property.

Let be a nonempty closed and convex subset of a Banach space and : → a mapping. The mapping is said to be closed if for any sequence such that and , then . A point is a fixed point of provided . In this paper, we use to denote the fixed point set of and use and to denote the strong convergence and weak convergence, respectively.

Recall that the mapping is said to be nonexpansive if

It is well known that if is a nonempty bounded closed and convex subset of a uniformly convex Banach space , then every nonexpansive self-mapping on has a fixed point. Further, the fixed point set of is closed and convex.

As we all know that if is a nonempty closed convex subset of a Hilbert space and : → is the metric projection of onto , then is nonexpansive. This fact actually characterizes Hilbert spaces and consequently, it is not available in more general Banach spaces. In this connection, Alber [3] recently introduced a generalized projection operator in a Banach space which is an analogue of the metric projection in Hilbert spaces.

Next, we assume that is a smooth Banach space. Consider the functional defined by

Observe that, in a Hilbert space , (1.4) is reduced to The generalized projection : → is a map that assigns to an arbitrary point , the minimum point of the functional that is, where is the solution to the minimization problem

Existence and uniqueness of the operator follow from the properties of the functional and strict monotonicity of the mapping (see, e.g., [1–4]). In Hilbert spaces, It is obvious from the definition of function that

Remark 1.1.

If is a reflexive, strictly convex and smooth Banach space, then for , if and only if . It is sufficient to show that if then . From (1.6), we have . This implies that From the definition of we have . Therefore, we have see [1, 2] for more details.

Let be a nonempty closed convex subset of and a mapping from into itself. A point in is said to be an asymptotic fixed point of [5] if contains a sequence which converges weakly to such that . The set of asymptotic fixed points of will be denoted by . A mapping from into itself is said to be relatively nonexpansive [3, 6, 7] if and for all and . The mapping is said to be hemirelatively nonexpansive [8–12] if and for all and . The asymptotic behavior of a relatively nonexpansive mappings was studied in [3, 6, 7].

Remark 1.2.

The class of hemirelatively nonexpansive mappings is more general than the class of relatively nonexpansive mappings which requires the restriction: . From Su et al. [11], we see that every hemirelatively nonexpansive mapping is relatively nonexpansive, but the inverse is not true. Hemirelatively nonexpansive mapping is also said to be quasi- -nonexpansive; see [13–17].

Recently, fixed point iterations of relatively nonexpansive mappings and hemirelatively nonexpansive mappings have been considered by many authors; see, for example [14–25] and the references therein. In 2005, Matsushita and Takahashi [8] considered fixed point problems of a single relatively nonexpansive mapping in a Banach space. To be more precise, they proved the following theorem.

Theorem 1 MT.

where is the duality mapping on . If is nonempty, then converges strongly to , where is the generalized projection from onto

In 2007, Plubtieng and Ungchittrakool [9] further improved Theorem MT by considering a pair of relatively nonexpansive mappings. To be more precise, they proved the following theorem.

Theorem PU

with the following restrictions:

for each and

for each , and .

Then the sequence converges strongly to , where is the generalized projection from onto

Very recently, Su et al. [11] improved Theorem PU partially by considering a pair of hemirelatively nonexpansive mappings. To be more precise, they obtained the following results.

Theorem SWX

with the following restrictions:

(1) ;

(2) ;

(3) for some .

Then the sequence converges strongly to , where is the generalized projection from onto

In this paper, motivated by Theorems MT, PU, and SWX, we consider the problem of finding a common fixed point of a pair of hemirelatively nonexpansive mappings by shrinking projection methods which were introduced by Takahashi et al. [26] in Hilbert spaces. Strong convergence theorems of common fixed points are established in a Banach space. The results presented in this paper mainly improve the corresponding results announced in Matsushita and Takahashi [8], Nakajo and Takahahsi [27], and Su et al. [11].

In order to prove our main results, we need the following lemmas.

Lemma 1.3 (see [3]).

Lemma 1.4 (see [3]).

The following lemma can be deduced from Matsushita and Takahashi [8].

Lemma 1.5.

Let be a strictly convex and smooth Banach space, a nonempty closed convex subset of and a hemirelatively nonexpansive mapping. Then is a closed convex subset of .

Lemma 1.6 (see [28]).

for all and with

## 2. Main Results

Theorem 2.1.

where , , , and are real sequences in satisfying the following restrictions:

and .

Then converges strongly to , where is the generalized projection from onto .

Proof.

Hence, we have as In view of the Kadec-Klee property of we obtain that as

It follows from the closedness of that By repeating (2.27)–(2.37), we can obtain that . This shows that

and hence by Lemma 1.3. This completes the proof.

Remark 2.2.

Theorem 2.1 improves Theorem SWX in the following aspects:

from the point of view on computation, we remove the set in Theorem SWX;

from the point of view on the framework of spaces, we extend Theorem SWX from a uniformly smooth and uniformly convex Banach space to a uniformly smooth and strictly convex Banach space which enjoys the Kadec-Klee property. Note that every uniformly convex Banach space enjoys the Kadec-Klee property.

If for each , then Theorem 2.1 is reduced to the following.

Corollary 2.3.

where , , and are real sequences in satisfying the following restrictions:

and .

Then converges strongly to , where is the generalized projection from onto .

If , then Corollary 2.3 is reduced to the following.

Corollary 2.4.

where is a real sequence in satisfying . Then converges strongly to , where is the generalized projection from onto .

## Declarations

### Acknowledgment

This project is supported by the National Natural Science Foundation of China (no. 10901140).

## Authors’ Affiliations

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