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# Some results on parallel iterative algorithms for strictly pseudocontractive mappings

- Xiaoye Yang
^{1}, - Yuan Qing
^{2}and - Mingliang Zhang
^{3}Email author

**2013**:74

https://doi.org/10.1186/1029-242X-2013-74

© Yang et al.; licensee Springer 2013

**Received:**17 September 2012**Accepted:**25 January 2013**Published:**28 February 2013

## Abstract

In this paper, a parallel iterative algorithm with mixed errors is investigated. Strong and weak convergence theorems of common fixed points of a finite family of strictly pseudocontractive mappings are established in a real Banach space.

**AMS Subject Classification:**47H05, 47H09, 47J25.

## Keywords

- implicit iterative algorithm
- fixed point
- pseudocontractive mapping
- strictly pseudocontractive mapping

## 1 Introduction and preliminaries

*E*and ${E}^{\ast}$ a real Banach space and a dual space of

*E*, respectively. Let $\u3008\cdot ,\cdot \u3009$ denote the pairing between

*E*and ${E}^{\ast}$. The normalized duality mapping $J:E\to {2}^{{E}^{\ast}}$ is defined by

In the sequel, we use *j* to denote the single-valued normalized duality mapping. Let *K* be a nonempty subset of *E* and $T:K\to K$ be a mapping. Recall the following.

*T*is said to be Lipschitz if there exists a positive constant

*L*such that

*T*is said to be nonexpansive if

*T*is said to be strictly pseudocontractive in the terminology of Browder and Petryshyn [1] if there exists $\lambda >0$ such that

*T*is said to be strongly pseudocontractive if there exists $k\in (0,1)$ such that

*T*is said to be pseudocontractive if there exists some $j(x-y)\in J(x-y)$ such that

*T*is said to be Lipschitz if there exists a positive constant

*L*such that

We remark that the class of strongly pseudocontractive mappings is independent of the class of strictly pseudocontractive mappings. This can be seen from the following examples.

**Example 1.1** [3]

Then *T* is a strictly pseudocontractive mapping but not a strongly pseudocontractive mapping.

**Example 1.2** [3]

**Example 1.3** [4]

Then *T* is a Lipschitz pseudocontractive mapping but not a strictly pseudocontractive mapping.

*E*is said to be smooth or is said to have a Gâteaux differentiable norm if the limit

exists for each $x,y\in U$. *E* is said to have a uniformly Gâteaux differentiable norm if for each $y\in U$, the limit is attained uniformly for all $x\in U$. *E* is said to be uniformly smooth or is said to have a uniformly Fréchet differentiable norm if the limit is attained uniformly for $x,y\in U$. It is known that if the norm of *E* is uniformly Gâteaux differentiable, then the duality mapping *J* is single valued and uniformly norm to weak^{∗} continuous on each bounded subset of *E*.

where ${T}_{n}={T}_{n(modN)}$ (here the mod*N* takes values in $\{1,2,\dots ,N\}$).

They obtained the following weak convergence theorem.

**Theorem XO** *Let* *H* *be a real Hilbert space*, *K* *be a nonempty closed convex subset of* *H*, *and* ${T}_{i}:K\to K$ *be a nonexpansive mapping such that* $F={\bigcap}_{i=1}^{N}F({T}_{i})\ne \mathrm{\varnothing}$. *Let* $\{{x}_{n}\}$ *be defined by* (1.6). *If* $\{{\alpha}_{n}\}$ *is chosen so that* ${\alpha}_{n}\to 0$ *as* $n\to \mathrm{\infty}$, *then* $\{{x}_{n}\}$ *converges weakly to a common fixed point of the family of* ${\{{T}_{i}\}}_{i=1}^{N}$.

They further remarked that it is yet unclear what assumptions on the mappings and/or the parameters $\{{\alpha}_{n}\}$ are sufficient to guarantee the strong convergence of the sequence $\{{x}_{n}\}$.

In 2004, Osilike [6] further extended the above results from Hilbert spaces to Banach spaces. To be more precise, he obtain the following results.

**Theorem O** *Let* *H* *be a real Hilbert space*, *K* *be a nonempty closed convex subset of* *H*, *and* ${T}_{i}:K\to K$ *be a strictly pseudocontractive mapping such that* $F={\bigcap}_{i=1}^{N}F({T}_{i})\ne \mathrm{\varnothing}$. *Let* $\{{x}_{n}\}$ *be defined by* (1.6). *If* $\{{\alpha}_{n}\}$ *is chosen so that* ${\alpha}_{n}\to 0$ *as* $n\to \mathrm{\infty}$, *then* $\{{x}_{n}\}$ *converges weakly to a common fixed point of the family of* ${\{{T}_{i}\}}_{i=1}^{N}$.

Subsequently, many authors have investigated the fixed point problem of strictly pseudocontractive mappings based on an implicit or non-implicit iterative algorithm in Banach spaces; see [7–32] and the references therein.

In 2007, Acedo and Xu proposed a parallel iterative algorithm for strictly pseudocontractive mappings in the framework of Hilbert spaces. Weak and strong convergence theorems for common fixed points of a family of strictly pseudocontractive mappings were established; see [26] for more details and the reference therein.

In this paper, motivated by the above results, we consider an implicitly parallel iterative algorithm for a finite family of strictly pseudocontractive mappings. Weak and strong convergence theorems are established in the framework of Banach spaces.

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

*E*is said to satisfy Opial’s condition [33] if, for each sequence $\{{x}_{n}\}$ in

*E*, the convergence ${x}_{n}\to x$ weakly implies that

Recall that the mapping $T:K\to K$ is semicompact if any sequence $\{{x}_{n}\}$ in *K* satisfying ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$ has a convergent subsequence.

**Lemma 1.1** [34]

*Let* *E* *be a real Banach space*, *K* *be a nonempty closed convex subset of* *E*, *and* $T:K\to K$ *be a continuous strongly pseudocontractive mapping*. *Then* *T* *has a unique fixed point in* *K*.

**Lemma 1.2** [11]

*Let* *E* *be a smooth Banach space and* *K* *be a nonempty convex subset of* *E*. *Let* $r\ge 1$ *be some integer*. *Let* ${T}_{i}:K\to K$ *be a strictly pseudocontractive mapping*. *Assume that* ${\bigcap}_{i=1}^{r}F({T}_{i})$ *is not empty*. *Assume that* ${\{{\mu}_{i}\}}_{i=1}^{r}$ *is a positive sequence such that* ${\sum}_{i=1}^{r}{\mu}_{i}=1$. *Then* ${\bigcap}_{i=1}^{r}F({T}_{i})=F({\sum}_{i=1}^{r}{\mu}_{i}{T}_{i})$.

**Lemma 1.3** [35]

*Let*$\{{a}_{n}\}$, $\{{b}_{n}\}$,

*and*$\{{c}_{n}\}$

*be three nonnegative sequences satisfying the following condition*:

*where* ${n}_{0}$ *is some nonnegative integer*, ${\sum}_{n=1}^{\mathrm{\infty}}{b}_{n}<\mathrm{\infty}$, *and* ${\sum}_{n=1}^{\mathrm{\infty}}{c}_{n}<\mathrm{\infty}$. *Then the limit* ${lim}_{n\to \mathrm{\infty}}{a}_{n}$ *exists*.

## 2 Main results

**Theorem 2.1**

*Let*

*E*

*be a smooth and reflexive Banach space which also satisfies Opial’s condition and*

*K*

*be a nonempty closed convex subset of*

*E*.

*Let*$N\ge 1$

*be some positive integer*.

*Let*${T}_{m}:K\to K$,

*where*$m\in \{1,\dots ,N\}$,

*be a*${\lambda}_{i}$-

*strictly pseudocontractive mapping and*$\{{u}_{n}\}$

*be a bounded sequence in*

*K*.

*Let*${\{{x}_{n}\}}_{n=0}^{\mathrm{\infty}}$

*be a sequence generated in the following algorithm*:

*where*$\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, $\{{\gamma}_{n}\}$,

*and*$\{{\delta}_{m}\}$

*are real number sequences in*$[0,1]$.

*Assume that*$F:={\bigcap}_{m=1}^{N}F({T}_{m})\ne \mathrm{\varnothing}$,

*and the above control sequences satisfy the following restrictions*:

- (a)
${\sum}_{m=1}^{N}{\delta}_{m}={\alpha}_{n}+{\beta}_{n}+{\gamma}_{n}=1$;

- (b)
${\sum}_{n=1}^{\mathrm{\infty}}{\gamma}_{n}<\mathrm{\infty}$;

- (c)
$0<a\le {\alpha}_{n}\le b<1$,

*where**a**and**b**are constants*.

*Then* $\{{x}_{n}\}$ *converges weakly to some point in* *F*.

*Proof*Put $T:={\sum}_{m=1}^{N}{\delta}_{m}{T}_{m}$. We show that

*T*is

*λ*-strictly pseudocontractive mapping, where $\lambda :=min\{{\lambda}_{m}:1\le m\le N\}$. Notice that

*T*is

*λ*-strictly pseudocontractive mapping. Next, we show that the implicit iterative algorithm (2.1) is well defined for the strictly pseudocontractive mappings. Define a mapping

*M*is an appropriate constant such that $M\ge {sup}_{n\ge 1}\{\frac{\parallel {u}_{n}-p\parallel}{a}\}$. In view of Lemma 1.3, we obtain that ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exits. Thanks to (2.2), we find from the restrictions (b) and (c) that

Since the space is reflexive and $\{{x}_{n}\}$ is bounded, there exists a subsequence $\{{x}_{{n}_{i}}\}$ of the sequence $\{{x}_{n}\}$, which weakly converges to some *x*∈. In view of Lemma 1.4, we find that $x\in F(T)=F$.

*x*. Suppose the contrary, then there exists some subsequence $\{{x}_{{n}_{j}}\}$ of the sequence $\{{x}_{n}\}$ which weakly converges to ${x}^{\prime}\ne x\in C$. It also follows from Lemma 1.4 that ${x}^{\prime}\in F$. Since ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exits for any $p\in F$. Put

This is a contradiction. This shows that $x={x}^{\prime}$. This proves that the sequence $\{{x}_{n}\}$ converges weakly to $x\in F$. This completes the proof. □

**Corollary 2.2**

*Let*

*E*

*be a smooth and reflexive Banach space which also satisfies Opial’s condition and let*

*K*

*be a nonempty closed convex subset of*

*E*.

*Let*$N\ge 1$

*be some positive integer*.

*Let*${T}_{m}:K\to K$,

*where*$m\in \{1,\dots ,N\}$,

*be a*${\lambda}_{i}$-

*strictly pseudocontractive mapping*.

*Let*${\{{x}_{n}\}}_{n=0}^{\mathrm{\infty}}$

*be a sequence generated in the following algorithm*:

*where*$\{{\alpha}_{n}\}$

*and*$\{{\delta}_{m}\}$

*are real number sequences in*$[0,1]$.

*Assume that*$F:={\bigcap}_{m=1}^{N}F({T}_{m})\ne \mathrm{\varnothing}$

*and the above control sequences satisfy the following restrictions*:

- (a)
${\sum}_{m=1}^{N}{\delta}_{m}=1$;

- (b)
$0<a\le {\alpha}_{n}\le b<1$,

*where**a**and**b**are constants*.

*Then* $\{{x}_{n}\}$ *converges weakly to some point in* *F*.

In Hilbert spaces, we find from Theorem 2.1 the following.

**Corollary 2.3**

*Let*

*E*

*be a Hilbert space and*

*K*

*be a nonempty closed convex subset of*

*E*.

*Let*$N\ge 1$

*be some positive integer*.

*Let*${T}_{m}:K\to K$,

*where*$m\in \{1,\dots ,N\}$,

*be a*${\lambda}_{i}$-

*strictly pseudocontractive mapping and*$\{{u}_{n}\}$

*be a bounded sequence in*

*K*.

*Let*${\{{x}_{n}\}}_{n=0}^{\mathrm{\infty}}$

*be a sequence generated in the following algorithm*:

*where*$\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, $\{{\gamma}_{n}\}$,

*and*$\{{\delta}_{m}\}$

*are real number sequences in*$[0,1]$.

*Assume that*$F:={\bigcap}_{m=1}^{N}F({T}_{m})\ne \mathrm{\varnothing}$

*and the above control sequences satisfy the following restrictions*:

- (a)
${\sum}_{m=1}^{N}{\delta}_{m}={\alpha}_{n}+{\beta}_{n}+{\gamma}_{n}=1$;

- (b)
${\sum}_{n=1}^{\mathrm{\infty}}{\gamma}_{n}<\mathrm{\infty}$;

- (c)
$0<a\le {\alpha}_{n}\le b<1$,

*where**a**and**b**are constants*.

*Then* $\{{x}_{n}\}$ *converges weakly to some point in* *F*.

Next, we give a strong convergence theorem.

**Theorem 2.4**

*Let*

*E*

*be a smooth and reflexive Banach space and*

*K*

*be a nonempty closed convex subset of*

*E*.

*Let*$N\ge 1$

*be some positive integer*.

*Let*${T}_{m}:K\to K$,

*where*$m\in \{1,\dots ,N\}$,

*be a*${\lambda}_{i}$-

*strictly pseudocontractive mapping and*$\{{u}_{n}\}$

*be a bounded sequence in*

*K*.

*Let*${\{{x}_{n}\}}_{n=0}^{\mathrm{\infty}}$

*be a sequence generated in the following algorithm*:

*where*$\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, $\{{\gamma}_{n}\}$,

*and*$\{{\delta}_{m}\}$

*are real number sequences in*$[0,1]$.

*Assume that*$F:={\bigcap}_{m=1}^{N}F({T}_{m})\ne \mathrm{\varnothing}$

*and the above control sequences satisfy the following restrictions*:

- (a)
${\sum}_{m=1}^{N}{\delta}_{m}={\alpha}_{n}+{\beta}_{n}+{\gamma}_{n}=1$;

- (b)
${\sum}_{n=1}^{\mathrm{\infty}}{\gamma}_{n}<\mathrm{\infty}$;

- (c)
$0<a\le {\alpha}_{n}\le b<1$,

*where**a**and**b**are constants*.

*If* ${\sum}_{m=1}^{N}{\delta}_{m}{T}_{m}$ *is semicompact*, *then* $\{{x}_{n}\}$ *converges strongly to some point in* *F*.

*Proof*Since ${\sum}_{m=1}^{N}{\delta}_{m}{T}_{m}$ is semicompact, we see that there exists a subsequence $\{{x}_{{n}_{i}}\}$ of $\{{x}_{n}\}$ such that ${x}_{{n}_{i}}\to {x}^{\ast}$. Notice that

This completes the proof. □

**Corollary 2.5**

*Let*

*E*

*be a smooth and reflexive Banach space and*

*K*

*be a nonempty closed convex subset of*

*E*.

*Let*$N\ge 1$

*be some positive integer*.

*Let*${T}_{m}:K\to K$,

*where*$m\in \{1,\dots ,N\}$,

*be a*${\lambda}_{i}$-

*strictly pseudocontractive mapping*.

*Let*${\{{x}_{n}\}}_{n=0}^{\mathrm{\infty}}$

*be a sequence generated in the following algorithm*:

*where*$\{{\alpha}_{n}\}$

*and*$\{{\delta}_{m}\}$

*are real number sequences in*$[0,1]$.

*Assume that*$F:={\bigcap}_{m=1}^{N}F({T}_{m})\ne \mathrm{\varnothing}$

*and the above control sequences satisfy the following restrictions*:

- (a)
${\sum}_{m=1}^{N}{\delta}_{m}=1$;

- (b)
$0<a\le {\alpha}_{n}\le b<1$,

*where**a**and**b**are constants*.

*If* ${\sum}_{m=1}^{N}{\delta}_{m}{T}_{m}$ *is semicompact*, *then* $\{{x}_{n}\}$ *converges strongly to some point in* *F*.

In Hilbert spaces, we find from Theorem 2.1 the following.

**Corollary 2.6**

*Let*

*E*

*be a Hilbert space and*

*K*

*be a nonempty closed convex subset of*

*E*.

*Let*$N\ge 1$

*be some positive integer*.

*Let*${T}_{m}:K\to K$,

*where*$m\in \{1,\dots ,N\}$,

*be a*${\lambda}_{i}$-

*strictly pseudocontractive mapping and*$\{{u}_{n}\}$

*be a bounded sequence in*

*K*.

*Let*${\{{x}_{n}\}}_{n=0}^{\mathrm{\infty}}$

*be a sequence generated in the following algorithm*:

*where*$\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, $\{{\gamma}_{n}\}$,

*and*$\{{\delta}_{m}\}$

*are real number sequences in*$[0,1]$.

*Assume that*$F:={\bigcap}_{m=1}^{N}F({T}_{m})\ne \mathrm{\varnothing}$

*and the above control sequences satisfy the following restrictions*:

- (a)
${\sum}_{m=1}^{N}{\delta}_{m}={\alpha}_{n}+{\beta}_{n}+{\gamma}_{n}=1$;

- (b)
${\sum}_{n=1}^{\mathrm{\infty}}{\gamma}_{n}<\mathrm{\infty}$;

- (c)
$0<a\le {\alpha}_{n}\le b<1$,

*where**a**and**b**are constants*.

*If* ${\sum}_{m=1}^{N}{\delta}_{m}{T}_{m}$ *is semicompact*, *then* $\{{x}_{n}\}$ *converges strongly to some point in* *F*.

## Declarations

### Acknowledgements

The authors are grateful to the reviewers’ suggestions which improved the contents of the article.

## Authors’ Affiliations

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