R E S E A R C H
Open Access
Strong convergence to a fixed point of a total
asymptotically nonexpansive mapping
Gang Eun Kim
**Correspondence:
[email protected] Department of Applied Mathematics, Pukyong National University, Busan, 608-737, Korea
Abstract
In this paper, we prove strong convergence for the modified Ishikawa iteration process of a total asymptotically nonexpansive mapping satisfying condition (A) in a real uniformly convex Banach space. Our result generalizes the results due to Rhoades (J. Math. Anal. Appl. 183:118-120, 1994).
MSC: 47H05; 47H10
Keywords: strong convergence; fixed point; modified Ishikawa iteration process; total asymptotically nonexpansive mapping
1 Introduction
LetXbe a real Banach space, letCbe a nonempty closed convex subset ofX, and letTbe a mapping ofCinto itself. ThenT is said to beasymptotically nonexpansive[] if there exists a sequence{kn},kn≥, withlimn→∞kn= , such that
Tnx–Tny≤knx–y (.)
for allx,y∈Candn≥.Tis said to beuniformly L-Lipschitzianif there exists a constant
L> such that
Tnx–Tny≤Lx–y
for all x,y∈C and n≥. If T is asymptotically nonexpansive, then it is uniformly
L-Lipschitzian. We denote byNthe set of all positive integers.Tis said to betotal asymp-totically nonexpansive(in brief, TAN) [] if there exist two nonnegative real sequences
{cn}and{dn}withcn,dn→ asn→ ∞,φ∈(R+) such that
Tnx–Tny≤ x–y+cnφx–y+dn, (.)
for allx,y∈C andn≥, whereR+:= [,∞) andφ∈(R+) if and only if φ is strictly
increasing, continuous onR+andφ() = . It is clear that if we takeφ(t) =tfor allt≥ anddn= for alln≥ in (.), it is reduced to (.). Approximating fixed points of the modified Ishikawa iterative scheme under total asymptotically nonexpansive mappings has been investigated by several authors; see, for example, Chidume and Ofoedu [, ], Kim [], Kim and Kim [] and others. For a mappingT ofCinto itself in a Hilbert space,
Schu [] considered the following modified Ishikawa iteration process (cf.Ishikawa []) in
Cdefined by
x∈C,
xn+= ( –αn)xn+αnTnyn, (.)
yn= ( –βn)xn+βnTnxn,
where{αn}and{βn}are two real sequences in [, ]. Ifβn= for alln≥, then iteration
process (.) becomes the following modified Mann iteration process (cf.Mann []):
x∈C,
xn+= ( –αn)xn+αnTnxn,
(.)
where{αn}is a real sequence in [, ].
Rhoades [] proved the following results which extended Theorems . and . of Schu [] to uniformly convex Banach spaces.
Theorem . Let X be a uniformly convex Banach space,let C be a nonempty bounded closed convex subset of X,and let T:C→C be a completely continuous asymptotically nonexpansive mapping with{kn}satisfying kn≥,
∞
n=(knr– ) <∞,r=max{,p}.Then, for any x∈C,the sequence{xn}defined by(.),where{αn}satisfies a≤αn≤ –a for all n≥and some a> ,converges strongly to some fixed point of T.
Theorem . Let X be a uniformly convex Banach space,let C be a nonempty bounded closed convex subset of E,and let T :C→C be a completely continuous asymptotically nonexpansive mapping with{kn}satisfying kn≥,∞n=(kr
n– ) <∞,r=max{,p}.Then, for any x∈C,the sequence{xn}defined by(.),where{αn},{βn}satisfy a≤( –αn), ( –
βn)≤ –a for all n≥and some a> ,converges strongly to some fixed point of T.
On the other hand, Kim [] proved the following result which generalized Theorem of Senter and Dotson [].
Theorem . Let X be a real uniformly convex Banach space,let C be a nonempty closed convex subset of X,and let T be a nonexpansive mapping of C into itself satisfying condition
(A)with F(T)=∅.Suppose that for any x in C,the sequence {xn} is defined by xn+=
( –αn)xn+αn[βnxn+ ( –βn)Txn],for all n≥,where{αn}and{βn}are sequences in[, ] such that∞n=αn( –αn) =∞and
∞
n=βn<∞.Then{xn}converges strongly to some fixed point of T.
In this paper, we prove that if T is a total asymptotically nonexpansive self-mapping satisfying condition (A), the iteration{xn} defined by (.) converges strongly to some fixed point ofT, which generalizes the results due to Rhoades [].
2 Preliminaries
set of all fixed points ofT,i.e.,F(T) ={x∈C:Tx=x}. We also denote bya∨b:=max{a,b}. A Banach spaceXis said to beuniformly convexif the modulus of convexityδX=δX(),
<≤, ofXdefined by
δX() =inf
–x+y
:x,y∈X,x ≤,y ≤,x–y ≥
satisfies the inequalityδX() > for every∈(, ]. When{xn}is a sequence inX, then xn→xwill denote strong convergence of the sequence{xn}tox.
Definition .[] A mappingT:C→CwithF(T)=∅is said to satisfy condition (A) if there exists a nondecreasing functionf : [,∞)→[,∞) withf() = andf(r) > for all
r∈(,∞) such that
x–Tx ≥fdx,F(T)
for allx∈C, whered(x,F(T)) =infz∈F(T)x–z.
3 Strong convergence theorem We first begin with the following lemma.
Lemma .[] Let{an},{bn}and{cn}be sequences of nonnegative real numbers such that
∞
n=bn<∞,
∞
n=cn<∞and
an+≤( +bn)an+cn
for all n≥.Thenlimn→∞anexists.
Lemma .[] Let X be a uniformly convex Banach space.Let x,y∈X.Ifx ≤,y ≤
andx–y ≥> ,thenλx+ ( –λ)y ≤ – λ( –λ)δ()for≤λ≤.
Lemma . Let C be a nonempty closed convex subset of a uniformly convex Banach space X,and let T:C→C be a TAN mapping with F(T)=∅.Suppose that{cn},{dn}andφsatisfy the following two conditions:
(I) ∃α,β> such thatφ(t)≤αtfor allt≥β. (II) ∞n=cn<∞,∞n=dn<∞.
Suppose that the sequence{xn} is defined by(.).Thenlimn→∞xn–z exists for any z∈F(T).
Proof For anyz∈F(T), we set
M:= ∨φ(β) <∞.
From (I) and strict increasing ofφ, we obtain
By using (.), we have
Tnxn–z≤ xn–z+cnφ
xn–z
+dn
≤ xn–z+cnφ(β) +αxn–z +dn
≤( +αcn)xn–z+κnM,
whereκn=cn+dnand
∞
n=κn<∞. Since
yn–z=βnTnxn+ ( –βn)xn–z
≤βnTnxn–z+ ( –βn)xn–z
≤βn
( +αcn)xn–z+κnM + ( –βn)xn–z
≤( +αcn)xn–z+κnM,
and thus
yn–z+cnφyn–z
≤( +αcn)xn–z+κnM+cn
φ(β) +αyn–z
≤( +αcn)xn–z+κnM+cnφ(β) +αcn( +αcn)xn–z+αcnκnM
≤( +σn)xn–z+δnM,
whereσn= αcn+αcn,δn=κn+cn+αcnκn,
∞
n=σn<∞and
∞
n=δn<∞. So, we have Tnyn–z≤ yn–z+cnφyn–z+dn
≤( +σn)xn–z+δnM+dn
≤( +σn)xn–z+ηnM,
whereηn=δn+dnand
∞
n=ηn<∞. Hence
xn+–z=( –αn)xn+αnTnyn–z
≤( –αn)xn–z+αnTnyn–z
≤( –αn)xn–z+αn
( +σn)xn–z+ηnM
≤( +σn)xn–z+ηnM.
By Lemma ., we see thatlimn→∞xn–zexists. Theorem . Let X be a uniformly convex Banach space,and let C be a nonempty closed convex subset of X.Let T:C→C be a uniformly continuous and TAN mapping with F(T)=∅.Suppose that{cn},{dn}andφsatisfy the following two conditions:
Suppose that for any xin C,the sequence{xn}defined by(.)satisfies
∞
n=αn( –αn) =∞ andlimβn= .Then{xn}converges strongly to some fixed point of T.
Proof For anyz∈F(T), by Lemma .,{xn}is bounded. We set
M:= ∨φ(β)∨sup
n≥
xn–z<∞.
By Lemma ., we see thatlimn→∞xn–z(≡r) exists. Without loss of generality, we
assumer> . As in the proof of Lemma ., we obtain
Tnyn–z≤( +σn)xn–z+ηnM
≤ xn–z+νnM,
whereνn=σn+ηnand
∞
n=νn<∞. By using Lemma . and Takahashi [], we obtain
xn+–z=( –αn)xn+αnTnyn–z
=( –αn)(xn–z) +αn
Tnyn–z
≤xn–z+νnM – αn( –αn)δX
Tnyn–xn
xn–z+νnM
.
Hence we obtain
αn( –αn)
xn–z+νnMδX
Tnyn–xn
xn–z+νnM
≤ xn–z–xn+–z+νnM.
Thus
αn( –αn)
xn–z+νnMδX
Tny n–xn
xn–z+νnM
<∞.
SinceδXis strictly increasing, continuous and
∞
n=αn( –αn) =∞, we obtain
lim inf
n→∞T ny
n–xn= . (.)
By using (.) in the proof of Lemma ., we have
Tn–xn––z≤ xn––z+cn–φ
xn––z
+dn–
≤ xn––z+cn–
φ(β) +αxn––z +dn–
≤( +αcn–)xn––z+ρn–M,
whereρn–=cn–+dn–and
∞
n=ρn–<∞. Thus
yn––z=βn–Tn–xn–+ ( –βn–)xn––z
≤βn–
( +αcn–)xn––z+ρn–M + ( –βn–)xn––z
≤( +αcn–)xn––z+ρn–M,
and hence
yn––z+cn–φ
yn––z
≤( +αcn–)xn––z+ρn–M+cn–
φ(β) +αyn––z
≤( +αcn–)xn––z+ρn–M+cn–φ(β) +αcn–( +αcn–)xn––z
+αcn–ρn–M
≤( +μn–)xn––z+ϕn–M,
where μn– = αcn– + αcn–, ϕn– = ρn– + cn– + αcn–ρn–,
∞
n=μn– < ∞ and
∞
n=ϕn–<∞. So, we have
Tn–yn––z≤ yn––z+cn–φ
yn––z
+dn–
≤( +μn–)xn––z+ϕn–M+dn–
≤ xn––z+ωn–M,
whereωn–=μn–+ϕn–+dn–and
∞
n=ωn–<∞. By using Lemma . and Takahashi
[], we obtain
xn–z=( –αn–)xn–+αn–Tn–yn––z
=( –αn–)(xn––z) +αn–
Tn–yn––z
≤xn––z+ωn–M
– αn( –αn)δX
Tn–yn
––xn–
xn––z+ωn–M
.
By the same method as above, we obtain
lim inf
n→∞T n–y
n––xn–= . (.)
Since{xn}is bounded andTis a TAN mapping, we obtain
yn–xn=βnTnxn+ ( –βn)xn–xn
≤βnTnxn–xn
≤βnM,
whereM=supn≥Tnxn–xn<∞. By usinglimβn= , we have
lim
n→∞xn–yn= . (.)
Since
by (.) and (.), we obtain
lim inf
n→∞T
nyn–yn= . (.)
By using (.) and (.), we obtain
lim inf
n→∞T n–yn
––yn–= . (.)
Since
Tn–xn––xn–≤Tn–xn––Tn–yn–+Tn–yn––xn–
≤ xn––yn–+cn–φ
xn––yn–
+dn–
+Tn–yn––xn–,
by using (.) and (.), we have
lim inf
n→∞T n–xn
––xn–= . (.)
Since
xn–xn–=( –αn–)xn–+αn–Tn–yn––xn–
=αn–Tn–yn––xn–
≤Tn–yn––yn–+yn––xn–,
by (.) and (.), we get
lim inf
n→∞ xn–xn–= . (.)
From
Tn–xn–xn≤Tn–xn–Tn–xn–+Tn–xn––xn–+xn––xn
≤xn–xn–+cn–φ
xn–xn–
+dn–+Tn–xn––xn–,
by (.) and (.), we obtain
lim inf
n→∞T
n–xn–xn= . (.)
Since
xn–Txn
≤ xn–yn+yn–Tnyn+Tnyn–Tnxn+Tnxn–Txn
≤yn–Tnyn+ xn–yn+cnφ
xn–yn
and by the uniform continuity ofT, (.), (.) and (.), we have
lim inf
n→∞ xn–Txn= . (.)
By using condition (A), we obtain
fdxn,F(T)
≤ xn–Txn (.)
for alln≥. As in the proof of Lemma ., we obtain
xn+–z ≤( +σn)xn–z+ηnM. (.)
Thus
inf
z∈F(T)xn+–z ≤( +σn)z∈infF(T)xn–z+ηnM.
By using Lemma ., we see that limn→∞d(xn,F(T)) (≡c) exists. We first claim that
limn→∞d(xn,F(T)) = . In fact, assume that c=limn→∞d(xn,F(T)) > . Then we can
choosen∈Nsuch that <c <d(xn,F(T)) for alln≥n. By using condition (A), (.)
and (.), we obtain
<f
c
≤fdxni,F(T)
≤ xni–Txni →
asi→ ∞. This is a contradiction. So, we obtainc= . Next, we claim that{xn}is a Cauchy sequence. Since∞n=σn<∞, we obtain
∞
n=( +σn) :=U<∞. Let> be given. Since
limn→∞d(xn,F(T)) = and
∞
n=ηn<∞, there existsn∈Nsuch that for alln≥n, we
obtain
dxn,F(T)<
U+ and
∞
i=n ηi<
M. (.)
Letn,m≥nandp∈F(T). Then, by (.), we obtain
xn–xm ≤ xn–p+xm–p
≤
n–
i=n
( +σi)xn–p+M
n–
i=n ηi+
m–
i=n
( +σi)xn–p+M
m–
i=n ηi
≤
∞
i=n
( +σi)xn–p+M
∞
i=n ηi
.
Taking the infimum over allp∈F(T) on both sides and by (.), we obtain
xn–xm ≤
∞
i=n
( +σi)d
xn,F(T)
+M ∞
i=n ηi
<
(U+ ) U+ +M
M
for alln,m≥n. This implies that{xn}is a Cauchy sequence. Letlimn→∞xn=q. Then d(q,F(T)) = . SinceF(T) is closed, we obtainq∈F(T). Hence{xn}converges strongly to
some fixed point ofT.
Remark . IfT:C→Cis completely continuous, then it satisfies demicompact and, if
T is continuous and demicompact, it satisfies condition (A); see Senter and Dotson [].
Remark . If {αn}is bounded away from both and , i.e., a≤αn≤b for alln≥
and somea,b∈(, ), then∞n=αn( –αn) =∞andlimn→∞βn= hold. However, the
converse is not true. For example, considerαn=n.
We give an example of a mappingT:C→Cwhich satisfies all the assumptions ofTin Theorem .,i.e.,T:C→Cis a uniformly continuous mapping withF(T)=∅which is TAN onC, not Lipschitzian and hence not asymptotically nonexpansive.
Example . LetX:=RandC:= [, ]. DefineT:C→Cby
Tx=
⎧ ⎨ ⎩
, x∈[, ];
√
√
–x, x∈[, ].
Note that Tnx= for allx∈Candn≥ andF(T) ={}. Clearly,T is both uniformly
continuous and TAN onC. We show thatT satisfies condition (A). In fact, ifx∈[, ], then|x– |=|x–Tx|. Similarly, ifx∈[, ], then
|x– |=x– ≤x–√
√
–x=|x–Tx|.
So, we getd(x,F(T)) =|x– | ≤ |x–Tx|for allx∈C. ButT is not Lipschitzian. Indeed, suppose not,i.e., there existsL> such that
|Tx–Ty| ≤L|x–y|
for allx,y∈C. If we takex= –(L+) > andy= , then
√
√
–x≤L( –x) ⇔
L ≤
–x
+x=
L+ L+ .
This is a contradiction.
Competing interests
The author declares that they have no competing interests.
Acknowledgements
The author would like to express their sincere appreciation to the anonymous referees for useful suggestions which improved the contents of this manuscript.
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