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# An implicit method for finding a common fixed point of a representation of nonexpansive mappings in Banach spaces

- Nawab Hussain
^{1}, - Mahmood Lashkarizadeh Bami
^{2}and - Ebrahim Soori
^{3}Email author

**2014**:238

https://doi.org/10.1186/1687-1812-2014-238

© Hussain et al.; licensee Springer. 2014

**Received:**31 August 2014**Accepted:**12 November 2014**Published:**4 December 2014

The Erratum to this article has been published in Fixed Point Theory and Applications 2015 2015:203

## Abstract

We introduce an implicit method for finding an element of the set of common fixed points of a representation of nonexpansive mappings. Then we prove the strong convergence of the proposed implicit scheme to the common fixed point of a representation of nonexpansive mappings.

**MSC:**90C33, 47H10.

## Keywords

- fixed point
- nonexpansive mapping
- representation
- semigroup
- sunny nonexpansive retraction

## 1 Introduction

*C*be a nonempty closed and convex subset of a Banach space

*E*and ${E}^{\ast}$ be the dual space of

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

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

*j*to denote the single-valued normalized duality mapping. Let $U=\{x\in E:\parallel x\parallel =1\}$.

*E*is said to be smooth or 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 Féchet 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*. A Banach space *E* is smooth if the duality mapping *J* of *E* is single-valued. We know that if *E* is smooth, then *J* is norm to weak-star continuous; for more details, see [1].

Let *C* be a nonempty closed and convex subset of a Banach space *E*. A mapping *T* of *C* into itself is called nonexpansive if $\parallel Tx-Ty\parallel \le \parallel x-y\parallel $ for all $x,y\in C$, and a mapping *f* is an *α*-contraction on *E* if $\parallel f(x)-f(y)\parallel \le \alpha \parallel x-y\parallel $, $x,y\in E$ such that $0\le \alpha <1$.

*S*as nonexpansive mappings from

*C*into itself, with respect to a left regular sequence of means defined on an appropriate subspace of bounded real-valued functions of the semigroup. On the other hand, our goal is to prove that there exists a sunny nonexpansive retraction

*P*of

*C*onto $Fix(\mathcal{S})$ and $x\in C$ such that the following sequence $\{{z}_{n}\}$ converges strongly to

*Px*:

## 2 Preliminaries

*S*be a semigroup. We denote by $B(S)$ the Banach space of all bounded real-valued functions defined on

*S*with supremum norm. For each $s\in S$ and $f\in B(S)$, we define ${l}_{s}$ and ${r}_{s}$ in $B(S)$ by

*X*be a subspace of $B(S)$ containing 1, and let ${X}^{\ast}$ be its topological dual. An element

*μ*of ${X}^{\ast}$ is said to be a mean on

*X*if $\parallel \mu \parallel =\mu (1)=1$. We often write ${\mu}_{t}(f(t))$ instead of $\mu (f)$ for $\mu \in {X}^{\ast}$ and $f\in X$. Let

*X*be left invariant (resp. right invariant),

*i.e.*, ${l}_{s}(X)\subset X$ (resp. ${r}_{s}(X)\subset X$) for each $s\in S$. A mean

*μ*on

*X*is said to be left invariant (resp. right invariant) if $\mu ({l}_{s}f)=\mu (f)$ (resp. $\mu ({r}_{s}f)=\mu (f)$) for each $s\in S$ and $f\in X$.

*X*is said to be left (resp. right) amenable if

*X*has a left (resp. right) invariant mean.

*X*is amenable if

*X*is both left and right amenable. As is well known, $B(S)$ is amenable when

*S*is a commutative semigroup (see p.29 of [1]). A net $\{{\mu}_{\alpha}\}$ of means on

*X*is said to be left regular if

for each $s\in S$, where ${l}_{s}^{\ast}$ is the adjoint operator of ${l}_{s}$.

Let *f* be a function of the semigroup *S* into a reflexive Banach space *E* such that the weak closure of $\{f(t):t\in S\}$ is weakly compact, and let *X* be a subspace of $B(S)$ containing all the functions $t\to \u3008f(t),{x}^{\ast}\u3009$ with ${x}^{\ast}\in {E}^{\ast}$. We know from [4] that for any $\mu \in {X}^{\ast}$, there exists a unique element ${f}_{\mu}$ in *E* such that $\u3008{f}_{\mu},{x}^{\ast}\u3009={\mu}_{t}\u3008f(t),{x}^{\ast}\u3009$ for all ${x}^{\ast}\in {E}^{\ast}$. We denote such ${f}_{\mu}$ by $\int f(t)\phantom{\rule{0.2em}{0ex}}\mathrm{d}\mu (t)$. Moreover, if *μ* is a mean on *X*, then from [5], $\int f(t)\phantom{\rule{0.2em}{0ex}}\mathrm{d}\mu (t)\in \overline{co}\{f(t):t\in S\}$.

*C*be a nonempty closed and convex subset of

*E*. Then a family $\mathcal{S}=\{{T}_{s}:s\in S\}$ of mappings from

*C*into itself is said to be a representation of

*S*as a nonexpansive mapping on

*C*into itself if $\mathcal{S}$ satisfies the following:

- (1)
${T}_{st}x={T}_{s}{T}_{t}x$ for all $s,t\in S$ and $x\in C$;

- (2)
for every $s\in S$, the mapping ${T}_{s}:C\to C$ is nonexpansive.

We denote by $Fix(\mathcal{S})$ the set of common fixed points of $\mathcal{S}$, that is, $Fix(\mathcal{S})={\bigcap}_{s\in S}\{x\in C:{T}_{s}x=x\}$.

**Theorem 2.1** [6]

*Let*

*S*

*be a semigroup*,

*let*

*C*

*be a closed*,

*convex subset of a reflexive Banach space*

*E*, $\mathcal{S}=\{{T}_{s}:s\in S\}$

*be a representation of*

*S*

*as a nonexpansive mapping from*

*C*

*into itself such that weak closure of*$\{{T}_{t}x:t\in S\}$

*is weakly compact for each*$x\in C$,

*and let*

*X*

*be a subspace of*$B(S)$

*such that*$1\in X$

*and the mapping*$t\to \u3008T(t)x,{x}^{\ast}\u3009$

*be an element of*

*X*

*for each*$x\in C$

*and*${x}^{\ast}\in E$,

*and*

*μ*

*be a mean on*

*X*.

*If we write*${T}_{\mu}x$

*instead of*$\int {T}_{t}x\phantom{\rule{0.2em}{0ex}}\mathrm{d}\mu (t)$,

*then the following hold*.

- (i)
${T}_{\mu}$

*is a nonexpansive mapping from**C**into**C*. - (ii)
${T}_{\mu}x=x$

*for each*$x\in Fix(\mathcal{S})$. - (iii)
${T}_{\mu}x\in \overline{co}\{{T}_{t}x:t\in S\}$

*for each*$x\in C$. - (iv)
*If**X**is*${r}_{s}$-*invariant for each*$s\in S$*and**μ**is right invariant*,*then*${T}_{\mu}{T}_{t}={T}_{\mu}$*for each*$t\in S$.

**Remark** From Theorem 4.1.6 in [1], every uniformly convex Banach space is strictly convex and reflexive.

Let *D* be a subset of *B*, where *B* is a subset of a Banach space *E*, and let *P* be a retraction of *B* onto *D*, that is, $Px=x$ for each $x\in D$. Then *P* is said to be sunny if for each $x\in B$ and $t\ge 0$ with $Px+t(x-Px)\in B$, $P(Px+t(x-Px))=Px$. A subset *D* of *B* is said to be a sunny nonexpansive retract of *B* if there exists a sunny nonexpansive retraction *P* of *B* onto *D*. We know that if *E* is smooth and *P* is a retraction of *B* onto *D*, then *P* is sunny and nonexpansive if and only if for each $x\in B$ and $z\in D$, $\u3008x-Px,J(z-Px)\u3009\le 0$. For more details, see [1].

**Lemma 2.2** [7]

*Let* *S* *be a semigroup*, *and let* *C* *be a compact convex subset of a real strictly convex and smooth Banach space* *E*. *Suppose that* $\mathcal{S}=\{{T}_{s}:s\in S\}$ *is a representation of* *S* *as a nonexpansive mapping from* *C* *into itself*. *Let* *X* *be a left invariant subspace of* $B(S)$ *such that* $1\in X$, *and the function* $t\mapsto \u3008{T}_{t}x,{x}^{\ast}\u3009$ *is an element of* *X* *for each* $x\in C$ *and* ${x}^{\ast}\in {E}^{\ast}$. *If* *μ* *is a left invariant mean on* *X*, *then* $Fix({T}_{\mu})={T}_{\mu}C=Fix(\mathcal{S})$ *and there exists a unique sunny nonexpansive retraction from* *C* *onto* $Fix(\mathcal{S})$.

Throughout the rest of this paper, the open ball of radius *r* centered at 0 is denoted by ${B}_{r}$. Let *C* be a nonempty closed convex subset of a Banach space *E*. For $\u03f5>0$ and a mapping $T:C\to C$, we let ${F}_{\u03f5}(T)$ be the set of *ϵ*-approximate fixed points of *T*, *i.e.*, ${F}_{\u03f5}(T)=\{x\in C:\parallel x-Tx\parallel \le \u03f5\}$.

## 3 Main result

In this section, we deal with a strong convergence approximation scheme for finding a common element of the set of common fixed points of a representation of nonexpansive mappings.

**Theorem 3.1**

*Let*

*S*

*be a semigroup*.

*Let*

*C*

*be a nonempty compact convex subset of a real strictly convex and reflexive and smooth Banach space*

*E*.

*Suppose that*$\mathcal{S}=\{{T}_{s}:s\in S\}$

*is a representation of*

*S*

*as a nonexpansive mapping from*

*C*

*into itself such that*$Fix(\mathcal{S})\ne \mathrm{\varnothing}$.

*Let*

*X*

*be a left invariant subspace of*$B(S)$

*such that*$1\in X$,

*and the function*$t\mapsto \u3008{T}_{t}x,{x}^{\ast}\u3009$

*is an element of*

*X*

*for each*$x\in C$

*and*${x}^{\ast}\in {E}^{\ast}$.

*Let*$\{{\mu}_{n}\}$

*be a left regular sequence of means on*

*X*.

*Suppose that*

*f*

*is an*

*α*-

*contraction on*

*C*.

*Let*${\u03f5}_{n}$

*be a sequence in*$(0,1)$

*such that*${lim}_{n}{\u03f5}_{n}=0$.

*Then there exists a unique sunny nonexpansive retraction*

*P*

*of*

*C*

*onto*$Fix(\mathcal{S})$

*and*$x\in C$

*such that the following sequence*$\{{z}_{n}\}$

*generated by*

*strongly converges to* *Px*.

*Proof* By Proposition 1.7.3 and Theorem 1.9.21 in [8], any compact subset *C* of a reflexive Banach space *E* is weakly compact, and from Proposition 1.9.18 in [8], any closed convex subset of a weakly compact subset *C* of a Banach space *E* is itself weakly compact, and by Proposition 1.9.13 in [8], any convex subset *C* of a normed space *E* is weakly closed if and only if *C* is closed. Therefore, weak closure of $\{{T}_{t}x:t\in S\}$ is weakly compact for each $x\in C$.

We divide the proof into five steps.

Step 1. The existence of ${z}_{n}$ which satisfies (1).

Therefore, by the Banach contraction principle [1], there exists a unique point ${z}_{n}\in C$ such that ${N}_{n}{z}_{n}={z}_{n}$.

Step 2. ${lim}_{n\to \mathrm{\infty}}\parallel {z}_{n}-{T}_{t}{z}_{n}\parallel =0$ for all $t\in S$.

*N*such that

Since $\u03f5>0$ is arbitrary, we get ${lim}_{n\to \mathrm{\infty}}\parallel {z}_{n}-{T}_{t}{z}_{n}\parallel =0$.

Step 3. $\mathfrak{S}\{{z}_{n}\}\subset Fix(\mathcal{S})$, where $\mathfrak{S}\{{z}_{n}\}$ denotes the set of strongly limit points of $\{{z}_{n}\}$.

therefore $z\in Fix(\mathcal{S})$.

*P*of

*C*onto $Fix(\mathcal{S})$ and $x\in C$ such that

*P*of

*C*onto $Fix(\mathcal{S})$. The Banach contraction mapping principle guarantees that

*fP*has a unique fixed point $x\in C$. We show that

*C*is a compact subset of

*E*, we can select a subsequence $\{{z}_{{n}_{j}}\}$ of $\{{z}_{n}\}$ with the following properties:

- (i)
${lim}_{j}\u3008x-Px,J({z}_{{n}_{j}}-Px)\u3009=\mathrm{\Gamma}$;

- (ii)
$\{{z}_{{n}_{j}}\}$ converges strongly to a point

*z*.

*E*is smooth, we have

Step 5. $\{{z}_{n}\}$ strongly converges to *Px*.

That is, ${z}_{n}\to Px$. □

**Remark 3.2** It would be an interesting problem to prove Theorem 3.1 for continuous representations instead of nonexpansive.

## Notes

## Declarations

### Acknowledgements

This article was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah. The first author acknowledges with thanks DSR, KAU for financial support. The authors would like to thank the referee of the paper for his helpful comments and invaluable suggestions. This research was supported by the Center of Excellence for Mathematics and the Office of Graduate Studies of the Lorestan University and the University of Isfahan.

## Authors’ Affiliations

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