# Common Fixed Points of Multistep Noor Iterations with Errors for a Finite Family of Generalized Asymptotically Quasi-Nonexpansive Mappings

Authors: S. Imnang; S. Suantai
Journal: Abstract and Applied Analysis
 (2009)

---

## Abstract

We introduce a general iteration scheme for a finite family
of generalized asymptotically quasi-nonexpansive mappings in Banach spaces.
The new iterative scheme includes the multistep Noor iterations with errors,
modified Mann and Ishikawa iterations, three-step iterative scheme of Xu and
Noor, and Khan and Takahashi scheme as special cases. Our results generalize
and improve the recent ones announced by Khan et al. (2008),
H. Fukhar-ud-din and S. H. Khan (2007), J. U. Jeong and S. H. Kim (2006),
and many others.


---

## Body

## 1. Introduction

LetC be a subset of real Banach space X. Let T be a self-mapping of C and let F(T) denote the fixed points set of T, that is,F(T):={x∈C:Tx=x}. Recall that a mapping T is said to be asymptotically nonexpansive on C if there exists a sequence {bn} in [0,∞) with limn→∞bn=0 such that for each x,y∈C,(1.1)∥Tnx-Tny∥≤(1+bn)∥x-y∥,∀n≥1.Ifbn=0 for all n≥1, then T is known as a nonexpansive mapping. T is called generalized asymptotically quasi-nonexpansive [1] if there exist sequences {bn},{cn} in [0,∞) with limn→∞bn=0=limn→∞cn such that(1.2)∥Tnx-p∥≤(1+bn)∥x-p∥+cn,∀n≥1,
for all x∈C and all p∈F(T). If cn=0 for all n≥1, then T is known as an asymptotically quasi-nonexpansive mapping. T  is called asymptotically nonexpansive mapping in the intermediate sense [2] provided that T is uniformly continuous and(1.3)limsupn→∞supx,y∈C(∥Tnx-Tny∥-∥x-y∥)≤0.T is said to be (L-γ)  uniform Lipschitz [3] if there are constants L>0 and γ>0 such that(1.4)∥Tnx-Tny∥≤L∥x-y∥γ,∀n≥1,
for all x,y∈C. A mapping T is called semicompact if any bounded sequence {xn} in C with limn→∞∥xn-Txn∥=0, there exists a subsequence {xni} of {xn} such that {xni} converges strongly to some x* in C.Remark 1.1.
LetT be asymptotically nonexpansive mapping in the intermediate sense. Put Gn=supx,y∈C(∥Tnx-Tny∥-∥x-y∥)∨0, ∀n≥1.
IfF(T)≠Ø, we obtain that ∥Tnx-p∥≤∥x-p∥+Gn for all x∈C and all p∈F(T). Since limn→∞Gn=0, therefore T is a generalized asymptotically quasi-nonexpansive mapping.Recall that a mappingT:C→C with F(T)≠Ø is said to satisfy condition (I) [4] if there is a nondecreasing function f:[0,∞)→[0,∞) with f(0)=0 and f(t)>0 for all t∈(0,∞) such that ∥x-Tx∥≥f(d(x,F(T))) for all x∈C, where d(x,F(T))=inf{∥x-p∥:p∈F(T)}.Fixed-point iteration processes for asymptotically quasi-nonexpansive mapping in Banach spaces including Mann and Ishikawa iterations processes have been studied extensively by many authors; see [3, 5–11]. Many of them are used widely to study the approximate solutions of the certain problems. In 1974, Senter and Dotson [4] studied the convergence of the Mann iteration scheme defined by x1∈C,(1.5)xn+1=αnTxn+(1-αn)xn,∀n≥1,
in a uniformly convex Banach space, where {αn} is a sequence satisfying 0<a≤αn≤b<1foralln≥1 and T is a nonexpansive (or a quasi-nonexpansive) mapping. They established a relation between condition (I) and demicompactness.Recall that a mappingT:C→C is demicompact if for every bounded sequence {xn} in C such that {xn-Txn} converges, there exists a subsequence say {xnj} of {xn} that converges strongly to some y in C. Every compact and semicompact mapping is demicompact. They actually showed that condition (I) is weaker than demicompactness for a nonexpansive mapping defined on bounded set.Xu and Noor [12], in 2002, introduced a three-step iterative scheme as follows:(1.6)zn=anTnxn+(1-an)xn,yn=bnTnzn+(1-bn)xn,xn+1=αnTnyn+(1-αn)xn,n≥1,
where {an},{bn},{αn} are appropriate sequences in [0,1]. The theory of three-step iterative scheme is very rich, and this scheme, in the context of one or more mappings, has been extensively studied (e.g., see Khan et al. [6], Plubtieng and Wangkeeree [7], Fukhar-ud-din and Khan [5], Petrot [13], and Suantai [14]). It has been shown in [15] that three-step method performs better than two-step and one-step methods for solving variational inequalities.In 2001, Khan and Takahashi [16] have approximated common fixed points of two asymptotically nonexpansive mappings by the modified Ishikawa iteration. Jeong and Kim [17] have approximated common fixed points of two asymptotically nonexpansive mappings. Plubtieng et al. [18], in 2006, modified Noor iterations with errors and have approximated common fixed points of three asymptotically nonexpansive mappings. Shahzad and Udomene [10] established convergence theorems for the modified Ishikawa iteration process of to asymptotically quasi-nonexpansive mappings to a common fixed point of the mappings. Plubtieng and Wangkeeree [7], in 2006, established strong convergence theorems of the modified multistep Noor iterations with errors for an asymptotically quasi-nonexpansive mapping and asymptotically nonexpansive mapping in the intermediate sense.Very recently, Khan et al. [6], in 2008, established convergence theorems for the modified multistep Noor iterations process of finite family of asymptotically quasi-nonexpansive mappings to a common fixed point of the mappings. For rerated results with errors terms, we refer to [5–7, 17–21]. Inspired and motivated by these facts, we introduce a new iteration process for a finite family of {Ti:i=1,2,…,k} of generalized asymptotically quasi-nonexpansive mappings as follows.LetTi:C→C(i=1,2,…,k) be mappings and F:=⋂i=1kF(Ti). For a given x1∈C, and a fixed k∈ℕ (ℕ denote the set of all positive integers), compute the iterative sequences {xn} and {yin} by(1.7)xn+1=ykn=αknTkny(k-1)n+βknxn+γknukn,y(k-1)n=α(k-1)nTk-1ny(k-2)n+β(k-1)nxn+γ(k-1)nu(k-1)n,⋮y3n=α3nT3ny2n+β3nxn+γ3nu3n,y2n=α2nT2ny1n+β2nxn+γ2nu2n,y1n=α1nT1ny0n+β1nxn+γ1nu1n,
where y0n=xn and {u1n},{u2n},…,{ukn} are bounded sequences in C with {αin},{βin}, and {γin} are appropriate real sequences in [0,1] such that αin+βin+γin=1 for all i=1,2,…,k and all n. Our iteration includes and extends the Mann iteration (1.5), three-step iteration by Xu and Noor (1.6), the multistep Noor iterations with errors by Plubtieng and Wangkeeree [7], and the iteration defined by Khan et al. [6] simultaneously.The purpose of this paper is to establish several strong convergence theorems of the iterative scheme (1.7) for a finite family of generalized asymptotically quasi-nonexpansive mappings when one mapping Ti satisfies a condition which is weaker than demicompactness and we also weak convergence theorem for a finite family of generalized asymptotically quasi-nonexpansive mappings in a uniformly convex Banach space satisfying Opial's property. Our results generalize and improve the corresponding ones announced by Khan et al. [6], Fukhar-ud-din and Khan [5], and many others.


## 2. Preliminaries

In the sequel, the following lemmas are needed to prove our main results.A mappingT with domain D(T) and range R(T) in X is said to be demiclosed at 0 if whenever {xn} is a sequence in D(T) such that {xn} converges weakly to x∈D(T) and {Txn} converging strongly to 0, we have Tx=0.A Banach spaceX is said to satisfy Opial's property if for each x in X and each sequence {xn} weakly convergent to x, the following condition holds for x≠y:(2.1)liminfn→∞∥xn-x∥<liminfn→∞∥xn-y∥.
It is well known that all Hilbert spaces and lp(1<p<∞) spaces have Opial's property while Lp spaces (p≠2) have not. A family {Ti:i=1,2,…,k} of self-mappings of C with F:=⋂i=1kF(Ti)≠Ø is said to satisfy the following conditions.(1)
Condition(A¯) [22]. If there is a nondecreasing function f:[0,∞)→[0,∞) with f(0)=0 and f(t)>0 for all t∈(0,∞) such that 1/k∑i=1k∥x-Tix∥≥f(d(x,F)) for all x∈C, where d(x,F)=inf{∥x-p∥:p∈F}.(2)
Condition(B¯) [22]. If there is a nondecreasing function f:[0,∞)→[0,∞) with f(0)=0 and f(t)>0 for all t∈(0,∞) such that max1≤i≤k{∥x-Tix∥}≥f(d(x,F)) for all x∈C.(3)
Condition(C¯) [22]. If there is a nondecreasing function f:[0,∞)→[0,∞) with f(0)=0 and f(t)>0 for all t∈(0,∞) such that ∥x-Tlx∥≥f(d(x,F)) for all x∈C and for at least one Tl,l=1,2,…,k.Note that(B¯) and (C¯) are equivalent, condition (B¯) reduces to condition (I) when all but one of Ti's are identities, and in addition, it also condition (A¯).It is well known that every continuous and demicompact mapping must satisfy condition (I) (see [4]). Since every completely continuous T:C→C is continuous and demicompact so that it satisfies condition (I). Thus we will use condition (C¯) instead of the demicompactness and complete continuity of a family {Ti:i=1,2,…,k}.Lemma 2.1 (see [8, Lemma 1]).
Let{an},{bn}, and {δn} be sequences of nonnegative real numbers satisfying the inequality
(2.2)an+1≤(1+δn)an+bn,∀n=1,2,….
If ∑n=1∞δn<∞ and ∑n=1∞bn<∞, then(i)
limn→∞an exists;(ii)
limn→∞an=0 whenever liminfn→∞an=0.Lemma 2.2 (see [7, Lemma 3.1]).
LetX be a uniformly convex Banach space, {xn}, {yn}⊂X, real numbers a≥0,α,β∈(0,1), and let {αn} be a real sequence number which satisfies(i)
0<α≤αn≤β<1,foralln≥n0 and for some n0∈ℕ;(ii)
limsupn→∞∥xn∥≤a and limsupn→∞∥yn∥≤a;(iii)
limn→∞∥αnxn+(1-αn)yn∥=a. Then limn→∞∥xn-yn∥=0.Lemma 2.3 (see [14, Lemma 2.7]).
LetX be a Banach space which satisfies Opial's property and let {xn} be a sequence in X. Let u,v∈X be such that limn→∞∥xn-u∥ and limn→∞∥xn-v∥ exist. If {xnk} and {xmk} are subsequences of {xn} which converge weakly to u and v, respectively, then u=v.


## 3. Convergence Theorems in Banach Spaces

Our first result is the strong convergence theorems of the iterative scheme (1.7) for a finite family of generalized asymptotically quasi-nonexpansive mappings in a Banach space. In order to prove our main results, the following lemma is needed.Lemma 3.1.
LetX be a Banach space and C a nonempty closed and convex subset of X, and {Ti:i=1,2,…,k} a finite family of generalized asymptotically quasi-nonexpansive self-mappings of C with the sequences {bin},{cin}⊂[0,∞) such that ∑n=1∞bin<∞ and ∑n=1∞cin<∞ for all i=1,2,…,k. Assume that F≠Ø and ∑n=1∞γin<∞ for each i=1,2,…,k. For a given x1∈C, let the sequences {xn} and {yin} be defined by (1.7). Then(a)
there exist sequences{vn} and {ein} in [0,∞) such that ∑n=1∞vn<∞, ∑n=1∞ein<∞, and ∥yin-p∥≤(1+vn)i∥xn-p∥+ein, for all i=1,2,…,k and all p∈F;(b)
limn→∞∥xn-p∥ exists for all p∈F;(c)
there exist constantM>0 and {si} in [0,∞) such that ∑i=1∞si<∞ and ∥xn+m-p∥≤M∥xn-p∥+∑i=n∞si for all p∈F and n,m∈ℕ.Proof.
(a) Letp∈F, vn=max1≤i≤k{bin} and dn=max1≤i≤k{cin} for all n.
Since∑n=1∞bin<∞ and ∑n=1∞cin<∞, for all i=1,2,…,k, therefore ∑n=1∞vn<∞ and ∑n=1∞dn<∞. For each n≥1, we note that
(3.1)∥y1n-p∥=∥α1nT1ny0n+β1nxn+γ1nu1n-p∥≤α1n∥T1nxn-p∥+β1n∥xn-p∥+γ1n∥u1n-p∥≤α1n(1+b1n)∥xn-p∥+α1nc1n+β1n∥xn-p∥+γ1n∥u1n-p∥≤α1n(1+vn)∥xn-p∥+α1ndn+β1n(1+vn)∥xn-p∥+γ1n∥u1n-p∥≤(1+vn)∥xn-p∥+e1n,
where e1n=α1ndn+γ1n∥u1n-p∥. Since {u1n} is bounded, ∑n=1∞γ1n<∞ and ∑n=1∞dn<∞, we obtain that ∑n=1∞e1n<∞. It follows from (3.1) that
(3.2)∥y2n-p∥≤α2n∥T2ny1n-p∥+β2n∥xn-p∥+γ2n∥u2n-p∥≤α2n(1+vn)∥y1n-p∥+α2ndn+β2n∥xn-p∥+γ2n∥u2n-p∥≤α2n(1+vn)((1+vn)∥xn-p∥+e1n)+α2ndn+β2n(1+vn)2∥xn-p∥+γ2n∥u2n-p∥=(α2n+β2n)(1+vn)2∥xn-p∥+α2n(1+vn)e1n+α2ndn+γ2n∥u2n-p∥≤(1+vn)2∥xn-p∥+e2n,
where e2n=α2n(1+vn)e1n+α2ndn+γ2n∥u2n-p∥. Since {u2n}, {vn} are bounded, ∑n=1∞e1n<∞, ∑n=1∞dn<∞, and ∑n=1∞γ2n<∞, it follows that ∑n=1∞e2n<∞. Moreover, we see that
(3.3)∥y3n-p∥≤α3n∥T3ny2n-p∥+β3n∥xn-p∥+γ3n∥u3n-p∥≤α3n(1+vn)∥y2n-p∥+α3ndn+β3n∥xn-p∥+γ3n∥u3n-p∥≤α3n(1+vn)((1+vn)2∥xn-p∥+e2n)+α3ndn+β3n(1+vn)3∥xn-p∥+γ3n∥u3n-p∥=(α3n+β3n)(1+vn)3∥xn-p∥+α3n(1+vn)e2n+α3ndn+γ3n∥u3n-p∥≤(1+vn)3∥xn-p∥+e3n,
where e3n=α3n(1+vn)e2n+α3ndn+γ3n∥u3n-p∥. Since {u3n}, {vn} are bounded, ∑n=1∞e2n<∞, ∑n=1∞dn<∞, and ∑n=1∞γ3n<∞, it follows that ∑n=1∞e3n<∞. By continuing the above method, there are nonnegative real sequences {ein} in [0,∞) such that ∑n=1∞ein<∞ and
(3.4)∥yin-p∥≤(1+vn)i∥xn-p∥+ein∀i=1,2,…,k.
This completes the proof of (a).
(b) From part (a), for the casei=k, we have
(3.5)∥xn+1-p∥≤(1+vn)k∥xn-p∥+ekn,∀n,p∈F.
It follows from Lemma 2.1(i) that limn→∞∥xn-p∥ exists, for all p∈F.
(c) Ift≥0, then 1+t≤et and so, (1+t)k≤ekt, for k=1,2,…. Thus, from (3.5), it follows that
(3.6)∥xn+m-p∥≤(1+vn+m-1)k∥xn+m-1-p∥+ek(n+m-1)≤exp{kvn+m-1}∥xn+m-1-p∥+ek(n+m-1)≤⋯≤exp{k∑i=nn+m-1vi}∥xn-p∥+∑i=nn+m-1eki≤exp{k∑i=1∞vi}∥xn-p∥+∑i=n∞eki≤M∥xn-p∥+∑i=n∞si,
where M=exp{k∑i=1∞vi} and si=eki.Theorem 3.2.
LetX be a Banach space and C a nonempty closed and convex subset of X and {Ti:i=1,2,…,k} a finite family of generalized asymptotically quasi-nonexpansive self-mappings of C with the sequences {bin},{cin}⊂[0,∞) such that ∑n=1∞bin<∞ and ∑n=1∞cin<∞ for all i=1,2,…,k. Assume that F≠∅ is closed and ∑n=1∞γin<∞ for each i=1,2,…,k. Then the iterative sequence {xn} defined by (1.7) converges strongly to a common fixed point of the family of mappings if and only if liminfn→∞d(xn,F)=0.Proof.
We prove only the sufficiency because the necessity is obvious. From (3.5), we have ∥xn+1-p∥≤(1+vn)k∥xn-p∥+ekn,forallnandallp∈F.
Hence, we have(3.7)d(xn+1,F)≤(1+vn)kd(xn,F)+ekn=(1+∑r=1kk(k-1)…(k-r+1)r!vnr)d(xn,F)+ekn.
Since ∑n=1∞vn<∞, it follows that ∑n=1∞∑r=1k(k(k-1)⋯(k-r+1)/r!)vnr<∞. Since ∑n=1∞ekn<∞ and liminfn→∞d(xn,F)=0, it follows from Lemma 2.1(ii) that limn→∞d(xn,F)=0. Next, we prove that {xn} is a Cauchy sequence. From Lemma 3.1(c), we have
(3.8)∥xn+m-p∥≤M∥xn-p∥+∑i=n∞si,∀p∈F,n,m∈ℕ.
Since limn→∞d(xn,F)=0 and ∑i=1∞si<∞, therefore for ϵ>0, there exists n0∈ℕ such that
(3.9)d(xn,F)<ϵ4M,∑i=n0∞si<ϵ4,∀n≥n0.
Therefore, there exists z1 in F such that
(3.10)∥xn0-z1∥<ϵ4M.
From (3.8) to (3.10), for all n≥n0 and m≥1, we have
(3.11)∥xn+m-xn∥≤∥xn+m-z1∥+∥xn-z1∥≤M∥xn0-z1∥+∑i=n0∞si+M∥xn0-z1∥+∑i=n0∞si<Mϵ4M+ϵ4+Mϵ4M+ϵ4=ϵ.
This shows that {xn} is a Cauchy sequence, hence xn→z∈C. It remains to show that z∈F. Notice that
(3.12)|d(z,F)-d(xn,F)|≤∥z-xn∥,∀n.
Since limn→∞d(xn,F)=0, we obtain that z∈F.The following corollary follows from Theorem3.2.Corollary 3.3.
LetX be a Banach space and C a nonempty closed and convex subset of X and {Ti:i=1,2,…,k} a finite family of generalized asymptotically quasi-nonexpansive self-mappings of C with the sequences {bin},{cin}⊂[0,∞) such that ∑n=1∞bin<∞ and ∑n=1∞cin<∞ for all i=1,2,…,k. Assume that F≠∅ is closed and ∑n=1∞γin<∞ for each i=1,2,…,k. Then the iterative sequence {xn}, defined by (1.7), converges strongly to a point p∈F if and only if there exists a subsequence {xnj} of {xn} converging to p.Since an asymptotically quasi-nonexpansive mapping is generalized asymptotically quasi-nonexpansive mapping, so we have the following result.Corollary 3.4.
LetX be a Banach space and C a nonempty closed and convex subset of X and {Ti:i=1,2,…,k} a finite family of asymptotically quasi-nonexpansive self-mappings of C with the sequences {bin}⊂[0,∞) such that ∑n=1∞bin<∞ for all i=1,2,…,k. Assume that F≠Ø and ∑n=1∞γin<∞ for each i=1,2,…,k. Then the iterative sequence {xn}, defined by (1.7), converges strongly to a common fixed point of the family of mappings if and only if liminfn→∞d(xn,F)=0.Remark 3.5.
Theorem3.2 generalizes and extends Theorem 2.2 of Khan et al. [6], for a finite family of asymptotically quasi-nonexpansive mappings, Theorem 1 of Fukhar-ud-din and Khan [5], and Theorem 3.2 of Shahzad and Udomene [10] for two asymptotically quasi-nonexpanaive mappings to the more general class of generalized asymptotically quasi-nonexpansive mappings.Theorem 3.6.
LetX be a Banach space and C a nonempty closed and convex subset of X and {Ti:i=1,2,…,k} a finite family of generalized asymptotically quasi-nonexpansive self-mappings of C with the sequences {bin},{cin}⊂[0,∞) such that ∑n=1∞bin<∞ and ∑n=1∞cin<∞ for all i=1,2,…,k. Suppose that F≠∅ is closed. Let x1∈C and {xn} be the sequence defined by (1.7). If ∑n=1∞γin<∞, limn→∞∥xn-Tixn∥=0 for all i=1,2,…,k and {Ti:i=1,2,…,k} satisfies condition (C¯), then {xn} converges strongly to a common fixed point of the family of mappings.Proof.
Fromlimn→∞∥xn-Tixn∥=0 for all i=1,2,…,k and {Ti:i=1,2,…,k} satisfying condition (C¯), there is a nondecreasing function f:[0,∞)→[0,∞) with f(0)=0 and f(t)>0 for all t∈(0,∞) such that ∥xn-Ti0xn∥≥f(d(xn,F)) for some i0∈{1,2,…,k}, it follows that limn→∞d(xn,F)=0. From Theorem 3.2, we obtain that {xn} converges strongly to a common fixed point of the family of mappings.


## 4. Convergence Theorems in Uniformly Convex Banach Spaces

In this section, we establish weak and strong convergence theorems of the iterative scheme (1.7) for a finite family of generalized asymptotically quasi-nonexpansive and (L-γ) uniform Lipschitz mappings in a uniformly convex Banach space. In order to prove our main results, we need the following lemma.Lemma 4.1.
LetC be a nonempty closed and convex subset of a uniformly convex Banach space X and {Ti,i=1,2,…,k} a finite family of (L-γ) uniform Lipschitz and generalized asymptotically quasi-nonexpansive self-mappings of C with the sequences {bin},{cin}⊂[0,∞) such that ∑n=1∞bin<∞ and ∑n=1∞cin<∞ for all i=1,2,…,k. Assume that F≠Ø and ∑n=1∞γin<∞ for all i=1,2,…,k. For a given x1∈C let {xn} and {yin} be the sequences defined by (1.7) with 0<η≤αin≤ρ<1, for all i=1,2,…,k and all n≥n0 and for some n0∈ℕ. Then (i)
limn→∞∥Tjny(j-1)n-xn∥=0 for all j=1,2,…,k,(ii)
limn→∞∥Tjxn-xn∥=0 for all j=1,2,…,k,(iii)
limn→∞∥yjn-xn∥=0 for all j=1,2,…,k.Proof.
Letp∈F, vn=max1≤i≤k{bin} and dn=max1≤i≤k{cin} for all n.
(i) From Lemma3.1(b), we have that limn→∞∥xn-p∥ exists for all p∈F. Suppose that
(4.1)limn→∞∥xn-p∥=a.
From (3.4) and (4.1), we get that
(4.2)limsupn→∞∥yjn-p∥≤a,for1≤j≤k-1.
For each j∈{1,2,…,k-1} and n∈ℕ, we have
(4.3)∥yjn-p∥≤αjn∥Tjny(j-1)n-p∥+βjn∥xn-p∥+γjn∥ujn-p∥≤αjn(1+vn)∥y(j-1)n-p∥+αjn(1+vn)dn+(1-αjn)(1+vn)∥xn-p∥+γjn(1+vn)∥ujn-p∥.
By using (4.3) and (1.7), for each j=1,2,…,k-1, we have
(4.4)∥xn+1-p∥=∥αkn(Tkny(k-1)n-p)+βkn(xn-p)+γkn(ukn-p)∥≤αkn(1+vn)∥y(k-1)n-p∥+(1-αkn)(1+vn)∥xn-p∥+αkn(1+vn)dn+γkn∥ukn-p∥(1+vn)≤(αknα(k-1)n)(1+vn)2∥y(k-2)n-p∥+(1-αknα(k-1)n)(1+vn)2∥xn-p∥+(αkn+α(k-1)n)(1+vn)2dn+(γkn∥ukn-p∥+γ(k-1)n∥u(k-1)n-p∥)(1+vn)2⋮≤(αknα(k-1)n⋯α(j+1)n)(1+vn)(k-j)∥yjn-p∥+(1-αknα(k-1)n⋯α(j+1)n)(1+vn)(k-j)∥xn-p∥+(αkn+α(k-1)n+⋯+α(j+1)n)(1+vn)(k-j)dn+(γkn∥ukn-p∥+γ(k-1)n∥u(k-1)n-p∥+⋯+γ(j+1)n∥u(j+1)n-p∥)(1+vn)k-j.
Since 0<η≤αin≤ρ<1, for all i=1,2,…,k and all n≥n0, we have that for all n≥n0 and all j=1,2,…,k-1,
(4.5)∥xn-p∥≤∥xn-p∥ηk-j-∥xn+1-p∥ηk-j(1+vn)k-j+∥yjn-p∥+ξjnηk-jdn+ϑjnηk-j,
where ξjn=αkn+α(k-1)n+⋯+α(j+1)n and ϑjn=γkn∥ukn-p∥+γ(k-1)n∥u(k-1)n-p∥+⋯+γ(j+1)n∥u(j+1)n-p∥. Since limn→∞∥xn-p∥=a and limn→∞ϑjn=limn→∞dn=limn→∞vn=0, it follows that
(4.6)a≤liminfn→∞∥yjn-p∥,∀j=1,2,…,k-1.
From (4.2) and (4.6), we have
(4.7)limn→∞∥yjn-p∥=a=limn→∞∥xn-p∥,∀j=1,2,…,k-1.
That is, for each j=1,2,…,k, we have
(4.8)limn→∞∥αjn(Tjny(j-1)n-p+γjn(ujn-xn))+(1-αjn)(xn-p+γjn(ujn-xn))∥=a.
Since
(4.9)∥Tjny(j-1)n-p+γjn(ujn-xn)∥≤∥Tjny(j-1)n-p∥+γjn∥ujn-xn∥≤(1+vn)∥y(j-1)n-p∥+dn+γjn∥ujn-xn∥,∥xn-p+γjn(ujn-xn)∥≤∥xn-p∥+γjn∥ujn-xn∥,
it follows that
(4.10)limsupn→∞∥Tjny(j-1)n-p+γjn(ujn-xn)∥≤a,(4.11)limsupn→∞∥xn-p+γjn(ujn-xn)∥≤a,∀j=1,2,…,k.
From (4.8) to (4.11), we can conclude from Lemma 2.2 that
(4.12)limn→∞∥Tjny(j-1)n-xn∥=0,∀j=1,2,…,k.
(ii) It follows from part (i) in the case j=1 that limn→∞∥T1nxn-xn∥=0. For j=2,3,…,k, we obtain from part (i) that
(4.13)∥Tjnxn-xn∥≤∥Tjnxn-Tjny(j-1)n∥+∥Tjny(j-1)n-xn∥≤L∥xn-y(j-1)n∥γ+∥Tjny(j-1)n-xn∥≤L(α(j-1)n∥Tj-1ny(j-2)n-xn∥+γ(j-1)n∥u(j-1)n-xn∥)γ+∥Tjny(j-1)n-xn∥→0asn→∞.
Therefore,
(4.14)limn→∞∥Tjnxn-xn∥=0,∀j=1,2,…,k.
Since
(4.15)∥xn-Tjxn∥≤∥xn+1-xn∥+∥xn+1-Tjn+1xn+1∥+∥Tjn+1xn+1-Tjn+1xn∥+∥Tjn+1xn-Tjxn∥≤αkn∥Tkny(k-1)n-xn∥+γkn∥ukn-xn∥+∥xn+1-Tjn+1xn+1∥+L(αkn∥Tkny(k-1)n-xn∥+γkn∥ukn-xn∥)γ+L∥Tjnxn-xn∥γ,
it follows from (i) and (4.14) that
(4.16)limn→∞∥Tjxn-xn∥=0,∀j=1,2,…,k.
(iii) Since limn→∞γjn=0 and
(4.17)∥yjn-xn∥≤αjn∥Tjny(j-1)n-xn∥+γjn∥ujn-xn∥
for all j∈{1,2,…,k}, (iii) is directly obtained by (i).Theorem 4.2.
LetC be a nonempty closed and convex subset of a uniformly convex Banach space X satisfying the Opial's property, and {Ti,i=1,2,…,k} a finite family of (L-γ) uniform Lipschitz and generalized asymptotically quasi-nonexpansive self-mappings of C with the sequences {bin},{cin}⊂[0,∞) such that ∑n=1∞bin<∞ and ∑n=1∞cin<∞ for all i=1,2,…,k. Assume that F≠Ø and ∑n=1∞γin<∞ for all i=1,2,…,k. For a given x1∈C let {xn} be the sequence defined by (1.7) with 0<η≤αin≤ρ<1, for all i=1,2,…,k and all n≥n0 and for some n0∈ℕ. If I-Ti, i=1,2,…,k, is demiclosed at 0, then {xn} converges weakly to a common fixed point of the family {Ti:i=1,2,…,k}.Proof.
By Lemma4.1(ii), we have limn→∞∥Tixn-xn∥=0, for all i=1,2,…,k. Since X is uniformly convex and {xn} is bounded, without loss of generality we may assume that xn→u weakly as n→∞ for some u∈C. Since I-Ti, i=1,2,…,k, is demiclosed at 0, we have u∈F. Suppose that there are subsequences {xnk} and {xmk} of {xn} that converge weakly to u and v, respectively. Again, as above, we can prove that u,v∈F. By Lemma 3.1(b), limn→∞∥xn-u∥ and limn→∞∥xn-v∥ exist. It follows from Lemma 2.3 that u=v. Therefore {xn} converges weakly to a common fixed point of {Ti:i=1,2,…,k}.Theorem 4.3.
Under the hypotheses of Lemma4.1, assume that the family {Ti:i=1,2,…,k} satisfies condition (C¯). Then {xn} and {yjn} converge strongly to a common fixed point of the family of mappings for all j=1,2,…,k.Proof.
From (3.5), we have
(4.18)∥xn+1-p∥≤(1+vn)k∥xn-p∥+ekn,∀n,p∈F.
Therefore,
(4.19)d(xn+1,F)≤(1+vn)kd(xn,F)+ekn=(1+∑r=1kk(k-1)⋯(k-r+1)r!vnr)d(xn,F)+ekn.
Since ∑n=1∞vn<∞, it follows that ∑n=1∞∑r=1k(k(k-1)⋯(k-r+1)/r!)vnr<∞. Since ∑n=1∞ekn<∞, we obtain from Lemma 2.1(i) that limn→∞d(xn,F) exists. By Lemma 4.1(ii), we have limn→∞∥Tixn-xn∥=0 for all i=1,2,…,k. Since {Ti:i=1,2,…,k} satisfies condition (C¯), there is a nondecreasing function f:[0,∞)→[0,∞) with f(0)=0 and f(t)>0 for all t∈(0,∞) such that ∥xn-Ti0xn∥≥f(d(xn,F)) for some i0∈{1,2,…,k}, it follows that limn→∞d(xn,F)=0. By Theorem 3.2, we can conclude that {xn} converges strongly to a common fixed point q of the family {Ti:i=1,2,…,k}. From Lemma 4.1(iii), we have limn→∞∥yjn-xn∥=0 for all j=1,2,…,k, and we obtain that limn→∞yjn=q for all j=1,2,…,k.Remark 4.4.
The family of generalized asymptotically quasi-nonexpansive mappings in Theorem4.2 and 4.3 can be replaced by a family of asymptotically quasi-nonexpansive mappings. Lemma 3.1 and 4.1 generalize and improve [6, Lemma 3.1], [19, Lemmas 3.4 and 3.5], [18, Lemma 2.3], [7, Lemma 4.2], and [17, Lemma 3.3] to a finite family of (L-γ) uniform Lipschitz and generalized asymptotically quasi-nonexpansive mappings. Theorem 4.2 generalizes and improves [6, Theorems 3.2 and 4.2], [18, Theorem 2.9], [17, Theorem 3.1], and [21, Theorem 1] to the more general class of a finite family of (L-γ) uniform Lipschitz and generalized asymptotically quasi-nonexpansive mappings. Theorem 4.3 generalizes and improves [6, Theorem 3.3], [7, Theorem 4.3], [5, Theorem 2], [18, Theorem 2.4], [19, Theorem 4.2], [17, Theorem 3.2], [10, Theorem 3.4], [20, Theorem 2] and [21, Theorem 2] by using condition (C¯) instead of condition (A¯) or semicompactness or completely continuous or compactness to the more general class of a finite family of (L-γ) uniform Lipschitz and generalized asymptotically quasi-nonexpansive mappings.Remark 4.5.
From Remark1.1, Theorems 3.2 to 4.3 hold true for a finite family {Ti}i=1k of asymptotically nonexpansive mappings in the intermediate sense.



---

*Source: 728510-2009-10-21.xml*