# Some results on a modified Mann iterative scheme in a reflexive Banach space

- Yan Hao
^{1}Email author

**2013**:227

https://doi.org/10.1186/1687-1812-2013-227

© Hao; licensee Springer. 2013

**Received: **10 April 2013

**Accepted: **12 August 2013

**Published: **28 August 2013

## Abstract

The purpose of this paper is to study Mann iterative schemes. Strong convergence of a modified Mann iterative scheme is obtained in a reflexive Banach space.

## Keywords

*ϕ*-nonexpansive mapping in the intermediate senserelatively nonexpansive mappinggeneralized projection

## 1 Introduction-Preliminaries

Normal Mann iterative scheme is an important iterative scheme to study the class of nonexpansive mappings [1]. However, the normal Mann iterative scheme is only weak convergence for nonexpansive mappings; see [2–4]. In many disciplines, including economics [5] and image recovery [6], problems arise in infinite dimensional spaces. In such problems, strong convergence is often much more desirable than weak convergence, for it translates the physically tangible property. Strong convergence of iterative sequences properties has a direct impact when the process is executed directly in the underlying infinite dimensional space. Recently, with the aid of projections, many authors studied Mann iterative schemes. Theoretically, the advantage of projection methods is that strong convergence of iterative sequences can be guaranteed without any compact assumptions. The purpose of this paper is to study a modified Mann iterative scheme. Strong convergence of the scheme is obtained in a reflexive Banach space.

*E*be a real Banach space, let

*C*be a nonempty subset of

*E*, and let $T:C\to C$ be a mapping. In this paper, we use $F(T)$ to denote the fixed point set of

*T*. Recall that

*T*is said to be asymptotically regular on

*C*iff for any bounded subset

*K*of

*C*,

Recall that *T* is said to be closed iff for any sequence $\{{x}_{n}\}\subset C$ such that ${lim}_{n\to \mathrm{\infty}}{x}_{n}={x}_{0}$ and ${lim}_{n\to \mathrm{\infty}}T{x}_{n}={y}_{0}$, then $T{x}_{0}={y}_{0}$. In this paper, we use → and ⇀ to denote the strong convergence and weak convergence, respectively.

*T*is said to be nonexpansive iff

*T*is said to be quasi-nonexpansive iff $F(T)\ne \mathrm{\varnothing}$, and

*T*is said to be asymptotically nonexpansive iff there exists a sequence $\{{k}_{n}\}\subset [1,\mathrm{\infty})$ with ${k}_{n}\to 1$ as $n\to \mathrm{\infty}$ such that

*T*is said to be asymptotically quasi-nonexpansive iff $F(T)\ne \mathrm{\varnothing}$ and there exists a sequence $\{{k}_{n}\}\subset [1,\mathrm{\infty})$ with ${k}_{n}\to 1$ as $n\to \mathrm{\infty}$ such that

The class of asymptotically nonexpansive mappings was introduced by Goebel and Kirk [7] in 1972. In uniformly convex Banach spaces, they proved that if *C* is nonempty, bounded, closed, and convex, then every asymptotically nonexpansive self-mapping *T* on *C* has a fixed point. Further, the fixed point set of *T* is closed and convex. Since 1972, a host of authors have studied the weak and strong convergence of iterative schemes for such a class of mappings.

*T*is said to be asymptotically nonexpansive in the intermediate sense iff it is continuous and the following inequality holds:

*T*is said to be asymptotically quasi-nonexpansive in the intermediate sense iff $F(T)\ne \mathrm{\varnothing}$ and the following inequality holds:

The class of the mappings that are asymptotically nonexpansive in the intermediate sense was considered by Bruck *et al.* [8] and Kirk [9]. It is worth mentioning that the class of mappings which are asymptotically nonexpansive in the intermediate sense may not be Lipschitz continuous. However, asymptotically nonexpansive mappings are Lipschitz continuous. For the existence of the mapping, we can find the details in [9].

In [10], Nakajo and Takahashi first investigated fixed point problems of nonexpansive mappings based on hybrid projection methods in the framework of Hilbert spaces. Subsequently, many authors investigated fixed point problems of nonlinear mappings based on the methods in the framework of Hilbert spaces. The advantage of the method is that strong convergence of iterative sequences can be guaranteed without any compact assumptions.

*E*be a Banach space with the dual ${E}^{\ast}$. We denote by

*J*the normalized duality mapping from

*E*to ${2}^{{E}^{\ast}}$ defined by

where $\u3008\cdot ,\cdot \u3009$ denotes the generalized duality pairing. A Banach space *E* is said to be strictly convex if $\parallel \frac{x+y}{2}\parallel <1$ for all $x,y\in E$ with $\parallel x\parallel =\parallel y\parallel =1$ and $x\ne y$. It is said to be uniformly convex if ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-{y}_{n}\parallel =0$ for any two sequences $\{{x}_{n}\}$ and $\{{y}_{n}\}$ in *E* such that $\parallel {x}_{n}\parallel =\parallel {y}_{n}\parallel =1$ and ${lim}_{n\to \mathrm{\infty}}\parallel \frac{{x}_{n}+{y}_{n}}{2}\parallel =1$. Let ${U}_{E}=\{x\in E:\parallel x\parallel =1\}$ be the unit sphere of *E*. Then the Banach space *E* is said to be smooth provided ${lim}_{t\to 0}\frac{\parallel x+ty\parallel -\parallel x\parallel}{t}$ exists for each $x,y\in {U}_{E}$. It is also said to be uniformly smooth if the above limit is attained uniformly for $x,y\in {U}_{E}$. It is well known that if *E* is uniformly smooth, then *J* is uniformly norm-to-norm continuous on each bounded subset of *E*. It is also well known that *E* is uniformly smooth if and only if ${E}^{\ast}$ is uniformly convex.

Recall that a Banach space *E* enjoys the Kadec-Klee property if for any sequence $\{{x}_{n}\}\subset E$, and $x\in E$ with ${x}_{n}\rightharpoonup x$, and $\parallel {x}_{n}\parallel \to \parallel x\parallel $, then $\parallel {x}_{n}-x\parallel \to 0$ as $n\to \mathrm{\infty}$. For more details on the Kadec-Klee property, the readers can refer to [11] and the references therein. It is well known that if *E* is a uniformly convex Banach space, then *E* enjoys the Kadec-Klee property.

As we all know, if *C* is a nonempty closed convex subset of a Hilbert space *H* and ${P}_{C}:H\to C$ is the metric projection of *H* onto *C*, then ${P}_{C}$ is nonexpansive. This fact actually characterizes Hilbert spaces and, consequently, it is not available in more general Banach spaces. In this connection, Alber [12] recently introduced a generalized projection operator ${\mathrm{\Pi}}_{C}$ in a Banach space *E* which is an analogue of the metric projection in Hilbert spaces.

*E*is a smooth Banach space. Consider the functional defined by

*H*, the equality is reduced to $\varphi (x,y)={\parallel x-y\parallel}^{2}$, $x,y\in H$. The generalized projection ${\mathrm{\Pi}}_{C}:E\to C$ is a map that assigns to an arbitrary point $x\in E$ the minimum point of the functional $\varphi (x,y)$, that is, ${\mathrm{\Pi}}_{C}x=\overline{x}$, where $\overline{x}$ is the solution to the minimization problem

*J*; see, for example, [11–13]. In Hilbert spaces, ${\mathrm{\Pi}}_{C}={P}_{C}$. It is obvious from the definition of a function

*ϕ*that

**Remark 1.1** If *E* is a reflexive, strictly convex, and smooth Banach space, then for $x,y\in E$, $\varphi (x,y)=0$ if and only if $x=y$. It is sufficient to show that if $\varphi (x,y)=0$, then $x=y$. From (2.1), we have $\parallel x\parallel =\parallel y\parallel $. This implies that $\u3008x,Jy\u3009={\parallel x\parallel}^{2}={\parallel Jy\parallel}^{2}$. From the definition of *J*, we have $Jx=Jy$. Therefore, we have $x=y$; see [11] for more details.

Let *C* be a nonempty closed convex subset of *E*, and let *T* be a mapping from *C* into itself. A point *p* in *C* is said to be an asymptotic fixed point of *T* [14] if *C* contains a sequence $\{{x}_{n}\}$ which converges weakly to *p* such that ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$. The set of asymptotic fixed points of *T* will be denoted by $\tilde{F}(T)$. A mapping *T* from *C* into itself is said to be relatively nonexpansive [15, 16] if $\tilde{F}(T)=F(T)\ne \mathrm{\varnothing}$ and $\varphi (p,Tx)\le \varphi (p,x)$ for all $x\in C$ and $p\in F(T)$. The mapping *T* is said to be relatively asymptotically nonexpansive [17, 18] if $\tilde{F}(T)=F(T)\ne \mathrm{\varnothing}$ and there exists a sequence $\{{k}_{n}\}\subset [1,\mathrm{\infty})$ with ${k}_{n}\to 1$ as $n\to \mathrm{\infty}$ such that $\varphi (p,Tx)\le {k}_{n}\varphi (p,x)$ for all $x\in C$, $p\in F(T)$ and $n\ge 1$. The asymptotic behavior of relatively nonexpansive mappings was studied in [14–16].

The mapping *T* is said to be quasi-*ϕ*-nonexpansive [19] if $F(T)\ne \mathrm{\varnothing}$ and $\varphi (p,Tx)\le \varphi (p,x)$ for all $x\in C$ and $p\in F(T)$. *T* is said to be asymptotically quasi-*ϕ*-nonexpansive [20–22] if $F(T)\ne \mathrm{\varnothing}$ and there exists a sequence $\{{k}_{n}\}\subset [0,\mathrm{\infty})$ with ${k}_{n}\to 1$ as $n\to \mathrm{\infty}$ such that $\varphi (p,Tx)\le {k}_{n}\varphi (p,x)$ for all $x\in C$, $p\in F(T)$ and $n\ge 1$.

**Remark 1.2** The class of asymptotically quasi-*ϕ*-nonexpansive mappings is more general than the class of relatively asymptotically nonexpansive mappings, which requires the restriction: $F(T)=\tilde{F}(T)$.

*T*is said to be asymptotically quasi-

*ϕ*-nonexpansive in the intermediate sense [23] if $F(T)\ne \mathrm{\varnothing}$ and

**Remark 1.3** The class of asymptotically quasi-*ϕ*-nonexpansive mappings in the intermediate sense is a generalization of the class of asymptotically quasi-nonexpansive mappings in the intermediate sense in the framework of Banach spaces.

*T*is an asymptotically quasi-

*ϕ*-nonexpansive mapping in the intermediate sense with the fixed point set $\{0\}$. We also have the following

where ${\xi}_{n}=\frac{1}{{2}^{2n}}+\frac{1}{{2}^{n-1}}$. Hence, we have $\varphi ({T}^{n}x,{T}^{n}y)\le \varphi (x,y)+{\xi}_{n}$, $\mathrm{\forall}x,y\in [0,1]$.

Recently, Matsushita and Takahashi [24] first investigated fixed point problems of relatively nonexpansive mappings based on hybrid projection methods. A strong convergence theorem was established in a uniformly convex and uniformly smooth Banach space. To be more precise, they proved the following result.

**Theorem MT**

*Let*

*E*

*be a uniformly convex and uniformly smooth Banach space*,

*let*

*C*

*be a nonempty closed convex subset of*

*E*,

*let*

*T*

*be a relatively nonexpansive mapping from*

*C*

*into itself*,

*and let*$\{{\alpha}_{n}\}$

*be a sequence of real numbers such that*$0\le {\alpha}_{n}<1$

*and*${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{\alpha}_{n}<1$.

*Suppose that*$\{{x}_{n}\}$

*is given by*

*where* *J* *is the duality mapping on* *E*. *If* $F(T)$ *is nonempty*, *then* $\{{x}_{n}\}$ *converges strongly to* ${P}_{F(T)}x$, *where* ${P}_{F(T)}$ *is the generalized projection from* *C* *onto* $F(T)$.

Recently, Su and Qin [25] introduced a monotone projection method for computing fixed points of nonexpansive mappings. A strong convergence theorem was established in the framework of Hilbert spaces; for more details, see [25]. Takahashi *et al.* [26] further introduced the shrinking projection method for nonexpansive mappings; for more details, see [26]. Subsequently, a lot results were obtained on the two methods in Hilbert spaces or Banach spaces; see [27–38] and the references therein.

Motivated by the above results, some fixed point theorems of asymptotically quasi-*ϕ*-nonexpansive mappings based on hybrid projection methods were established in the framework of Banach spaces.

In this paper, motivated by Matsushita and Takahashi [24], Su and Qin [25] and Takahashi *et al.* [26], we investigate a fixed point problem of an asymptotically quasi-*ϕ*-nonexpansive mapping in the intermediate sense. A strong convergence theorem is established in a reflexive, strictly convex, and smooth Banach space such that both *E* and ${E}^{\ast}$ have the Kadec-Klee property. The results improve and extend the corresponding results in the literature.

For our main results, we need the following lemmas.

**Lemma 1.4** [12]

*Let*

*E*

*be a reflexive*,

*strictly convex*,

*and smooth Banach space*,

*let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of*

*E*,

*and let*$x\in E$.

*Then*

**Lemma 1.5** [12]

*Let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of a smooth Banach space*

*E*,

*and let*$x\in E$.

*Then*${x}_{0}={\mathrm{\Pi}}_{C}x$

*if and only if*

## 2 Main results

**Theorem 2.1**

*Let*

*E*

*be a reflexive*,

*strictly convex*,

*and smooth Banach space such that both*

*E*

*and*${E}^{\ast}$

*have the Kadec*-

*Klee property*.

*Let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of E*.

*Let*$T:C\to C$

*be an asymptotically quasi*-

*ϕ*-

*nonexpansive mapping in the intermediate sense*.

*Assume that*

*T*

*is asymptotically regular on*

*C*

*and closed*,

*and*$F(T)$

*is nonempty and bounded*.

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

*be a sequence generated in the following manner*:

*where*

*If the sequence* $\{{\alpha}_{n}\}$ *satisfies the restriction* ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{\alpha}_{n}<1$, *then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${\mathrm{\Pi}}_{F(T)}{x}_{1}$, *where* ${\mathrm{\Pi}}_{F(T)}$ *is the generalized projection from* *C* *onto* $F(T)$.

*Proof* The proof is split into six steps.

Step 1. Show that $F(T)$ is closed and convex so that ${\mathrm{\Pi}}_{F(T)}x$ is well defined for any $x\in C$.

*T*that

*t*and $(1-t)$ on both sides of (2.1) and (2.2), respectively, yields that ${lim}_{n\to \mathrm{\infty}}\varphi (p,{T}^{n}p)=0$. In light of (1.1), we arrive at

*E*, we find that there exists an element $q\in E$ such that $Jq={q}^{\ast}$. It follows that

This implies that $p=q$, that is, $Jp={q}^{\ast}$. It follows that $J({T}^{n}p)\rightharpoonup Jp\in {E}^{\ast}$. In view of the Kadec-Klee property of ${E}^{\ast}$, we obtain from (2.4) that ${lim}_{n\to \mathrm{\infty}}\parallel J({T}^{n}p)-Jp\parallel =0$. Since ${J}^{-1}:{E}^{\ast}\to E$ is demicontinuous, we see that ${T}^{n}p\rightharpoonup p$. By virtue of the Kadec-Klee property of *E*, we see from (2.3) that ${T}^{n}p\to p$ as $n\to \mathrm{\infty}$. Hence, $T{T}^{n}p={T}^{n+1}p\to p$ as $n\to \mathrm{\infty}$. In view of the closedness of *T*, we can obtain that $p\in F(T)$. This shows that $F(T)$ is convex. Since *T* is closed, we can easily conclude that $F(T)$ is also closed. This completes the proof that $F(T)$ is convex and closed.

Step 2. Show that ${C}_{n}$ is closed and convex.

It is obvious that ${C}_{1}=C$ is closed and convex. Suppose that ${C}_{h}$ is closed and convex for some $h\in \mathbb{N}$. We now show that ${C}_{h+1}$ is also closed and convex.

*t*and $(1-t)$ on both sides of (2.7) and (2.8), respectively, yields that

This implies that ${C}_{h+1}$ is closed and convex. Then, for each $n\ge 1$, ${C}_{n}$ is closed and convex. This shows that ${\mathrm{\Pi}}_{{C}_{n+1}}{x}_{1}$ is well defined.

Step 3. Show that $F(T)\subset {C}_{n}$.

This shows that $w\in {C}_{h+1}$. This implies that $\mathcal{F}\subset {C}_{n}$.

Step 4. Show that ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-\overline{x}\parallel =0$, where $\overline{x}$ is some point in *C*.

which implies that ${lim}_{n\to \mathrm{\infty}}\varphi ({x}_{n},{x}_{1})=\varphi (\overline{x},{x}_{1})$. Hence, we have ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}\parallel =\parallel \overline{x}\parallel $. In view of the Kadec-Klee property of *E*, we find that ${x}_{n}\to \overline{x}$ as $n\to \mathrm{\infty}$. This completes Step 4.

Step 5. Show that $\overline{x}\in F(T)$.

*E*and ${E}^{\ast}$ are reflexive. We may assume that $J{y}_{n}\rightharpoonup {y}^{\ast}\in {E}^{\ast}$. In view of the reflexivity of

*E*, we see that $J(E)={E}^{\ast}$. This shows that there exists an element $y\in E$ such that $Jy={y}^{\ast}$. It follows that

*E*has the Kadec-Klee property, we find that

*T*, we find from (2.16) that

that is, $T{T}^{n}{x}_{n}-\overline{x}\to 0$ as $n\to \mathrm{\infty}$. It follows from the closedness of *T* that $T\overline{x}=\overline{x}$. This completes Step 5.

Step 6. Show that $\overline{x}={\mathrm{\Pi}}_{F(T)}{x}_{1}$.

It follows from Lemma 1.5 that $\overline{x}={\mathrm{\Pi}}_{F(T)}{x}_{1}$. This completes the proof of Theorem 2.1. □

**Remark 2.2** The sets ${C}_{n+1}$ become increasingly complicated, which may render the algorithm unimplementable. One may use an inner-loop for calculating an approximation of ${\mathrm{\Pi}}_{{C}_{n+1}}$ at each iterative step. The advantage of the algorithm is that strong convergence of iterative sequences can be guaranteed in a reflexive, strictly convex, and smooth Banach space such that both *E* and ${E}^{\ast}$ have the Kadec-Klee property without any compact assumptions.

If *T* is quasi-*ϕ*-nonexpansive, then Theorem 2.1 is reduced to the following.

**Corollary 2.3**

*Let*

*E*

*be a reflexive*,

*strictly convex*,

*and smooth Banach space such that both*

*E*

*and*${E}^{\ast}$

*have the Kadec*-

*Klee property*.

*Let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of*

*E*.

*Let*$T:C\to C$

*be a quasi*-

*ϕ*-

*nonexpansive mapping with a nonempty fixed point set*.

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

*be a sequence generated in the following manner*:

*If the sequence* $\{{\alpha}_{n}\}$ *satisfies the restriction* ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{\alpha}_{n}<1$, *then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${\mathrm{\Pi}}_{F(T)}{x}_{1}$, *where* ${\mathrm{\Pi}}_{F(T)}$ *is the generalized projection from* *C* *onto* $F(T)$.

**Remark 2.4** Corollary 2.3 mainly improves the corresponding results [24] in the following aspects: (1) from the relatively nonexpansive mapping to the quasi-*ϕ*-nonexpansive mapping; (2) from a uniformly convex and uniformly smooth Banach space to a reflexive, strictly convex, and smooth Banach space such that both *E* and ${E}^{\ast}$ have the Kadec-Klee property. The algorithm is also different from the one in [24].

If *E* is a Hilbert space, then we have the following result.

**Corollary 2.5**

*Let*

*E*

*be a Hilbert space*.

*Let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of E*.

*Let*$T:C\to C$

*be an asymptotically quasi*-

*nonexpansive mapping in the intermediate sense*.

*Assume that*

*T*

*is asymptotically regular on*

*C*

*and closed and*$F(T)$

*is nonempty and bounded*.

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

*be a sequence generated in the following manner*:

*where*

*If the sequence* $\{{\alpha}_{n}\}$ *satisfies the restriction* ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{\alpha}_{n}<1$, *then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${P}_{F(T)}{x}_{1}$, *where* ${P}_{F(T)}$ *is the metric projection from* *C* *onto* $F(T)$.

*Proof* In Hilbert spaces, we find that *J* is the identity and $\varphi (x,y)={\parallel x-y\parallel}^{2}$. We can immediately derive from Theorem 2.1 the desired conclusion. □

**Remark 2.6** Corollary 2.5 can be viewed as an improvement of the corresponding result in Su and Qin [38]. The mapping is extended from asymptotically nonexpansive mappings to asymptotically quasi-nonexpansive mappings in the intermediate sense.

If *T* is quasi-nonexpansive, then we have the following result.

**Corollary 2.7**

*Let*

*E*

*be a Hilbert space*.

*Let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of E*.

*Let*$T:C\to C$

*be a quasi*-

*nonexpansive mapping with a nonempty fixed point set*.

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

*be a sequence generated in the following manner*:

*If the sequence* $\{{\alpha}_{n}\}$ *satisfies the restriction* ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{\alpha}_{n}<1$, *then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${P}_{F(T)}{x}_{1}$, *where* ${P}_{F(T)}$ *is the metric projection from* *C* *onto* $F(T)$.

*Proof* In Hilbert spaces, we find that *J* is the identity and $\varphi (x,y)={\parallel x-y\parallel}^{2}$. We can immediately derive from Theorem 2.1 the desired conclusion. □

**Remark 2.8** Corollary 2.7 can be viewed as an improvement of the corresponding result in Nakajo and Takahashi [10]. The mapping has been extended from a nonexpansive mapping to a quasi-nonexpansive mapping. The sets ${Q}_{n}$ have also been relaxed.

## Declarations

### Acknowledgements

The study was supported by the Natural Science Foundation of Zhejiang Province (Y6110270). The author is grateful to the editor and the three anonymous reviewers’ suggestions which improved the contents of the article.

## Authors’ Affiliations

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