# Convergence theorems for modified generalized *f*-projections and generalized nonexpansive mappings

- Qingqing Cheng
^{1}, - Yongfu Su
^{1}Email author and - Jingling Zhang
^{1}

**2014**:305

https://doi.org/10.1186/1029-242X-2014-305

© Cheng et al.; licensee Springer. 2014

**Received: **18 March 2014

**Accepted: **14 July 2014

**Published: **19 August 2014

## Abstract

The purpose of this paper is to study a sequence of modified generalized *f*-projections in a reflexive, smooth, and strictly convex Banach space and show that Mosco convergence of their ranges implies their pointwise convergence to the generalized *f*-projection onto the limit set. Furthermore, we prove a strong convergence theorem for a countable family of *α*-nonexpansive mappings in a uniformly convex and smooth Banach space using the properties of a modified generalized *f*-projection operator. Our main results generalize the results of Ziming Wang, Yongfu Su, and Jinlong Kang and enrich the research contents of *α*-nonexpansive mappings.

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

## Keywords

*α*-nonexpansive mappingsmonotone hybrid algorithmmodified generalized

*f*-projection operatorMosco convergencecommon fixed point

## 1 Introduction

*E*be a real Banach space and

*C*be a nonempty closed convex subset of

*E*. A mapping $T:C\to C$ is said to be nonexpansive if

Lots of iterative schemes for nonexpansive mappings have been introduced (see [1–3]); furthermore, many strong convergence theorems for nonexpansive mappings have been proved. On the other hand, there are many nonlinear mappings which are more general than the nonexpansive mapping. Compared to the existing problem of a fixed point of those mappings, the iterative methods for finding a fixed point are also very useful in studying the fixed point theory and the theory of equations in other fields.

In 2007, Gobel and Pineda [4] introduced and studied a new mapping, called *α*-nonexpansive mapping. The mapping is more general than the nonexpansive mapping.

**Definition 1.1**For a given multi-index, $\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{n})$ satisfies ${\alpha}_{i}\ge 0$, $i=1,2,\dots ,n$ and ${\sum}_{i=1}^{n}{\alpha}_{i}=1$. A mapping $T:C\to C$ is said to be

*α*-nonexpansive if

In order to show that the class of *α*-nonexpansive mappings is more general than the one of nonexpansive mappings, we give an example [4].

**Example 1.2**Let $E={R}^{1}$, and

Then *T* is not nonexpansive but *α*-nonexpansive.

*Proof*Obviously,

*T*is not nonexpansive. Taking $x=\frac{1}{2}$, $y=0$, by the definition of

*Tx*, we have

where $\alpha =({\alpha}_{1},{\alpha}_{2})=(0,1)$. Then *T* is an *α*-nonexpansive mapping but not a nonexpansive one. □

If *T* is a nonexpansive self-mapping, we can imply that *T* must be an *α*-nonexpansive one, where $\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{n})=(\frac{1}{n},\dots ,\frac{1}{n})$.

*T*satisfies the Lipschitz condition

*α*-nonexpansive mapping

*T*, $\alpha =({\alpha}_{1},{\alpha}_{2},{\alpha}_{3},\dots {\alpha}_{n})$, it is obvious that the mapping

is nonexpansive. However, the nonexpansiveness of ${T}_{\alpha}$ is much weaker than (1.2), for instance, it does not entail the continuity of *T* (see [4]).

In 2010, Klin-eam and Suantai [5] introduced the relation of fixed point sets between an *α*-nonexpansive operator and a ${T}_{\alpha}$ operator. They gave the following theorem.

**Theorem 1.3** (see Theorem 3.1 of Klin-eam and Suantai [5])

*Let* *C* *be a closed convex subset of a Banach space* *E* *and for all* $n\in N$, *let* $\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{n})$ *such that* ${\alpha}_{i}\ge 0$, $i=1,2,\dots ,n$, ${\alpha}_{1}>0$, *and* ${\sum}_{i=1}^{n}{\alpha}_{i}=1$. *Let* *T* *be an* *α*-*nonexpansive mapping from* *C* *into itself*. *If* ${\alpha}_{1}>\frac{1}{\sqrt[n-1]{2}}$, *then* $F(T)=F({T}_{\alpha})$, *where* $F(T)$ *is the fixed point set of* *T*.

At the same time, they have succeeded in proving the demiclosedness principle for the *α*-nonexpansive mappings.

**Theorem 1.4** (see Theorem 3.4 of Klin-eam and Suantai [5])

*Let* *C* *be a closed convex subset of a Banach space* *E* *and for all* $n\in N$, *let* $\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{n})$ *such that* ${\alpha}_{i}\ge 0$, $i=1,2,\dots ,n$, ${\alpha}_{1}>0$, *and* ${\sum}_{i=1}^{n}{\alpha}_{i}=1$. *Let* *T* *be an* *α*-*nonexpansive mapping from* *C* *into itself*. *If* ${\alpha}_{1}>\frac{1}{\sqrt[n-1]{2}}$, *if* $\{{x}_{n}\}\subset C$ *converges weakly to* *x* *and* $\{{x}_{n}-T{x}_{n}\}$ *converges strongly to* 0 *as* $n\to \mathrm{\infty}$, *then* $x\in F(T)$.

*et al.*[6] proposed the following hybrid algorithm for an

*α*-nonexpansive mapping in a Banach space:

*C*is a nonempty closed convex subset of a Hilbert space

*H*and recall that the (nearest point) projection ${P}_{C}$ from

*H*onto

*C*assigns to each $x\in H$, and the unique point ${P}_{C}x\in C$ satisfies the property $\parallel x-{P}_{C}x\parallel ={min}_{y\in C}\parallel x-y\parallel $, it is well known that ${P}_{C}$ is nonexpansive. This fact actually characterizes Hilbert spaces and, consequently, it is not available in more general Banach spaces. We consider the functional defined by

*J*is the normalized duality mapping and the Banach space is smooth. In this connection, Alber [7] introduced a generalized projection ${\mathrm{\Pi}}_{C}$ from

*E*to

*C*as follows:

*ϕ*that

*E*is a Hilbert space, then $\varphi (y,x)={\parallel y-x\parallel}^{2}$ and ${\mathrm{\Pi}}_{C}$ becomes the metric projection of

*E*onto

*C*. 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 $\varphi (y,x)$, that is, ${\mathrm{\Pi}}_{C}x=\overline{x}$, where $\overline{x}$ is the solution to the minimization problem

The existence and uniqueness of the operator ${\mathrm{\Pi}}_{C}$ follow from the properties of the functional $\varphi (y,x)$ and strict monotonicity of the normalized duality mapping *J* [8]. It is well known that the metric projection operator plays an important role in nonlinear functional analysis, optimization theory, fixed point theory, nonlinear programming, game theory, variational inequality, and complementarity problems, *etc.* [8, 9]. In 1994, Alber [7] introduced and studied the generalized projections from Hilbert spaces to uniformly convex and uniformly smooth Banach spaces. Moreover, Alber [8] presented some applications of the generalized projections to approximately solve variational inequalities and von Neumann intersection problem in Banach spaces. In 2005, Li [9] extended the generalized projection operator from uniformly convex and uniformly smooth Banach spaces to reflexive Banach spaces and studied some properties of the generalized projection operator with applications to solve the variational inequality in Banach spaces. Later, Wu and Huang [10] introduced a new generalized *f*-projection operator in Banach spaces. They extended the definition of generalized projection operators introduced by Abler [7] and proved some properties of the generalized *f*-projection operator. In 2009, Fan *et al.* [11] presented some basic results for the generalized *f*-projection operator and discussed the existence of solutions and approximation of the solutions for generalized variational inequalities in noncompact subsets of Banach spaces.

The purpose of this paper is to study a sequence of modified generalized *f*-projections in a reflexive, smooth, and strictly convex Banach space and show that Mosco convergence of their ranges implies their pointwise convergence to the generalized *f*-projection onto the limit set. Furthermore, we prove strong convergence theorem for a countable family of *α*-nonexpansive mappings in a uniformly convex and smooth Banach space using the properties of a modified generalized *f*-projection operator. Our main results generalize the results of Wang *et al.* [6] and enrich the research contents of *α*-nonexpansive mappings.

## 2 Preliminaries

*E*is said to be strictly convex if $\frac{\parallel x+y\parallel}{2}<1$ for $x,y\in E$ with $\parallel x\parallel =\parallel y\parallel =1$ and $x\ne y$. It is said to be uniformly convex if for each $\u03f5>0$ there is $\delta >0$ such that for $x,y\in E$ with $\parallel x\parallel ,\parallel y\parallel \le 1$ and $\parallel x-y\parallel \ge \u03f5$, $\parallel x+y\parallel \le 2(1-\delta )$ holds. The space

*E*is said to be smooth if the limit

exists for all $x,y\in S(E)=\{x\in E:\parallel x\parallel =1\}$. And *E* is said to be uniformly smooth if the limit (2.1) exists uniformly for all $x,y\in S(E)$.

**Remark 2.1**The following basic properties of a Banach space

*E*can be found in Cioranescu [12]:

- (i)
if

*E*is uniformly convex, then*E*is reflexive and strictly convex; - (ii)
a Banach space

*E*is uniformly smooth if and only if ${E}^{\ast}$ is uniformly convex; - (iii)
each uniformly convex Banach space

*E*has the Kadec-Klee property,*i.e.*, for any sequence $\{{x}_{n}\}\subset E$, if ${x}_{n}\rightharpoonup x\in E$ and $\parallel {x}_{n}\parallel \to \parallel x\parallel $, then ${x}_{n}\to x$.

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

*J*the normalized duality mapping from

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

*J*can be found in Takahashi [13] or Vainberg [14]. We list some properties below for easy reference:

- (i)
*J*is a monotone and bounded operator in arbitrary Banach spaces; - (ii)
*J*is a strictly monotone operator in strictly convex Banach spaces; - (iii)
*J*is a continuous operator in smooth Banach spaces; - (iv)
*J*is a uniformly continuous operator on each bounded set in uniformly smooth Banach spaces; - (v)
*J*is a bijection in smooth, reflexive, and strictly convex Banach spaces; - (vi)
*J*is the identity operator in Hilbert spaces.

*f*-projector operator, together with its properties. Let $G:C\times {E}^{\ast}\to R\cup \{+\mathrm{\infty}\}$ be a functional defined as follows:

*ρ*is a positive number and $f:C\to R\cup \{+\mathrm{\infty}\}$ is proper, convex, and lower semi-continuous. From the definitions of

*G*and

*f*, it is easy to see the following properties:

- (i)
$G(\xi ,\phi )$ is convex and continuous with respect to

*φ*when*ξ*is fixed; - (ii)
$G(\xi ,\phi )$ is convex and lower semi-continuous with respect to

*ξ*when*φ*is fixed.

**Definition 2.2** ([10])

*E*be a real Banach space with its dual ${E}^{\ast}$. Let

*C*be a nonempty, closed, and convex subset of

*E*. We say that ${\mathrm{\Pi}}_{C}^{f}:{E}^{\ast}\to {2}^{C}$ is a generalized

*f*-projection operator if

For the generalized *f*-projection operator, Wu and Huang [10] proved the following basic properties.

**Lemma 2.3** ([10])

*Let*

*E*

*be a real reflexive Banach space with its dual*${E}^{\ast}$,

*and let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of*

*E*.

*Then the following statements hold*:

- (i)
${\mathrm{\Pi}}_{C}^{f}\phi $

*is a nonempty closed convex subset of**C**for all*$\phi \in {E}^{\ast}$. - (ii)
*If**E**is smooth*,*then for all*$\phi \in {E}^{\ast}$, $x\in {\mathrm{\Pi}}_{C}^{f}\phi $*if and only if*$\u3008x-y,\phi -Jx\u3009+\rho f(y)-\rho f(x)\ge 0,\phantom{\rule{1em}{0ex}}\mathrm{\forall}y\in C.$ - (iii)
*If**E**is strictly convex and*$f:C\to R\cup \{+\mathrm{\infty}\}$*is positive homogeneous*(*i*.*e*., $f(tx)=tf(x)$*for all*$t>0$*such that*$tx\in C$,*where*$x\in C$),*then*${\mathrm{\Pi}}_{C}^{f}$*is a single*-*valued mapping*.

Fan *et al.* [11] showed that the condition *f* is positive homogeneous, which appeared in Lemma 2.3, can be removed.

**Lemma 2.4** ([11])

*Let* *E* *be a real reflexive Banach space with its dual* ${E}^{\ast}$, *and let* *C* *be a nonempty*, *closed*, *and convex subset of* *E*. *Then if* *E* *is strictly convex*, *then* ${\mathrm{\Pi}}_{C}^{f}$ *is a single*-*valued mapping*.

*J*is a single-valued mapping when

*E*is a smooth Banach space. There exists a unique element $\phi \in {E}^{\ast}$ such that $\phi =Jx$ for each $x\in E$. This substitution in (2.2) gives

Now, we consider the second generalized *f*-projection operator in a Banach space.

**Definition 2.5**Let

*E*be a real Banach space and

*C*be a nonempty, closed, and convex subset of

*E*. We say that ${\mathrm{\Pi}}_{C}^{f}:E\to {2}^{C}$ is a generalized

*f*-projection operator if

We know that the following lemmas hold for the operator ${\mathrm{\Pi}}_{C}^{f}$.

**Lemma 2.6** ([15])

*Let*

*C*

*be a nonempty*,

*closed*,

*and convex subset of a smooth and reflexive Banach space*

*E*.

*Then the following statements hold*:

- (i)
${\mathrm{\Pi}}_{C}^{f}x$

*is a nonempty closed and convex subset of**C**for all*$x\in E$. - (ii)
*For all*$x\in E$, $\stackrel{\u02c6}{x}\in {\mathrm{\Pi}}_{C}^{f}x$*if and only if*$\u3008\stackrel{\u02c6}{x}-y,Jx-J\stackrel{\u02c6}{x}\u3009+\rho f(y)-\rho f(x)\ge 0,\phantom{\rule{1em}{0ex}}\mathrm{\forall}y\in C.$ - (iii)
*If**E**is strictly convex*,*then*${\mathrm{\Pi}}_{C}^{f}$*is a single*-*valued mapping*.

*f*-projection operator. Let $G:C\times {E}^{\ast}\to R$ be a functional defined as follows:

*ρ*is a positive number and $f:C\to R$ is convex and weakly continuous. From the definitions of

*G*and

*f*, it is easy to see the following properties:

- (i)
$G(\xi ,\phi )$ is convex and continuous with respect to

*φ*when*ξ*is fixed; - (ii)
$G(\xi ,\phi )$ is convex and weakly lower semi-continuous with respect to

*ξ*when*φ*is fixed.

Obviously, the other definitions and lemmas hold respectively.

Next, we give the following example [16] which shows that metric projection, generalized projection and generalized *f*-projection are different.

**Example 2.7**Let $X={R}^{3}$ be provided with the norm

This is a smooth strictly convex Banach space and $C=\{x\in {R}^{3}|{x}_{2}=0,{x}_{3}=0\}$ is a closed and convex subset of *X*. It is a simple computation; we get ${P}_{C}(1,1,1)=(1,0,0)$, ${\mathrm{\Pi}}_{C}(1,1,1)=(2,0,0)$.

*f*is convex and weakly continuous. Simple computations show that

*E*be a Banach space, and let ${C}_{1},{C}_{2},{C}_{3},\dots $ be a sequence of weakly closed subsets of

*E*. We denote by $s-L{i}_{n}{C}_{n}$ the set of limit points of $\{{C}_{n}\}$, that is, $x\in s-L{i}_{n}{C}_{n}$ if and only if there exists $\{{x}_{n}\}\subset E$ such that $\{{x}_{n}\}$ converges strongly to

*x*and that ${x}_{n}\in {C}_{n}$ for all $n\in N$. Similarly, we denote by $w-L{s}_{n}{C}_{n}$ the set of cluster points of $\{{C}_{n}\}$, $y\in w-L{s}_{n}{C}_{n}$ if and only if there exists $\{{y}_{{n}_{i}}\}$ such that $\{{y}_{{n}_{i}}\}$ converges weakly to

*y*and that $\{{y}_{{n}_{i}}\}\in {C}_{{n}_{i}}$ for all $i\in N$. Using these definitions, we define the Mosco convergence [2] of ${C}_{{n}_{i}}$. If ${C}_{0}$ satisfies

Notice that the inclusion $s-L{i}_{n}{C}_{n}\subset w-L{s}_{n}{C}_{n}$ is always true. Therefore, in order to show the existence of $M-{lim}_{n\to \mathrm{\infty}}{C}_{n}$, it is sufficient to prove $w-L{s}_{n}{C}_{n}\subset s-L{i}_{n}{C}_{n}$. For more details, see [17].

## 3 Main results

### 3.1 Generalized Mosco convergence theorems

**Theorem 3.1** *Let* *E* *be a smooth*, *reflexive*, *and strictly convex Banach space and* *C* *be a nonempty closed convex subset of E*. *Let* ${C}_{1},{C}_{2},{C}_{3},\dots $ *be nonempty closed convex subsets of* *C*, $f:E\to R$ *be a convex and weakly continuous mapping with* $C\subset int(D(f))$. *If* ${C}_{0}=M-{lim}_{n\to \mathrm{\infty}}{C}_{n}$ *exists and is nonempty*, *then* ${C}_{0}$ *is a closed convex subset of* *C* *and*, *for each* $x\in C$, $\{{\mathrm{\Pi}}_{{C}_{n}}^{f}x\}$ *converges weakly to* ${\mathrm{\Pi}}_{{C}_{0}}^{f}x$.

*Proof*It is easy to prove that ${C}_{0}$ is closed and convex if ${C}_{n}$ is a closed convex subset of

*C*for each $n\in N$. Fix $x\in C$. For the sake of simplicity, we write ${x}_{n}$ instead of ${\mathrm{\Pi}}_{{C}_{n}}^{f}x$ for $n\in N$. Since ${C}_{0}=M-{lim}_{n\to \mathrm{\infty}}{C}_{n}$, we have that for each $y\in {C}_{0}$, there exists $\{{y}_{n}\}\subset E$ such that ${y}_{n}\to y$ as $n\to \mathrm{\infty}$ and that ${y}_{n}\in {C}_{n}$ for each $n\in N$. From Lemma 2.6, we have

for a sufficiently large number $i\in N$. As $i\to \mathrm{\infty}$, we obtain $+\mathrm{\infty}\le \parallel x-{y}_{{n}_{i}}\parallel <+\mathrm{\infty}$. This is a contradiction. Hence we have that $\{{x}_{n}\}$ is bounded.

Since $\{{x}_{n}\}$ is bounded, there exists a subsequence, again denoted by $\{{x}_{n}\}$, such that it converges weakly to ${x}_{0}\in C$. From the definition of ${C}_{0}$, we get ${x}_{0}\in {C}_{0}$.

*f*, we have

Hence we get ${\mathrm{\Pi}}_{{C}_{0}}^{f}x={x}_{0}$.

According to our consideration above, each sequence $\{{x}_{n}\}$ has, in turn, a subsequence which converges weakly to the unique point ${\mathrm{\Pi}}_{{C}_{0}}^{f}x$. Therefore, the sequence $\{{x}_{n}\}$ converges weakly to ${\mathrm{\Pi}}_{{C}_{0}}^{f}x$. This completes the proof. □

A Banach space *E* is said to have the Kadec-Klee property if a sequence $\{{x}_{n}\}$ of *E* satisfying that ${x}_{n}\rightharpoonup {x}_{0}$ and $\parallel {x}_{n}\parallel \to \parallel {x}_{0}\parallel $ converges strongly to ${x}_{0}$. It is known that ${E}^{\ast}$ has a Fréchet differentiable norm if and only if *E* is reflexive, strictly convex, and has the Kadec-Klee property; see, for example, [10].

**Theorem 3.2** *Let* *E* *be a smooth Banach space such that* ${E}^{\ast}$ *has a Fréchet differentiable norm*. *Let* *C* *be a nonempty closed convex subset of* *E*. *Let* ${C}_{1},{C}_{2},{C}_{3},\dots $ *be nonempty closed convex subsets of* *C*, $f:E\to R$ *be a convex and weakly continuous mapping with* $C\subset int(D(f))$. *If* ${C}_{0}=M-{lim}_{n\to \mathrm{\infty}}{C}_{n}$ *exists and is nonempty*, *then* ${C}_{0}$ *is a closed convex subset of* *C* *and*, *for each* $x\in C$, $\{{\mathrm{\Pi}}_{{C}_{n}}^{f}x\}$ *converges strongly to* ${\mathrm{\Pi}}_{{C}_{0}}^{f}x$.

*Proof*Fix $x\in C$ arbitrarily. We write ${x}_{n}={\mathrm{\Pi}}_{{C}_{n}}^{f}x$ and ${x}_{0}={\mathrm{\Pi}}_{{C}_{0}}^{f}x$. By Theorem 3.1, we obtain ${x}_{n}\rightharpoonup {x}_{0}$. Since ${E}^{\ast}$ has a Fréchet differentiable norm,

*E*has the Kadec-Klee property. Therefore, it is sufficient to prove that $\parallel {x}_{n}\parallel \to \parallel {x}_{0}\parallel $ as $n\to \mathrm{\infty}$. Since ${x}_{0}\in {C}_{0}$, there exists a sequence $\{{y}_{n}\}\subset C$ such that ${y}_{n}\to {x}_{0}$ as $n\to \mathrm{\infty}$ and ${y}_{n}\in {C}_{n}$ for each $n\in N$. It follows that

*f*is weakly continuous, we get

Using the Kadec-Klee property of *E*, we obtain that $\{{x}_{n}\}$ converges strongly to ${x}_{0}$. This completes the proof. □

**Definition 3.3** ([18])

Let *C* be a closed convex subset of a Banach space *E*, let ${\{{T}_{n}\}}_{n=1}^{\mathrm{\infty}}$ be a countable family of mappings of *C* into itself with the nonempty common fixed point set *F*. The ${\{{T}_{n}\}}_{n=1}^{\mathrm{\infty}}$ is said to be uniformly closed if ${x}_{n}\to x$ and $\parallel {x}_{n}-{T}_{n}{x}_{n}\parallel \to 0$ as $n\to \mathrm{\infty}$ implies $x\in F$.

### 3.2 Strong convergence theorems

**Lemma 3.4** (see Lemma 3.3 of Klin-eam and Suantai [5])

*Let* *C* *be a closed convex subset of a Banach space* *E* *and for all* $n\in N$, *let* $\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{n})$ *such that* ${\alpha}_{i}\ge 0$, $i=1,2,\dots ,n$, ${\alpha}_{1}>0$, *and* ${\sum}_{i=1}^{n}{\alpha}_{i}=1$. *Let* *T* *be an* *α*-*nonexpansive mapping from* *C* *into itself*. *If* ${\alpha}_{1}>\frac{1}{\sqrt[n-1]{2}}$, *let* $\{{x}_{m}\}$ *be a bounded sequence in* *C*, *then* $\parallel {x}_{m}-T{x}_{m}\parallel \to 0$ *if and only if* $\parallel {x}_{m}-{T}_{\alpha}{x}_{m}\parallel \to 0$ *as* $m\to \mathrm{\infty}$.

**Lemma 3.5** ([6])

*Let* *C* *be a closed convex subset of a Banach space* *E*, *and for all* $n\in N$, *let* $\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{n})$ *such that* ${\alpha}_{i}\ge 0$, $i=1,2,\dots ,n$, ${\alpha}_{1}>0$, *and* ${\sum}_{i=1}^{n}{\alpha}_{i}=1$. *Let* *T* *be an* *α*-*nonexpansive mapping from* *C* *into itself*. *If* ${\alpha}_{1}>\frac{1}{\sqrt[n-1]{2}}$, *let* $\{{x}_{m}\}\subset C$ *converge strongly to* *x* *and* $\parallel {x}_{m}-T{x}_{m}\parallel \to 0$ *converge strongly to* 0 *as* $m\to \mathrm{\infty}$, *then* $x\in F(T)$.

**Lemma 3.6** ([6])

*Let* *C* *be a closed convex subset of a uniformly convex and smooth Banach space* *E*, *and for all* $n\in N$, *let* $\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{n})$ *such that* ${\alpha}_{i}\ge 0$, $i=1,2,\dots ,n$, ${\alpha}_{1}>0$, *and* ${\sum}_{i=1}^{n}{\alpha}_{i}=1$. *Let* *T* *be an* *α*-*nonexpansive mapping from* *C* *into itself*. *If* ${\alpha}_{1}>\frac{1}{\sqrt[n-1]{2}}$, *then* $F(T)$ *is closed and convex*.

**Theorem 3.7**

*Let*

*C*

*be a closed convex subset of a uniformly convex and smooth Banach space*

*E*,

*let*${\{{T}_{n}\}}_{n=1}^{\mathrm{\infty}}$

*be a uniformly closed countable family of*${\alpha}_{n}$-

*nonexpansive mappings of*

*C*

*into itself such that*$F:={\bigcap}_{n=1}^{\mathrm{\infty}}F({T}_{n})\ne \mathrm{\varnothing}$,

*let*${\alpha}_{n}=({\alpha}_{n1},{\alpha}_{n2},\dots ,{\alpha}_{n{N}_{0}})$

*such that*${\alpha}_{ni}\ge 0$, $i=1,2,\dots ,{N}_{0}$, ${\alpha}_{n1}>0$,

*and*${\sum}_{i=1}^{{N}_{0}}{\alpha}_{ni}=1$.

*Let*$f:E\to R$

*be a convex and weakly continuous mapping with*$C\subset int(D(f))$.

*For any given Gauss*${x}_{0}\in E$, ${C}_{1}=C$,

*and*${x}_{1}={\mathrm{\Pi}}_{{C}_{1}}^{f}{x}_{0}$,

*define a sequence*$\{{x}_{n}\}$

*in*

*C*

*by the following algorithm*:

*where* $0<a\le {\beta}_{n}\le 1$ *for all* $n\in N$. *If* ${\alpha}_{n1}>\frac{1}{\sqrt[{N}_{0}-1]{2}}$, *then* $\{{x}_{n}\}$ *converges strongly to* ${x}^{\ast}={\mathrm{\Pi}}_{F}^{f}{x}_{0}$.

*Proof* Step 1. We show that ${C}_{n}$ is closed and convex for each $n\ge 0$.

so ${C}_{n}$ is convex for each $n\ge 0$.

It implies that $p\in {C}_{n}$ for all $n\ge 0$. So, we have $F\subset {C}_{n}$ for all $n\ge 0$.

*E*such that $\overline{C}={\bigcap}_{n=1}^{\mathrm{\infty}}{C}_{n}$ is nonempty, it follows that

Since $\{{x}_{n}\}$ is uniformly closed, then ${x}^{\ast}\in F$.

Step 4. We show that ${x}^{\ast}={\mathrm{\Pi}}_{F}^{f}{x}_{0}$. Since ${x}^{\ast}={\mathrm{\Pi}}_{\overline{C}}^{f}{x}_{0}\in F$ and *F* is a nonempty closed convex subset of $\overline{C}={\bigcap}_{n=1}^{\mathrm{\infty}}{C}_{n}$, we conclude that ${x}^{\ast}={\mathrm{\Pi}}_{F}^{f}{x}_{0}$. This completes the proof. □

**Corollary 3.8** ([6])

*Let*

*C*

*be a closed convex subset of a uniformly convex and smooth Banach space*

*E*,

*let*

*T*

*be an*

*α*-

*nonexpansive mapping of*

*C*

*into itself such that*$F(T)\ne \mathrm{\varnothing}$,

*let*$\alpha =({\alpha}_{1},{\alpha}_{2},\dots ,{\alpha}_{{N}_{0}})$

*such that*${\alpha}_{i}\ge 0$, $i=1,2,\dots ,{N}_{0}$, ${\alpha}_{1}>0$,

*and*${\sum}_{i=1}^{{N}_{0}}{\alpha}_{i}=1$.

*For any given Gauss*${x}_{0}\in E$, ${C}_{1}=C$,

*and*${x}_{1}={\mathrm{\Pi}}_{{C}_{1}}{x}_{0}$,

*define a sequence*$\{{x}_{n}\}$

*in*

*C*

*by the following algorithm*:

*where* $0<a\le {\beta}_{n}\le 1$ *for all* $n\in N$. *If* ${\alpha}_{1}>\frac{1}{\sqrt[{N}_{0}-1]{2}}$, *then* $\{{x}_{n}\}$ *converges strongly to* ${x}^{\ast}={\mathrm{\Pi}}_{F}{x}_{0}$.

*Proof* Substituting *T* to ${T}_{n}$ in the proof of Theorem 3.7 and putting $f(x)\equiv 0$, we can draw from Theorem 3.7 the desired conclusion immediately. □

**Remark 3.9** Theorem 3.7 extends the main results of [6] from a single mapping to a countable family of mappings and from the generalized projection operator to the modified generalized *f*-projection operator by a new method.

## Declarations

### Acknowledgements

This project is supported by the National Natural Science Foundation of China under grant (11071279).

## Authors’ Affiliations

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