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# New iteration scheme for numerical reckoning fixed points of nonexpansive mappings

- Dipti Thakur
^{1}, - Balwant Singh Thakur
^{1}and - Mihai Postolache
^{2}Email author

**2014**:328

https://doi.org/10.1186/1029-242X-2014-328

© Thakur et al.; licensee Springer. 2014

**Received:**27 May 2014**Accepted:**19 August 2014**Published:**2 September 2014

## Abstract

The purpose of this paper is to introduce a new three step iteration scheme for approximation of fixed points of the nonexpansive mappings. We show that our iteration process is faster than all of the Picard, the Mann, the Agarwal *et al.*, and the Abbas *et al.* iteration processes. We support our analytic proof by a numerical example in which we approximate the fixed point by a computer using Matlab program. We also prove some weak convergence and strong convergence theorems for the nonexpansive mappings.

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

## Keywords

- fixed point
- nonexpansive mapping
- strong and weak convergence theorems

## 1 Introduction

*T*, the fixed point mapping, may be nonlinear. A solution ${x}^{\ast}$ of the problem (1.1) is called a

*fixed point*of the mapping

*T*. Consider a

*fixed point iteration*, which is given by

The iterative method (1.2) is also called a *Richardson iteration*, a *Picard iteration*, or *the method of successive substitution*. The standard result for a fixed point iteration is the *contraction mapping theorem*. Indeed, the contraction mapping theorem holds on an arbitrary complete metric space; that is, if *E* is a complete metric space with metric *d* and $T:E\to E$ such that $d(Tx,Ty)\le kd(x,y)$ for some $0\le k<1$ and all $x,y\in E$, then *T* has a unique fixed point ${x}^{\ast}$ and the iterates (1.2) converge to the fixed point ${x}^{\ast}$. The Picard iteration has been successfully employed in approximating the fixed point of contraction mappings and its variants. This success, however, has not extended to nonexpansive mappings *T* even when the existence of a fixed point of *T* is known. Consider the simple example of a self mapping in $[0,1]$ defined by $Tx=1-x$ for $0\le x\le 1$. Then *T* is a nonexpansive mapping with a unique fixed point at $x=\frac{1}{2}$. If one chooses as a starting value $x=a$, $a\ne \frac{1}{2}$, then the successive iterations of *T* yield the sequence $\{1-a,a,1-a,a,\dots \}$. Thus when a fixed point of nonexpansive mappings exists, other approximation techniques are needed to approximate it.

*I*is the identity operator. This average mapping is nonexpansive because

*T*is nonexpansive, and both have the same fixed point set. Krasnosel’skii [1] was first to notice the regularization effect of this average mapping. Schaefer [2] proved a convergence result for a general ${T}_{\lambda}=\lambda I+(1-\lambda )T$ ($0<\lambda <1$). An approximation of fixed points of a nonexpansive mapping using Mann’s algorithm [3] has extensively been studied in the literature (see,

*e.g.*, [4, 5] and references therein). Mann’s algorithm generates, for an arbitrary ${x}_{0}\in C$, a sequence $\{{x}_{n}\}$ according to the following:

where $\{{\alpha}_{n}\}$ is a real control sequence in the interval $(0,1)$.

for all $n\ge 1$, where $\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, and $\{{\gamma}_{n}\}$ are sequences in $(0,1)$.

*et al.*[8] introduced the following iteration process: for an arbitrary ${x}_{0}\in C$ construct a sequence $\{{x}_{n}\}$ by

where $\{{\alpha}_{n}\}$ and $\{{\beta}_{n}\}$ are in $(0,1)$. They showed that this process converges at a rate that is the same as that of the Picard iteration and faster than the Mann iteration for contractions.

where $\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, and $\{{\gamma}_{n}\}$ are in $(0,1)$. They showed that this process converges faster than the Agarwal *et al.* [8] iteration process.

where $\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, and $\{{\gamma}_{n}\}$ are real sequences in $(0,1)$.

The purpose of this paper is to prove that our process (1.8) converges faster than all of the Picard, the Mann, the Ishikawa, the Noor, the Agarwal *et al.*, and the Abbas *et al*. iteration processes for contractions in the sense of Berinde [10]. We also prove weak and strong convergence theorems for nonexpansive mapping using iteration (1.8). In the last section, using a numerical example, we compare the behavior of iteration (1.8) with respect to the above mentioned iteration processes.

## 2 Rate of convergence

Berinde [10] proposed a method to compare the fastness of two sequences.

**Definition 2.1**Let $\{{a}_{n}\}$ and $\{{b}_{n}\}$ be two sequences of real numbers that converge to

*a*and

*b*, respectively, and assume that there exists

- (i)
If $l=0$, then it can be said that $\{{a}_{n}\}$ converges faster to

*a*than $\{{b}_{n}\}$ to*b*. - (ii)
If $0<l<\mathrm{\infty}$, then it can be said that $\{{a}_{n}\}$ and $\{{b}_{n}\}$ have the same rate of convergence.

*p*, the error estimates

are available, where $\{{a}_{n}\}$ and $\{{b}_{n}\}$ are sequences of positive numbers (converging to zero).

Then, in view of Definition 2.1, Berinde [10] adopted the following concept.

**Definition 2.2** Let $\{{u}_{n}\}$ and $\{{v}_{n}\}$ be two fixed point iteration procedures that converge to the same fixed point *p* and satisfy (2.2) and (2.3), respectively. If $\{{a}_{n}\}$ converges faster than $\{{b}_{n}\}$, then it can be said that $\{{u}_{n}\}$ converges faster than $\{{v}_{n}\}$ to *p*.

In recent years, Definition 2.2 has been used as a standard tool to compare the fastness of two fixed point iterations. Using this technique Sahu [11] established that the Agarwal *et al.* iteration (1.6) converges faster than the Mann (1.3) and the Picard (1.2) iterations and supported the claim by the following example.

**Example 1** Let $X=\mathbb{R}$ and $K=[0,\mathrm{\infty})$. Let $T:K\to K$ be a mapping defined by $Tx={(3x+18)}^{\frac{1}{3}}$ for all $x\in K$. For ${x}_{0}=1,000$ and ${\alpha}_{n}={\beta}_{n}=\frac{1}{2}$, $n=0,1,2,\dots $ , Agarwal *et al.* iteration is faster than both the Mann and the Picard iteration.

Using a similar technique Abbas and Nazir [9] established that the Abbas *et al.* iteration (1.7) converges faster than the Agarwal *et al.* iteration (1.6) and hence it converges faster than the Mann (1.3) and the Picard (1.2) iterations also. An example is also given in support of the claim.

**Example 2** Let $X=\mathbb{R}$ and $K=[1,50]$. Let $T:K\to K$ be a mapping defined by $Tx=\sqrt{{x}^{2}-8x+40}$ for all $x\in K$. For ${x}_{0}=30$ and ${\alpha}_{n}={\beta}_{n}={\gamma}_{n}=\frac{1}{2}$, $n=0,1,2,\dots $ , the Abass *et al.* iteration (1.7) is faster than the Agarwal *et al.* iteration (1.6). Since Sahu [11] already has shown that the iteration (1.6) is faster than the Mann iteration (1.3), the iteration (1.7) is faster than the iterations (1.2), (1.3), and (1.6).

We now show that our process (1.8) converges faster than (1.7) in the sense of Berinde [10].

**Theorem 2.3** *Let* *C* *be a nonempty closed convex subset of a norm space* *E*. *Let* *T* *be a contraction with a contraction factor* $k\in (0,1)$ *and fixed point* *p*. *Let* $\{{u}_{n}\}$ *be defined by the iteration process* (1.7) *and* $\{{x}_{n}\}$ *by* (1.8), *where* $\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, *and* $\{{\gamma}_{n}\}$ *are in* $[\epsilon ,1-\epsilon ]$ *for all* $n\in \mathbb{N}$ *and for some* *ε* *in* $(0,1)$. *Then* $\{{x}_{n}\}$ *converges faster than* $\{{u}_{n}\}$. *That is*, *our process* (1.8) *converges faster than* (1.7).

*Proof*As proved in Theorem 3 of Abbas and Nazir [9],

Consequently $\{{x}_{n}\}$ converges faster than $\{{u}_{n}\}$. □

Now, we present an example which shows that the new iteration process (1.8) converges at a rate faster than the existing iteration schemes mentioned above.

**Example 3**Let $E=\mathbb{R}$ and $C=[1,50]$. Let $T:C\to C$ be a mapping defined by $T(x)=\sqrt{{x}^{2}-8x+40}$ for all $x\in C$. Choose ${\alpha}_{n}=0.85$, ${\beta}_{n}=0.65$, ${\gamma}_{n}=0.45$, with the initial value ${x}_{1}=40$. Our corresponding iteration process, the Abbas and Nazir iteration process (1.7), the Agarwal

*et al.*iteration process (1.6), the Noor iteration process (1.7), the Ishikawa iteration process (1.4), the Mann iteration process (1.3), and the Picard iteration processes (1.2) are, respectively, given in Table 1.

**Comparative results**

Step | Picard | Mann | Ishikawa | Noor | Agarwal | Abbas | New iter. |
---|---|---|---|---|---|---|---|

1 | 40.0000000000 | 40.0000000000 | 40.0000000000 | 40.0000000000 | 40.0000000000 | 40.0000000000 | 40.0000000000 |

2 | 36.3318042492 | 36.8820336118 | 34.8751575132 | 33.9816211055 | 34.3249281505 | 34.2399531822 | 32.9458774280 |

3 | 32.7008496221 | 33.7905308732 | 29.8335259837 | 28.0882816012 | 28.7529148550 | 28.5873914017 | 26.0696692526 |

4 | 29.1159538575 | 30.7306375124 | 24.9067432334 | 22.3811620460 | 23.3289757744 | 23.0905148078 | 19.4826041425 |

5 | 25.5892777970 | 27.7090706072 | 20.1467307646 | 16.9736024952 | 18.1321892967 | 17.8350979079 | 13.4242477938 |

6 | 22.1381326176 | 24.7347891266 | 15.6449263114 | 12.0962209155 | 13.3147454600 | 12.9887334680 | 8.4745882697 |

7 | 18.7880774656 | 21.8200359935 | 11.5741197024 | 8.2289280979 | 9.1939307941 | 8.9032413368 | 5.7279660470 |

8 | 15.5784221001 | 18.9820007784 | 8.2638548016 | 6.0182077910 | 6.3717274607 | 6.2123720180 | 5.0765141830 |

9 | 12.5721859009 | 16.2455313784 | 6.1736938982 | 5.2517005165 | 5.2434387591 | 5.2064678069 | 5.0064676549 |

10 | 9.8733161157 | 13.6475866165 | 5.3185408455 | 5.0576355955 | 5.0298139084 | 5.0252795464 | 5.0005330507 |

11 | 7.6482574613 | 11.2442765494 | 5.0768890301 | 5.0129587850 | 5.0033662656 | 5.0028941692 | 5.0000438381 |

12 | 6.1081734180 | 9.1201110370 | 5.0179832209 | 5.0029016212 | 5.0003761718 | 5.0003285479 | 5.0000036046 |

13 | 5.3333287129 | 7.3913650188 | 5.0041744485 | 5.0006491038 | 5.0000419870 | 5.0000372607 | 5.0000002964 |

14 | 5.0771808572 | 6.1732610225 | 5.0009673150 | 5.0001451769 | 5.0000046858 | 5.0000042253 | 5.0000000244 |

15 | 5.0160062399 | 5.4814708358 | 5.0002240577 | 5.0000324684 | 5.0000005229 | 5.0000004791 | 5.0000000020 |

16 | 5.0032258274 | 5.1725897008 | 5.0000518932 | 5.0000072614 | 5.0000000584 | 5.0000000543 | 5.0000000002 |

17 | 5.0006461643 | 5.0576419946 | 5.0000120186 | 5.0000016240 | 5.0000000065 | 5.0000000062 | 5.0000000000 |

18 | 5.0001292729 | 5.0187159301 | 5.0000027835 | 5.0000003632 | 5.0000000007 | 5.0000000007 | 5.0000000000 |

19 | 5.0000258562 | 5.0060176595 | 5.0000006447 | 5.0000000812 | 5.0000000001 | 5.0000000001 | 5.0000000000 |

20 | 5.0000051713 | 5.0019286052 | 5.0000001493 | 5.0000000182 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

21 | 5.0000010343 | 5.0006174572 | 5.0000000346 | 5.0000000041 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

22 | 5.0000002069 | 5.0001976174 | 5.0000000080 | 5.0000000009 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

23 | 5.0000000414 | 5.0000632408 | 5.0000000019 | 5.0000000002 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

24 | 5.0000000083 | 5.0000202374 | 5.0000000004 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

25 | 5.0000000017 | 5.0000064760 | 5.0000000001 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

26 | 5.0000000003 | 5.0000020723 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

27 | 5.0000000001 | 5.0000006631 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

28 | 5.0000000000 | 5.0000002122 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

29 | 5.0000000000 | 5.0000000679 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

30 | 5.0000000000 | 5.0000000217 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

31 | 5.0000000000 | 5.0000000070 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

32 | 5.0000000000 | 5.0000000022 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

33 | 5.0000000000 | 5.0000000007 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

34 | 5.0000000000 | 5.0000000002 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

35 | 5.0000000000 | 5.0000000001 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 | 5.0000000000 |

All sequences converge to ${x}^{\ast}=5$. Comparison shows that our iteration process (1.8) converges fastest among all the iterations considered in the example.

## 3 Convergence theorems

**Lemma 3.1** *Let* *C* *be a nonempty closed convex subset of a norm space* *E*. *Let* *T* *be a nonexpansive self mapping on* *C*, $\{{x}_{n}\}$ *defined by* (1.8) *and* $F(T)\ne \mathrm{\varnothing}$. *Then* ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ *exists for all* $p\in F$.

*Proof*Let $p\in F(T)$ for all $n\in \mathbb{N}$. From (1.8), we have

Thus ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exists for all $p\in F(T)$. □

We need following lemma to establish our next result.

**Lemma 3.2** [12]*Suppose that* *E* *is a uniformly convex Banach space and* $0<p\le {t}_{n}\le q<1$ *for all* $n\in \mathbb{N}$. *Let* $\{{x}_{n}\}$ *and* $\{{y}_{n}\}$ *be two sequences of* *E* *such that* ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}\parallel {x}_{n}\parallel \le r$, ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}\parallel {y}_{n}\parallel \le r$ *and* ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}\parallel {t}_{n}{x}_{n}+(1-{t}_{n}){y}_{n}\parallel =r$ *hold for some* $r\ge 0$. *Then* ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-{y}_{n}\parallel =0$.

We now establish a result which will be of key importance for the main result.

**Lemma 3.3** *Let* *C* *be a nonempty closed convex subset of a uniformly convex Banach space E*. *Let* *T* *be a nonexpansive self mapping on* *C*, $\{{x}_{n}\}$ *defined by* (1.8), *where* $\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, *and* $\{{\gamma}_{n}\}$ *are in* $[\epsilon ,1-\epsilon ]$ *for all* $n\in \mathbb{N}$ *and for some* *ε* *in* $(0,1)$ *and* $F(T)\ne \mathrm{\varnothing}$. *Then* ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$.

*Proof* By Lemma 3.1, ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exists. Assume that ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel =c$.

*T*is a nonexpansive mapping, it follows that

*T*is a nonexpansive mapping, we have from (3.1)

This completes the proof. □

**Lemma 3.4** [13]

*Let*

*C*

*be a nonempty bounded closed convex subset of a uniformly convex Banach space and*$T:C\to E$

*be a nonexpansive mapping*.

*Then there is a strictly increasing and continuous convex function*$g:[0,\mathrm{\infty})\to [0,\mathrm{\infty})$

*with*$g(0)=0$

*such that*

*for all* $x,y\in C$ *and* $t\in [0,1]$.

**Lemma 3.5** *For any* ${p}_{1},{p}_{2}\in F(T)$, ${lim}_{n\to \mathrm{\infty}}\parallel t{x}_{n}+(1-t){p}_{1}-{p}_{2}\parallel $ *exists*, *for all* $t\in [0,1]$ *under the conditions of Lemma * 3.3.

*Proof*By Lemma 3.1, ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exists for all $p\in F(T)$ and hence $\{{x}_{n}\}$ is bounded. Thus there exists a real number $r>0$ such that $\{{x}_{n}\}\subseteq D\equiv \overline{{B}_{r}(0)}\cap C$, so that

*D*is a closed convex nonempty subset of

*C*. Set

for all $t\in [0,1]$. Then ${lim}_{n\to \mathrm{\infty}}{a}_{n}(0)=\parallel {p}_{1}-{p}_{2}\parallel $ and, from Lemma 3.1, ${lim}_{n\to \mathrm{\infty}}{a}_{n}(1)=\parallel {x}_{n}-{p}_{2}\parallel $ exist.

Now it remains to show that ${lim}_{n\to \mathrm{\infty}}{a}_{n}(t)$ exists for $t\in (0,1)$.

for all $x\in D$.

Since ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exists for all $p\in F(T)$, we get ${lim}_{n,m\to \mathrm{\infty}}g({b}_{n,m})=0$ and by the property of *g*, we get ${lim}_{n,m\to \mathrm{\infty}}{b}_{n,m}=0$.

This implies that ${lim}_{n\to \mathrm{\infty}}{a}_{n}(t)$ exists for all $t\in (0,1)$, *i.e.*, ${lim}_{n\to \mathrm{\infty}}\parallel t{x}_{n}+(1-t){p}_{1}-{p}_{2}\parallel $ exists for all $t\in [0,1]$. □

*E*be a Banach space and ${S}_{E}=\{x\in E:\parallel x\parallel =1\}$ unit sphere on

*E*. The Banach space

*E*is said to be smooth if

exists for each *x* and *y* in ${S}_{E}$. In this case, the norm of *E* is called Gâteaux differentiable.

*E*is called Fréchet differentiable normed (see,

*e.g.*, [14]); for each

*x*in

*E*, the above limit exists and is attained uniformly for

*y*in

*E*, and in this case it is also well known that

for all $x,h\in E$, where *J* is the Fréchet derivative of the function $\frac{1}{2}{\parallel \cdot \parallel}^{2}$ at $x\in E$, $\u3008\cdot ,\cdot \u3009$ is the dual pairing between *E* and ${E}^{\ast}$, and *b* is an increasing function defined on $[0,\mathrm{\infty})$ such that ${lim}_{t\to 0}\frac{b(t)}{t}=0$.

**Lemma 3.6** *Assume that the conditions of Lemma * 3.3 *are satisfied*. *Then*, *for any* ${p}_{1},{p}_{2}\in F(T)$, ${lim}_{n\to \mathrm{\infty}}\u3008{x}_{n},J({p}_{1}-{p}_{2})\u3009$ *exists*; *in particular*, $\u3008p-q,J({p}_{1}-{p}_{2})\u3009=0$ *for all* $p,q\in {\omega}_{w}({x}_{n})$, *the set of all weak limits of* $\{{x}_{n}\}$.

The proof of Lemma 3.6 is similar to the proof of Lemma 2.3 of Khan and Kim [15].

*E*is said to satisfy the Opial condition [16] if for each sequence $\{{x}_{n}\}$ in

*E*, ${x}_{n}\rightharpoonup x$ implies that

for all $y\in E$ with $y\ne x$.

A Banach space *E* is said to have the *Kadec-Klee property* if for every sequence $\{{x}_{n}\}$ in *E*, ${x}_{n}\rightharpoonup x$ and $\parallel {x}_{n}\parallel \to \parallel x\parallel $ together imply ${x}_{n}\to x$ as $n\to \mathrm{\infty}$.

We need the following to prove our next result.

**Definition 3.7** A mapping $T:C\to E$ is demiclosed at $y\in E$ if for each sequence $\{{x}_{n}\}$ in *C* and each $x\in E$, ${x}_{n}\rightharpoonup x$, and $T{x}_{n}\to y$ imply that $x\in C$ and $Tx=y$.

**Lemma 3.8** [17]

*Let* *C* *be a nonempty closed convex subset of a uniformly convex Banach space* *E*, *and* *T* *a nonexpansive mapping on* *C*. *Then* $I-T$ *is demiclosed at zero*.

**Lemma 3.9** [8]

*Let* *E* *be a reflexive Banach space satisfying the Opial condition*, *C* *a nonempty convex subset of* *E*, *and* $T:C\to E$ *an operator such that* $I-T$ *demiclosed at zero and* $F(T)\ne \mathrm{\varnothing}$. *Let* $\{{x}_{n}\}$ *be a sequence in* *C* *such that* ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$ *and* ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ *exists for all* $p\in F(T)$. *Then* $\{{x}_{n}\}$ *converges weakly to a fixed point of* *T*.

**Lemma 3.10** [18]

*Let* *E* *be a real reflexive Banach space such that its dual* ${E}^{\ast}$ *has the Kadec*-*Klee property*. *Let* $\{{x}_{n}\}$ *be a bounded sequence in* *E* *and* ${x}^{\ast},{y}^{\ast}\in {\omega}_{w}({x}_{n})$, *here* ${\omega}_{w}({x}_{n})$ *denotes the* *w*-*limit set of* $\{{x}_{n}\}$. *Suppose* ${lim}_{n\to \mathrm{\infty}}\parallel t{x}_{n}+(1-t){x}^{\ast}-{y}^{\ast}\parallel $ *exists for all* $t\in [0,1]$. *Then* ${x}^{\ast}={y}^{\ast}$.

We now establish a weak convergence result.

**Theorem 3.11**

*Let*

*E*

*be a uniformly convex Banach space and let*

*C*,

*T*,

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

*be as in Lemma*3.3

*and*$F(T)\ne \mathrm{\varnothing}$.

*Assume that any of the following conditions hold*:

- (a)
*E**satisfies the Opial condition*, - (b)
*E**has a Fréchet differentiable norm*, - (c)
*the dual*${E}^{\ast}$*of**E**satisfies the Kadec*-*Klee property*.

*Then* $\{{x}_{n}\}$ *converges weakly to a point of* $F(T)$.

*Proof* Let $p\in F(T)$, by Lemma 3.1, ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exists.

We prove that $\{{x}_{n}\}$ has a unique weak subsequential limit in $F(T)$.

Let *u* and *v* be weak limits of the subsequences $\{{x}_{{n}_{i}}\}$ and $\{{x}_{{n}_{j}}\}$ of $\{{x}_{n}\}$, respectively. By Lemma 3.3, ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$, and also $I-T$ is demiclosed with respect to zero, hence by Lemma 3.8, we obtain $Tu=u$. In a similar manner, we have $v\in F(T)$.

Next, we prove the uniqueness.

This is a contradiction, so $u=v$.

Next, assume (b) holds.

By Lemma 3.6, $\u3008p-q,J({p}_{1}-{p}_{2})\u3009=0$, for all $p,q\in {\omega}_{w}({x}_{n})$. Therefore, ${\parallel u-v\parallel}^{2}=\u3008u-v,J(u-v)\u3009=0$ implies $u=v$.

Finally, assume that (c) is true.

Since ${lim}_{n\to \mathrm{\infty}}\parallel t{x}_{n}+(1-t)u-v\parallel $ exists for all $t\in [0,1]$ by Lemma 3.5, $u=v$ by Lemma 3.10, and $\{{x}_{n}\}$ converges weakly to a fixed point of $F(T)$ and this completes the proof. □

A mapping $T:C\to C$ is said to be semicompact if any sequence $\{{x}_{n}\}$ in *C*, such that ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$, has a subsequence converging strongly to some $p\in C$.

Next we establish the following strong convergence results.

**Theorem 3.12** *Let* *E* *be a uniformly convex Banach space and let* *C*, *T*, *and* $\{{x}_{n}\}$ *be as in Lemma * 3.3. *If* *T* *is semicompact and* $F(T)\ne \mathrm{\varnothing}$, *then* $\{{x}_{n}\}$ *converges strongly to a fixed point of* *T*.

*Proof*By Lemma 3.3, we have ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$; since

*T*is semicompact, $\{{x}_{n}\}$ has a subsequence converging to some $p\in C$ as

*C*is closed. Continuity of

*T*gives ${lim}_{j\to \mathrm{\infty}}\parallel T{x}_{{n}_{j}}-Tp\parallel \to 0$. Then by Lemma 3.3,

This yields $p\in F(T)$. By Lemma 3.1, ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-p\parallel $ exists for all $p\in F(T)$, and therefore $\{{x}_{n}\}$ must itself converge to $p\in F(T)$ and this completes the proof. □

**Theorem 3.13** *Let* *E* *be a uniformly convex Banach space and let* *C*, *T*, $F(T)$, *and* $\{{x}_{n}\}$ *be as in Lemma * 3.3. *Then* $\{{x}_{n}\}$ *converges to a point of* $F(T)$ *if and only if* ${lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}d({x}_{n},F(T))=0$, *where* $d(x,F(T))=inf\{\parallel x-p\parallel :p\in F(T)\}$.

*Proof* Necessity is obvious. Suppose that ${lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}d({x}_{n},F(T))=0$. As proved in Lemma 3.3, ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-w\parallel $ exists for all $w\in F(T)$, therefore ${lim}_{n\to \mathrm{\infty}}d({x}_{n},F(T))$ exists. But by hypothesis, ${lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}d({x}_{n},F(T))=0$, therefore ${lim}_{n\to \mathrm{\infty}}d({x}_{n},F(T))=0$.

*C*. Since ${lim}_{n\to \mathrm{\infty}}d({x}_{n},F(T))=0$, for given $\epsilon >0$, there exists ${n}_{0}$ in ℕ such that, for all $n\ge {n}_{0}$,

Hence $\{{x}_{n}\}$ is a Cauchy sequence in *C*. Since *C* is a closed subset of a complete space, ${lim}_{n\to \mathrm{\infty}}{x}_{n}=p\in C$. Since $F(T)$ is closed, ${lim}_{n\to \mathrm{\infty}}d({x}_{n},F(T))=0$ gives $d(p,F(T))=0$, *i.e.*, $p\in F(T)$. □

**Definition 3.14** A mapping $T:C\to C$, where *C* is a subset of a normed space *E*, is said to satisfy Condition (*I*) [19] if there exists a nondecreasing function $f:[0,\mathrm{\infty})\to [0,\mathrm{\infty})$ with $f(0)=0$, $f(r)>0$ for all $r\in (0,1)$ such that $\parallel x-Tx\parallel \ge f(d(x,F(T)))$ for all $x\in C$ where $d(x,F(T))=inf\{\parallel x-p\parallel :p\in F(T)\}$.

Applying Theorem 3.13, we obtain strong convergence of the process (1.8) under Condition (*I*) as follows.

**Theorem 3.15** *Let* *e* *be a uniformly convex Banach space and let* *C*, *T*, *and* $\{{x}_{n}\}$ *be as in Lemma * 3.3. *Let* *T* *satisfy Condition* (*I*), *then* $\{{x}_{n}\}$ *converges strongly to a fixed point of* *T*.

*Proof*We proved in Lemma 3.3 that

*I*) and (3.17), we get

*i.e.*, ${lim}_{n\to \mathrm{\infty}}f(d({x}_{n},F(T)))=0$. Since $f:[0,\mathrm{\infty})\to [0,\mathrm{\infty})$ is a nondecreasing function satisfying $f(0)=0$, $f(r)>0$ for all $r\in (0,\mathrm{\infty})$, we have

Now all the conditions of Theorem 3.13 are satisfied, therefore, by its conclusion, $\{{x}_{n}\}$ converges strongly to a point of $F(T)$. □

## Declarations

### Acknowledgements

The second author would like to thank the Rajiv Gandhi National Fellowship of India for the grant (F1-17.1/2011-12/RGNF-ST-CHH-6632).

## Authors’ Affiliations

## References

- Krasnosel’skii MA:
**Two remarks on the method of successive approximations.***Usp. Mat. Nauk*1955,**10:**123-127.MathSciNetGoogle Scholar - Schaefer H:
**Über die methode sukzessiver approximationen.***Jahresber. Dtsch. Math.-Ver.*1957,**59**(1):131-140.MathSciNetMATHGoogle Scholar - Mann WR:
**Mean value methods in iteration.***Proc. Am. Math. Soc.*1953,**4:**506-510. 10.1090/S0002-9939-1953-0054846-3View ArticleMathSciNetMATHGoogle Scholar - Byrne C:
**A unified treatment of some iterative algorithms in signal processing and image reconstruction.***Inverse Probl.*2004,**20**(1):103-120. 10.1088/0266-5611/20/1/006MathSciNetView ArticleMATHGoogle Scholar - Kim TH, Xu HK:
**Strong convergence of modified Mann iterations.***Nonlinear Anal.*2005,**61**(1-2):51-60. 10.1016/j.na.2004.11.011MathSciNetView ArticleMATHGoogle Scholar - Ishikawa S:
**Fixed points by a new iteration method.***Proc. Am. Math. Soc.*1974,**44:**147-150. 10.1090/S0002-9939-1974-0336469-5MathSciNetView ArticleMATHGoogle Scholar - Noor MA:
**New approximation schemes for general variational inequalities.***J. Math. Anal. Appl.*2000,**251**(1):217-229. 10.1006/jmaa.2000.7042MathSciNetView ArticleMATHGoogle Scholar - Agarwal RP, O’Regan D, Sahu DR:
**Iterative construction of fixed points of nearly asymptotically nonexpansive mappings.***J. Nonlinear Convex Anal.*2007,**8**(1):61-79.MathSciNetMATHGoogle Scholar - Abbas M, Nazir T:
**A new faster iteration process applied to constrained minimization and feasibility problems.***Mat. Vesn.*2014,**66**(2):223-234.MathSciNetGoogle Scholar - Berinde V:
**Picard iteration converges faster than Mann iteration for a class of quasicontractive operators.***Fixed Point Theory Appl.*2004,**2:**97-105.MathSciNetView ArticleMATHGoogle Scholar - Sahu DR:
**Applications of the**S**-iteration process to constrained minimization problems and split feasibility problem.***Fixed Point Theory*2011,**12**(1):187-204.MathSciNetMATHGoogle Scholar - Schu J:
**Weak and strong convergence to fixed points of asymptotically nonexpansive mappings.***Bull. Aust. Math. Soc.*1991,**43**(1):153-159. 10.1017/S0004972700028884MathSciNetView ArticleMATHGoogle Scholar - Bruck RE:
**A simple proof of the mean ergodic theorem for nonlinear contractions in Banach spaces.***Isr. J. Math.*1979,**32**(2-3):107-116. 10.1007/BF02764907MathSciNetView ArticleMATHGoogle Scholar - Takahashi W, Kim GE:
**Approximating fixed points of nonexpansive mappings in Banach spaces.***Math. Jpn.*1998,**48**(1):1-9.MathSciNetMATHGoogle Scholar - Khan SH, Kim JK:
**Common fixed points of two nonexpansive mappings by a modified faster iteration scheme.***Bull. Korean Math. Soc.*2010,**47**(5):973-985. 10.4134/BKMS.2010.47.5.973MathSciNetView ArticleMATHGoogle Scholar - Opial Z:
**Weak convergence of the sequence of successive approximations for nonexpansive mappings.***Bull. Am. Math. Soc.*1967,**73:**591-597. 10.1090/S0002-9904-1967-11761-0MathSciNetView ArticleMATHGoogle Scholar - Goebel K, Kirk WA
**Cambridge Studies in Advanced Mathematics 28.**In*Topics in Metric Fixed Point Theory*. Cambridge University Press, Cambridge; 1990.View ArticleGoogle Scholar - Kaczor W:
**Weak convergence of almost orbits of asymptotically nonexpansive semigroups.***J. Math. Anal. Appl.*2002,**272**(2):565-574. 10.1016/S0022-247X(02)00175-0MathSciNetView ArticleMATHGoogle Scholar - Senter HF, Dotson WG:
**Approximating fixed points of nonexpansive mappings.***Proc. Am. Math. Soc.*1974,**44:**375-380. 10.1090/S0002-9939-1974-0346608-8MathSciNetView ArticleMATHGoogle Scholar

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