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# Some results on continuous pseudo-contractions in a reflexive Banach space

- Songtao Lv
^{1}Email author and - Yan Hao
^{2}

**2013**:538

https://doi.org/10.1186/1029-242X-2013-538

© Lv and Hao; licensee Springer. 2013

**Received:**17 April 2013**Accepted:**1 October 2013**Published:**13 November 2013

## Abstract

In this paper, we investigate fixed point problems of a continuous pseudo-contraction based on a viscosity iterative scheme. Strong convergence theorems are established in a reflexive Banach space which also enjoys a weakly continuous duality mapping.

**MSC:**47H05, 47H09, 47H10, 47J05, 47J25.

## Keywords

- fixed point
- Banach space
- weakly continuous duality mapping
- pseudo-contraction

## 1 Introduction and preliminaries

Fixed point problems of nonlinear operators, which include many important problems in nonlinear analysis and optimization such as the Nash equilibrium problem, variational inequalities, complementarity problems, vector optimization problems, and saddle point problems, recently have been studied as an effective and powerful tool for studying many real world problems which arise in economics, finance, medicine, image reconstruction, ecology, transportation, and network; see [1–20] and the references therein. Interest in pseudo-contractive operators, an important class of nonlinear operators, stems mainly from their firm connection with equations of evolution. It is known that many physically significant problems can be modeled by initial value problems of the form ${x}^{\prime}(t)+x(t)-Ax(t)=0$, $x(0)={x}_{0}$, where *A* is a pseudo-contractive operator in the framework of Banach spaces. Typical examples where such evolution equations occur can be found in the heat, wave or Schrödinger equations. Fixed points of a pseudo-contractive operator have been investigated by many authors; see [21–32] and the references therein. For the existence of fixed points of pseudo-contractive operators, we refer readers to [31] and [32] and the references therein. In this paper, we consider the mean regularization viscosity method for treating fixed points of pseudo-contractive operators in a Banach space.

*E*be a real Banach space, ${E}^{\ast}$ be the dual space of

*E*. Let $\phi :[0,\mathrm{\infty}]:={R}^{+}\to {R}^{+}$ be a continuous and strictly increasing function such that $\phi (0)=0$ and $\phi (t)\to \mathrm{\infty}$ as $t\to \mathrm{\infty}$. This function

*φ*is called a gauge function. The duality mapping ${J}_{\phi}:E\to {E}^{\ast}$ associated with a gauge function

*φ*is defined by

where $\u3008\cdot ,\cdot \u3009$ denotes the generalized duality pairing. In the case that $\phi (t)=t$, we write *J* for ${J}_{\phi}$ and call *J* the normalized duality mapping.

Recall that a Banach space *E* is said to have a weakly continuous duality mapping if there exists a gauge *φ* for which the duality mapping ${J}_{\phi}(x)$ is single-valued and weak-to-weak^{∗} sequentially continuous (*i.e.*, if $\{{x}_{n}\}$ is a sequence in *E* weakly convergent to a point *x*, then the sequence ${J}_{\phi}({x}_{n})$ converges weakly^{∗} to ${J}_{\phi}$). It is known that ${l}^{p}$ has a weakly continuous duality mapping with a gauge function $\phi (t)={t}^{p-1}$ for all $1<p<\mathrm{\infty}$. Set $\mathrm{\Phi}(t)={\int}_{0}^{t}\phi (\tau )\phantom{\rule{0.2em}{0ex}}d\tau $, $\mathrm{\forall}t\ge 0$. Then ${J}_{\phi}(x)=\partial \mathrm{\Phi}(\parallel x\parallel )$, $\mathrm{\forall}x\in X$, where *∂* denotes the sub-differential in the sense of convex analysis.

*C*be a nonempty closed and convex subset of

*E*. Let

*D*be a nonempty subset of

*C*, and let

*Q*be a mapping of

*C*into

*D*. Then

*Q*is said to be sunny if

whenever $Qx+t(x-Qx)\in C$ for $x\in C$ and $t\ge 0$. A mapping *Q* of *C* into itself is called a retraction if ${Q}^{2}=Q$. If a mapping *Q* of *C* into itself is a retraction, then $Qz=z$ for all $z\in R(Q)$, where $R(Q)$ is the range of *Q*. A nonempty subset *D* of *C* is called a sunny nonexpansive retract of *C* if there exists a sunny nonexpansive retraction from *C* onto *D*.

*T*. Recall that

*T*is contractive with the coefficient $k\in (0,1)$ if

The mapping *T* is said to be nonexpansive if the contractive coefficient $k=1$.

where $u\in C$ is a fixed point. Banach’s contraction mapping principle guarantees that ${T}_{t}$ has a unique fixed point ${x}_{t}$ in *C*. In the case of *T* having a fixed point, Browder [1] proved that if *E* is a Hilbert space, then ${x}_{t}$ converges strongly to a fixed point of *T* that is nearest to *u*. Reich [33] extended Browder’s result to the setting of Banach spaces and proved that if *E* is a uniformly smooth Banach space, then ${x}_{t}$ converges strongly to a fixed point of *T* and the limit defines the (unique) sunny nonexpansive retraction from *C* onto $F(T)$.

Xu [34] proved that Reich’s results hold in reflexive Banach spaces which have a weakly continuous duality map. To be more precise, he proved the following theorem.

**Theorem X**

*Let*

*E*

*be a reflexive Banach space and have a weakly continuous duality map*${J}_{\phi}$

*with a gauge*

*φ*.

*Let*

*C*

*be a closed convex subset of*

*E*,

*and let*$T:C\to C$

*be a nonexpansive mapping*.

*Fix*$u\in C$

*and*$t\in (0,1)$.

*Let*${x}_{t}\in C$

*be the unique solution in*

*C*

*to the equation*

*Then* *T* *has a fixed point if and only if* $\{{x}_{t}\}$ *remains bounded as* $t\to {0}^{+}$, *and in this case*, $\{{x}_{t}\}$ *converges as* $t\to {0}^{+}$ *strongly to a fixed point of* *T*.

The mapping *T* is said to be pseudo-contractive if the coefficient $k=1$.

Recently, Zegeye and Shahzad [35] improved Theorem X about the mapping *T* from nonexpansive mappings to pseudo-contractions by the viscosity approximation method which was first introduced by Moudafi [36]. More precisely, they proved the following results.

**Theorem ZS**

*Let*

*K*

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

*E*.

*Let*$T:K\to E$

*be a continuous pseudo*-

*contractive mapping and*$f:K\to E$

*be a contraction*(

*with constant*

*β*)

*both satisfying the weakly inward condition*.

*Then*,

*for*$t\in (0,1)$,

*there exists a sequence*$\{{y}_{t}\}\subset K$

*satisfying the following condition*:

*Suppose further that* $\{{y}_{t}\}$ *is bounded or* $(F(T)\ne \mathrm{\varnothing})$ *and* *E* *is a reflexive Banach space having a weakly continuous duality mapping* ${J}_{\phi}$ *for some gauge* *φ*. *Then* $\{{y}_{t}\}$ *converges strongly to a fixed point of* *T*.

We give some remarks about Moudafi’s viscosity approximation method which was recently studied by many authors. From Suzuki [37], we know that Moudafi’s viscosity approximation with a contraction is trivial. Since the mapping ${P}_{C}f$, where ${P}_{C}$ is a metric projection from *H* onto its nonempty closed and convex subset *C*, is a contraction, we can get the conclusion if Browder’s property is satisfied; see [37] for more details.

Next, we propose the following question.

What happens if the mapping *f* is a strong pseudo-contraction instead of a contraction? Does Theorem ZS still hold?

Since we do not know whether the mapping ${P}_{C}f$, where *f* is a strong pseudo-contraction, has a unique fixed point or not, we cannot answer the above question easily based on Suzuki’s results.

It is our purpose in this paper to consider the convergence of paths for a continuous pseudo-contraction by Moudafi’s viscosity approximation with continuous strong pseudo-contractions instead of contractions, which gives an answer to the above question.

To prove our main results, we need the following lemma and definitions.

Let *C* be a nonempty subset of a Banach space *E*. For $x\in C$, the inward set of *x*, ${I}_{C}(x)$, is defined by ${I}_{C}(x):=\{x+\lambda (u-x):u\in C,\lambda \ge 1\}$. A mapping $T:C\to E$ is called weakly inward if $Tx\in cl[{I}_{C}(x)]$ for all $x\in C$, where $cl[{I}_{C}(x)]$ denotes the closure of the inward set. Every self-map is trivially weakly inward.

The first part of the next lemma is an immediate consequence of the subdifferential inequality and the proof of the second part can be found in [38].

**Lemma 1.1**

*Assume that a Banach space*

*E*

*has a weakly continuous duality mapping*${J}_{\phi}$

*with a gauge*

*φ*.

- (i)
*For all*$x,y\in E$,*the following inequality holds*:$\mathrm{\Phi}(\parallel x+y\parallel )\le \mathrm{\Phi}(\parallel x\parallel )+\u3008y,{J}_{\phi}(x+y)\u3009.$ - (ii)
*Assume that a sequence*$\{{x}_{n}\}$*in**E**converges weakly to a point*$x\in E$.*Then the following identity holds*:$\underset{n\to \mathrm{\infty}}{lim\hspace{0.17em}sup}\mathrm{\Phi}(\parallel {x}_{n}-y\parallel )=\underset{n\to \mathrm{\infty}}{lim\hspace{0.17em}sup}\mathrm{\Phi}(\parallel {x}_{n}-x\parallel )+\mathrm{\Phi}(\parallel y-x\parallel ),\phantom{\rule{1em}{0ex}}\mathrm{\forall}x,y\in E.$

**Lemma 1.2** [31]

*Let* *E* *be a Banach space*, *C* *be a nonempty closed and convex subset of* *E* *and* $T:C\to C$ *be a continuous and strong pseudo*-*contraction*. *Then* *T* *has a unique fixed point in* *C*.

## 2 Main results

**Lemma 2.1**

*Let*

*C*

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

*E*,

*and let*$T:C\to C$

*be a continuous pseudo*-

*contraction*.

*Let*$f:C\to C$

*be a fixed continuous bounded and strong pseudo*-

*contraction with the pseudo*-

*contractive coefficient*$k\in (0,1)$.

*For*$t\in (0,1)$,

*define a mapping*${T}_{t}^{f}$

*by*

*Then*

- (i)
(2.1)

*has a unique solution*${x}_{t}\in C$*for every*$t\in (0,1)$; - (ii)
*If*$\{{x}_{t}\}$*is bounded*,*then*$\parallel {x}_{t}-T{x}_{t}\parallel \to 0$*as*$t\to 0$; - (iii)
*If*$F(T)\ne \mathrm{\varnothing}$,*where*$F(T)$*denotes the fixed point set of**T*,*then*$\{{x}_{t}\}$*is bounded and satisfies*$\u3008{x}_{t}-f({x}_{t}),{J}_{\phi}({x}_{t}-p)\u3009\le 0,\phantom{\rule{1em}{0ex}}\mathrm{\forall}p\in F(T).$

*Proof*(i) Indeed, for any $x,y\in C$ and $t\in (0,1)$, we have

This shows that ${T}_{t}^{f}:C\to C$ is a continuous and strong pseudo-contraction for each $t\in (0,1)$. By Lemma 1.2, we obtain that ${T}_{t}^{f}$ has a unique fixed point ${x}_{t}$ in *C* for each $t\in (0,1)$. Therefore, one obtains that (i) holds.

*f*and $\{{x}_{t}\}$, we have

This completes the proof. □

**Theorem 2.2**

*Let*

*E*

*be a reflexive Banach space which has a weakly continuous duality mapping*${J}_{\phi}$

*for some gauge*

*φ*,

*and let*

*C*

*be a nonempty closed and convex subset of*

*E*.

*Let*$T:C\to C$

*be a continuous pseudo*-

*contraction and*$f:C\to C$

*be a fixed bounded*,

*continuous and strong pseudo*-

*contraction with the coefficient*$k\in (0,1)$.

*Let*$\{{x}_{t}\}$

*be as in Lemma*2.1.

*If*$\{{x}_{t}\}$

*is bounded*,

*then*$\{{x}_{t}\}$

*converges strongly to a fixed point*

*p*

*of*

*T*

*as*$t\to 0$,

*which is the unique solution in*$F(T)$

*to the following variational inequality*:

*Proof*Since $\{{x}_{t}\}$ is bounded and

*E*is reflexive, there exists a subnet $\{{x}_{{t}_{\alpha}}\}$ of $\{{x}_{t}\}$ such that ${x}_{{t}_{\alpha}}\rightharpoonup p$ as ${t}_{\alpha}\to 0$. Put

*S*is a continuous strong pseudo-contraction. From Lemma 1.2, we see that

*S*has a unique fixed point

*x*in

*C*, that is, $x=\frac{1}{2}y+\frac{1}{2}Tx$. This implies that

*H*is a nonexpansive mapping. Indeed, for any $x,y\in C$, we have

On the other hand, from (2.6), we obtain that $g(Hp)=g(p)+\mathrm{\Phi}(\parallel Hp-p\parallel )$. It follows from (2.8) that $\mathrm{\Phi}(\parallel Hp-p\parallel )=0$. Hence, $p\in F(H)=F(T)$.

*p*. From (2.4), we see that

It follows that ${x}_{{t}_{\alpha}}\to p$.

*p*. If there exists another subset $\{{x}_{{t}_{\beta}}\}$ of $\{{x}_{t}\}$ such that $\{{x}_{{t}_{\beta}}\}\to q$, then

*q*is also a fixed point of

*T*. By using (iii) of Lemma 2.1, we have

Adding up the above inequalities, we obtain that $p=q$. This completes the proof. □

**Remark 2.3** In the case that *T* and *f* are non-self mappings, we remark that the results of Lemma 2.1 and Theorem 2.2 still hold under the assumption that both *T* and *f* satisfy the weak inward condition. We give an affirmative answer to the question purposed in Section 1.

**Remark 2.4** It is clear that every contraction is a continuous, bounded and strong pseudo-contraction. The main results in this paper develop the so-called viscosity approximation method which was first introduced by Moudafi [36] from the class of contractions to the class of strong pseudo-contractions.

If $f(x)=u\in C$ for all $x\in C$ in Theorem 2.2, we have the following result.

**Corollary 2.5** *Let* *E* *be a reflexive Banach space which has a weakly continuous duality mapping* ${J}_{\phi}$ *for some gauge* *φ*, *and let* *C* *be a nonempty closed and convex subset of* *E*. *Let* $T:C\to C$ *be a continuous pseudo*-*contraction*. *Fix* $u\in C$ *and* $t\in (0,1)$. *Let* $\{{x}_{t}\}$ *be the unique solution of the equation* ${x}_{t}=tu+(1-t)T{x}_{t}$. *If* $\{{x}_{t}\}$ *is bounded*, *then* $\{{x}_{t}\}$ *converges strongly to a fixed point* *p* *of* *T* *as* $t\to 0$.

**Remark 2.6** Corollary 2.5 improves Theorem X (Theorem 3.1 of Xu [34]). To be more precise, it improves the mapping *T* from a nonexpansive mapping to continuous pseudo-contractions.

**Remark 2.7** In the case that *T* is a non-self mapping, we remark that Corollary 2.5 still holds under the assumption that *T* satisfies the weak inward condition.

## Declarations

### Acknowledgements

The authors are grateful to the reviewers for useful suggestions which improved the contents of the article. The first author was supported by the Natural Science Foundation of Henan Province (122300410420). The second author was supported by the Natural Science Foundation of Zhejiang Province (Y6110270).

## Authors’ Affiliations

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