- Research Article
- Open Access

# Convergence Theorems of Modified Ishikawa Iterative Scheme for Two Nonexpansive Semigroups

- Kriengsak Wattanawitoon
^{1}and - Poom Kumam
^{1}Email author

**2010**:914702

https://doi.org/10.1155/2010/914702

© K. Wattanawitoon and P. Kumam. 2010

**Received:**26 September 2009**Accepted:**24 November 2009**Published:**25 November 2009

## Abstract

We prove convergence theorems of modified Ishikawa iterative sequence for two nonexpansive semigroups in Hilbert spaces by the two hybrid methods. Our results improve and extend the corresponding results announced by Saejung (2008) and some others.

## Keywords

- Hilbert Space
- Convergence Theorem
- Hybrid Method
- Nonexpansive Mapping
- Strong Convergence

## 1. Introduction

Let be a subset of real Hilbert spaces with the inner product and the norm . is called a nonexpansive mapping if

We denote by the set of fixed points of , that is, .

Let be a family of mappings from a subset of into itself. We call it a nonexpansive semigroup on if the following conditions are satisfied:

(iii)for each the mapping is continuous;

where the initial guess is taken in arbitrarily and the sequence is in the interval .

In 1967, Halpern [2] first introduced the following iterative scheme:

see also Browder [3]. He pointed out that the conditions and are necessary in the sence that, if the iteration (1.3) converges to a fixed point of , then these conditions must be satisfied.

On the other hand, in 2002, Suzuki [4] was the first to introduce the following implicit iteration process in Hilbert spaces:

for the nonexpansive semigroup. In 2005, Xu [5] established a Banach space version of the sequence (1.4) of Suzuki [4].

In 2007, Chen and He [6] studied the viscosity approximation process for a nonexpansive semigroup and prove another strong convergence theorem for a nonexpansive semigroup in Banach spaces, which is defined by

where is a fixed contractive mapping.

Recently He and Chen [7] is proved a strong convergence theorem for nonexpansive semigroups in Hilbert spaces by hybrid method in the mathematical programming. Very recently, Saejung [8] proved a convergence theorem by the new iterative method introduced by Takahashi et al. [9] without Bochner integrals for a nonexpansive semigroup with in Hilbert spaces:

where denotes the metric projection from onto a closed convex subset of .

In 1974, Ishikawa [10] introduced a new iterative scheme, which is defined recursively by

where the initial guess is taken in arbitrarily and the sequences and are in the interval .

In this paper, motivated by the iterative sequences (1.6) given by Saejung in [8] and Ishikawa [10], we introduce the modified Ishikawa iterative scheme for two nonexpansive semigroups in Hilbert spaces. Further, we obtain strong convergence theorems by using the hybrid methods. This result extends and improves the result of Saejung [8] and some others.

## 2. Preliminaries

This section collects some lemmas which will be used in the proofs for the main results in the next section.

It is known that every Hilbert space satisfies the Opial's condition [11], that is,

Recall that the metric (nearest point) projection from a Hilbert space to a closed convex subset of is defined as follows. Given is the only point in with the property

Lemma 2.1.

Lemma 2.2.

Lemma 2.3 (see [12, Lemma ]).

Let be a real sequence and let be a real number such that . Suppose that either of the following holds:

then is a cluster point of . Moreover, for , there exists such that for every integer with

## 3. Main Results

### 3.1. The Shrinking Projection Method

In this section, we prove strong convergence of an iterative sequence generated by the shrinking hybrid projection method in mathematical programming.

Theorem 3.1.

Proof.

Therefore, is nondecreasing. From , we also have , for all

Thus, , for all and . Then exists and is bounded.

Next, we show that as . From (3.6) we have

For , we have This implies that and hence Moreover, since

As and (3.19), we obtain and so Similarly, we have Thus .

Finally, we show that Since and

Hence as required. This completes the proof.

Corollary 3.2.

Proof.

Putting , in Theorem 3.1, we obtain the conclusion immediately.

Corollary 3.3 (see [8, Theorem ]).

Proof.

If for all and for every in Theorem 3.1 then (3.1) reduced to (3.25). By using Theorem 3.1, we get the following conclusion.

### 3.2. The CQ Hybrid Method

In this section, we consider the modified Ishikawa iterative scheme computing by the CQ hybrid method [13–15]. We use the same idea as Saejung's Theorem in [8] and our Theorem 3.1 to obtain the following result and the proof is omitted.

Theorem 3.4.

Proof.

First, we show that both and are closed and convex, and for all . It follows easily from the definition that and are just intersection of and the half-spaces see also [9]. As in the proof of the preceding theorem, we have for all . Clearly, . Suppose that for some , we have . In particular, that is, . It follows from the induction that for all . This proves the claim.

We first claim that Indeed, as and ,

By using the same argument of Saejung [8, Theorem , page 6] and in the proof of Theorem 3.1, we have and . And we can choose a subsequence of such that , , and as .

From (3.21), we obtain

By the Opial's condition of , we have and for all , that is, .

We note that

Hence the whole sequence must converge to , as required. This completes the proof.

Corollary 3.5.

Proof.

If for all , in Theorem 3.4 then (3.26) reduced to (3.36). So, we obtain the result immediately.

We also deduce the following corollary.

Corollary 3.6 (see [8, Theorem ]).

## Declarations

### Acknowledgments

The authors would like to thank the editors and the anonymous referees for their valuable suggestions which help to improve this paper. This research was supported by the Computational Science and Engineering Research Cluster, King Mongkut's University of Technology Thonburi (KMUTT) (National Research Universities under CSEC Project no. E01008).

## Authors’ Affiliations

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