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Research | Open | Published:

An approximation method for common fixed points of a finite family of asymptotic pointwise nonexpansive mappings

Abstract

In this article, we consider an iterative scheme to approximate a common fixed point for a finite family of asymptotic pointwise nonexpansive mappings. We obtain weak and strong convergence theorems of the proposed iteration in uniformly convex Banach spaces. The related results for complete CAT(0) spaces are also included.

MSC:47H09, 47H10.

1 Introduction

It is well known that many of the most important nonlinear problems of applied mathematics reduce to solving a given equation which in turn may be reduced to finding the fixed points of a certain operator. It is important not only to know the fixed points exist, but also to be able to construct that fixed points. Lau is a great mathematician who has published many good papers concerning to the existence and the approximation of fixed points for various types of mappings (see, e.g., [111]).

The existence of fixed points for nonexpansive mappings was studied independently by three authors in 1965 (see Browder [12], Göhde [13], and Kirk [14]). Since then the iteration methods for approximating fixed points of nonexpansive mappings has rapidly been developed and many of papers have appeared (see, e.g., [1521]). One of the popular classes of generalized nonexpansive mappings is the class of asymptotically nonexpansive mappings which was introduced by Goebel and Kirk [22] in 1972. Later on, Kirk and Xu [23] introduced the concept of asymptotic pointwise nonexpansive mappings which generalizes the concept of asymptotically nonexpansive mappings and proved the existence of fixed points for such maps in a uniformly convex Banach space. In 2011, Kozlowski [24] defined an iterative sequence for an asymptotic pointwise nonexpansive mapping T on a convex subset C of a Banach space X by x 1 C and

(1)

where { t k } and { s k } are sequences in [0,1] and { n k } is an increasing sequence of natural numbers. He proved, under some suitable assumptions, that the sequence { x k } defined by (1) converges weakly to a fixed point of T where X is a uniformly convex Banach space which satisfies the Opial condition and { x k } converges strongly to a fixed point of T provided T r is a compact mapping for some rN. Recently, Pasom and Panyanak [25] extended Kozlowski’s results to a finite family of asymptotic pointwise nonexpansive mappings T 1 ,, T m . Precisely, they proved weak and strong convergence theorems of the iterative process defined by

(2)

where { t i k } k = 1 are sequences in [0,1] for all i=1,2,,m, and { n k } be an increasing sequence of natural numbers. On the other hand, Kettapun et al.[26] studied the iterative process defined by

(3)

where T 1 ,, T m are asymptotically quasi-nonexpansive mappings on C.

In this article, motivated by the results mentioned above, we obtain weak and strong convergence theorems of the iterative process defined by

(4)

where T 1 ,, T m are asymptotic pointwise nonexpansive mappings on C, { t i k } k = 1 are sequences in [0,1] for all i=1,2,,m, and { n k } be an increasing sequence of natural numbers.

2 Preliminaries and lemmas

Let C be a nonempty subset of a metric space (X,d) and T be a mapping on C. A point x in C is called a fixed point of T if x=Tx. We shall denote by F(T) the set of fixed points of T. The mapping T:CC is said to be

  1. (i)

    nonexpansive if d(Tx,Ty)(x,y)for allx,yC,

  2. (ii)

    asymptotically nonexpansive if there is a sequence { k n } of positive numbers with the property lim n k n =1 and such that

    d ( T n x , T n y ) k n d(x,y),for allx,yCandn1,
  3. (iii)

    asymptotically quasi-nonexpansive if there is a sequence { k n } of positive numbers with the property lim n k n =1 and such that

    d ( T n x , p ) k n d(x,p),for allxC,pF(T)andn1,
  4. (iv)

    asymptotic pointwise nonexpansive if there exists a sequence of functions α n :C[0,) such that lim sup n α n (x)1 and

    d ( T n x , T n y ) α n (x)d(x,y),for allx,yCandn1.

The following implications hold.

T is nonexpansive T is asymptotically nonexpansive T is asymptotically quasi-nonexpansive T is asymptotic pointwise nonexpansive

The existence of fixed points for asymptotic pointwise nonexpansive mappings in uniformly convex Banach spaces was proved by Kirk and Xu [23] as the following result.

Theorem 2.1 Let C be a nonempty bounded closed and convex subset of a uniformly convex Banach space X. Then every asymptotic pointwise nonexpansive mappingT:CChas a fixed point. Moreover, F(T)is closed and convex.

For common fixed points of a family of commuting mappings, Pasom and Panyanak [27] obtained the following result.

Theorem 2.2 Let C be a nonempty bounded closed convex subset of a uniformly convex Banach space X. Then every commuting familySof asymptotic pointwise nonexpansive mappings on C has a nonempty closed convex common fixed point set.

Let C be a nonempty subset of a metric space (X,d). We shall denote by T(C) the class of all asymptotic pointwise nonexpansive mappings from C into C. Let T 1 ,, T m T(C), without loss of generality, we can assume that there exists a sequence of mappings α n :C[0,) such that for all x,yC, i=1,,m, and nN,

d ( T i n x , T i n y ) α n (x)d(x,y)and lim sup n α n (x)1.
(5)

Let a n (x)=max{ α n (x),1}. Again, without loss of generality, we can assume that

d ( T i n x , T i n y ) a n (x)d(x,y), lim n a n (x)=1and a n (x)1,
(6)

for all x,yC, i=1,,m, and nN. We define b n (x)= a n (x)1, then for each xC we have lim n b n (x)=0.

Definition 2.3[24]

Define T r (C) as a class of all TT(C) such that

(7)
(8)

Let C be a nonempty subset of a Banach space X and T 1 ,, T m T r (C). Let { t i k } k = 1 (0,1) be bounded away from 0 and 1 for all i=1,2,,m and { n k } be an increasing sequence of natural numbers. Let x 1 C and define a sequence { x k } in C as

(9)

We say that the sequence { x k } in (9) is well defined if lim sup k a n k ( x k )=1. As in [24], we observe that lim k a k (x)=1 for every xC. Hence, we can always choose a subsequence { a n k } which makes { x k } well defined.

Definition 2.4 A strictly increasing sequence { n k }N is called quasi-periodic if the sequence { n k + 1 n k } is bounded, or equivalently if there exists a number pN such that any block of p consecutive natural numbers must contain a term of the sequence { n k }. The smallest of such numbers p will be called a quasi-period of { n k }.

Recall that a mapping T:CC is called semi-compact if for any sequence { x n } in C such that

lim n d( x n ,T x n )=0,

there exists a subsequence { x n j } of { x n } and qC such that lim j x n j =q. A family of mapping { T i :i=1,2,,m} on C is said to satisfy Condition ( A ) if there exists a nondecreasing function f:[0,)[0,) with f(0)=0 and f(r)>0 for all r>0 such that d(x, T j x)f(dist(x,F)), for some j=1,,m for all xC, where dist(x,F)=inf{d(x,p):pF= i = 1 m F( T i )}.

Lemma 2.5[28], Lemma 2.2]

Let{ s n }and{ u n }be sequences of nonnegative real numbers satisfy:

s n + 1 (1+ u n ) s n ,for allnN,and n = 1 u n <.

Then (i) lim n s n exists (ii) if lim inf n s n =0, then lim n s n =0.

Lemma 2.6[29], Lemma 1]

Suppose { r k } is a bounded sequence of real numbers and { d k , l } is a doubly index sequence of real numbers which satisfy

lim sup k lim sup l d k , l 0,and r k + l r k + d k , l

for eachk,lN. Then lim k r k =afor someaR.

Lemma 2.7[30, 31]

Let X be a uniformly convex Banach space and let{ t n }be a sequence in[a,b]for somea,b(0,1). Suppose that{ u n }and{ v n }are sequences in X such that

lim sup n u n r, lim sup n v n r,and lim n t n u n + ( 1 t n ) v n =r,

for somer0. Then lim n u n v n =0.

Lemma 2.8[24], Lemma 3.1]

Let C be a nonempty closed convex subset of a uniformly convex Banach space X and letT T r (C). If lim n x n T x n =0then for anymN, lim n x n T m x n =0.

Lemma 2.9[24], Theorem 3.1]

Let X be a uniformly convex Banach space with the Opial property and let C be a nonempty closed convex subset of X. LetT T r (C)and letωX, { x n }Xbe such that weak- lim n x n =ωand lim n x n T x n =0. ThenωF(T).

3 Results in Banach spaces

3.1 Results for bounded domains

Recall that a subset C of a metric space (X,d) is said to be bounded if

diam(C):=sup { d ( x , y ) : x , y C } <.

Lemma 3.1 Let C be a nonempty closed convex subset of a Banach space X and T 1 ,, T m T r (C). Let { t i k } k = 1 [0,1]and{ n k }Nbe such that{ x k }in (9) is well defined. Assume thatF:= i = 1 m F( T i ). Then for eachpF, there are sequences of nonnegative real numbers{ γ k }and{ δ k } (depending on p) such that k = 1 γ k <, k = 1 δ k <and the following statements hold:

  1. (i)

    T i n k y ( i 1 ) k p(1+ γ k ) y ( i 1 ) k p, for all i=1,,m;

  2. (ii)

    y i k p ( 1 + γ k ) i x k p, for all i=1,,m1;

  3. (iii)

    x k + 1 p(1+ δ k ) x k p;

  4. (iv)

    if C is bounded, then lim k x k p exists.

Proof Let pF and γ k = b n k (p) for all kN. Then k = 1 γ k <.

  1. (i)

    For i=1,2,,m, we have

    T i n k y ( i 1 ) k p (1+ γ k ) y ( i 1 ) k p.
  2. (ii)

    By (9), we obtain

    y 1 k p = ( 1 t 1 k ) ( x k p ) + t 1 k ( T 1 n k x k p ) ( 1 t 1 k ) x k p + t 1 k T 1 n k x k p ( 1 t 1 k ) x k p + t 1 k ( 1 + γ k ) x k p ( 1 + γ k ) x k p .

We assume that y j k p ( 1 + γ k ) j x k p holds for some j=1,,m2. From part (i), we have

y ( j + 1 ) k p = ( 1 t ( j + 1 ) k ) ( y j k p ) + t ( j + 1 ) k ( T j + 1 n k y j k p ) ( 1 t ( j + 1 ) k ) y j k p + t ( j + 1 ) k T j + 1 n k y j k p ( 1 t ( j + 1 ) k ) y j k p + t ( j + 1 ) k ( 1 + γ k ) y j k p ( 1 + γ k ) y j k p ( 1 + γ k ) ( 1 + γ k ) j x k p = ( 1 + γ k ) j + 1 x k p .

By mathematical induction, we obtain

y i k p ( 1 + γ k ) i x k p,for alli=1,,m1.
  1. (iii)

    By part (ii), we get

    x k + 1 p = ( 1 t m k ) ( y ( m 1 ) k p ) + t m k ( T m n k y ( m 1 ) k p ) ( 1 t m k ) y ( m 1 ) k p + t m k T m n k y ( m 1 ) k p ( 1 t m k ) y ( m 1 ) k p + t m k ( 1 + γ k ) y ( m 1 ) k p ( 1 + γ k ) y ( m 1 ) k p ( 1 + γ k ) ( 1 + γ k ) m 1 x k p ( 1 + γ k ) m x k p ( 1 + δ k ) x k p ,

where δ k = ( m 1 ) γ k + ( m 2 ) γ k 2 ++ ( m m ) γ k m . Since k = 1 γ k <, then k = 1 δ k <.

  1. (iv)

    By part (iii), we have x k + 1 p x k p+diam(C) δ k for all kN. Thus, for each lN,

    x k + l p x k p+diam(C) i = k k + l 1 δ i .

Since i = 1 δ i <, lim sup k lim sup l i = k k + l 1 δ i =0. The conclusion follows from Lemma 2.6 by letting r k = x k p and d k , l =diam(C) i = k k + l 1 δ i . □

Lemma 3.2 Let C be a nonempty bounded closed convex subset of a uniformly convex Banach space X and T 1 ,, T m T r (C). Let { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (9) is well defined. Assume thatF:= i = 1 m F( T i ). Then

  1. (i)

    lim k y ( i 1 ) k T i n k y ( i 1 ) k =0, for all i=1,2,,m;

  2. (ii)

    lim k x k T i n k y ( i 1 ) k =0, for all i=1,2,,m;

  3. (iii)

    If the set J={kN: n k + 1 =1+ n k } is quasi-periodic, then lim k x k T i x k =0, for all i=1,2,,m.

Proof (i) Let pF, then by Lemma 3.1(iv) we have lim k x k p exists. Let

lim k x k p=c.
(10)

By (10) and Lemma 3.1(ii), we get that

lim sup k y i k pc,for alli=1,,m1.
(11)

Note that

x k + 1 p ( 1 t m k ) y ( m 1 ) k p + t m k T m n k y ( m 1 ) k p ( 1 t m k ) y ( m 1 ) k p + t m k ( 1 + γ k ) y ( m 1 ) k p ( 1 + γ k ) y ( m 1 ) k p = ( 1 + γ k ) ( 1 t ( m 1 ) k ) ( y ( m 2 ) k p ) + t ( m 1 ) k ( T m 1 n k y ( m 2 ) k p ) ( 1 + γ k ) ( ( 1 t ( m 1 ) k ) y ( m 2 ) k p + t ( m 1 ) k ( 1 + r k ) y ( m 2 ) k p ) ( 1 + γ k ) 2 y ( m 2 ) k p ( 1 + γ k ) m i y i k p ,

for all i=1,,m1. So that

c lim inf k y i k p,for alli=1,,m1.
(12)

From (11) and (12), we have

lim k y i k p=c,for alli=1,2,,m1.
(13)

That is,

lim k ( 1 t i k ) ( y ( i 1 ) k p ) + t i k ( T i n k y ( i 1 ) k p ) =c,for alli=1,2,,m1.
(14)

By Lemma 3.1(i) and (13), we get that

lim sup k T i n k y ( i 1 ) k p c,for allj=1,2,,m1.
(15)

By (11), (14), (15), and Lemma 2.7, we obtain

lim k y ( i 1 ) k T i n k y ( i 1 ) k =0,for alli=1,2,,m1.
(16)

For the case i=m, by Lemma 3.1(i), we have

T m n k y ( m 1 ) k p (1+ γ k ) y ( m 1 ) k p.

This implies by (13) that

lim sup k T m n k y ( m 1 ) k p c.
(17)

Moreover,

lim k ( 1 t m k ) ( y ( m 1 ) k p ) + t m k ( T m n k y ( m 1 ) k p ) = lim k x k + 1 p=c.

Again, by Lemma 2.7, we get that

lim k y ( m 1 ) k T m n k y ( m 1 ) k =0.
(18)

Thus, (16) and (18) imply that

lim k y ( i 1 ) k T i n k y ( i 1 ) k =0,for alli=1,,m.
(19)
  1. (ii)

    From (9), we have

    y i k y ( i 1 ) k = t i k T i n k y ( i 1 ) k y ( i 1 ) k ,for alli=1,,m1.

By (19), we obtain

lim k y i k y ( i 1 ) k =0,fori=1,,m1.
(20)

From

x k y i k x k y 1 k + y 1 k y 2 k ++ y ( i 1 ) k y i k ,for alli=1,,m1,

it follows by (20) that

lim k x k y i k =0,for alli=1,,m1.
(21)

From

x k T i n k y ( i 1 ) k x k y ( i 1 ) k + y ( i 1 ) k T i n k y ( i 1 ) k ,

it implies by (19) and (21) that

lim k x k T i n k y ( i 1 ) k =0,for alli=1,2,,m.
(22)

(iii) For i=1, from (ii) we have

lim k T 1 n k x k x k =0.
(23)

If i=2,3,,m, then

T i n k x k x k T i n k x k T i n k y ( i 1 ) k + T i n k y ( i 1 ) k x k a n k ( x k ) x k y ( i 1 ) k + T i n k y ( i 1 ) k x k .

By (21), (22), and lim sup k a n k ( x k )=1, we get

lim sup k T i n k x k x k =0for alli=2,3,,m.
(24)

By (23) and (24), we have

lim k T i n k x k x k =0for alli=1,2,,m.
(25)

From (9), we have

x k + 1 x k ( 1 t m k ) y ( m 1 ) k x k + t m k T m n k y ( m 1 ) k x k ( 1 t m k ) y ( m 1 ) k x k + t m k ( T m n k y ( m 1 ) k y ( m 1 ) k + y ( m 1 ) k x k ) = y ( m 1 ) k x k + t m k T m n k y ( m 1 ) k y ( m 1 ) k .

From (19) and (21),

lim k x k + 1 x k =0.
(26)

The proof of the remaining part is identical to the proof of [25], Lemma 4.8(iii)] upon replacing d(,) with . □

By using Lemma 3.1 and the argument in the proof of [26], Theorem 3.2], we can obtain the following result.

Lemma 3.3 Let C be a nonempty bounded closed convex subset of a Banach space X and T 1 ,, T m T r (C). Let { t i k } k = 1 [0,1]and{ n k }Nbe such that{ x k }in (9) is well defined. Assume thatF:= i = 1 m F( T i ). Then{ x k }converges strongly to a point in F if and only if lim inf k dist( x k ,F)=0.

Theorem 3.4 Let X be a uniformly convex Banach space with the Opial property and C be a nonempty bounded closed convex subset of X. Let T 1 ,, T m T r (C)be such thatF:= i = 1 m F( T i ). { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (9) is well defined. If the setJ={kN: n k + 1 =1+ n k }is quasi-periodic, then the sequence{ x k }converges weakly to a common fixed point of the family{ T i :i=1,,m}.

Proof We have by Lemma 3.1 that lim n x k p exists for every pF. We shall prove that { x k } has a unique weak subsequential limit in F. For this, we suppose that there are subsequences { x k l } and { x k j } of { x k } which converge weakly to u and v, respectively. By Lemma 3.2(iii), lim k T i x k x k =0 for all i=1,,m. It follows from Lemma 2.9 that u,vF( T i ) for all i=1,,m. That is u,vF. Finally, we prove that u=v. Suppose not, then by the Opial property we get that

lim k x k u = lim l x k l u < lim l x k l v = lim k x k v = lim j x k j v < lim j x k j u = lim k x k u .

This is a contradiction. Therefore, the proof is complete. □

Theorem 3.5 Let X be a uniformly convex Banach space and C be a nonempty bounded closed convex subset of X. Let T 1 ,, T m T r (C)be such that T i l is semi-compact for somei{1,,m}andlN. { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (9) is well defined. Suppose thatF:= i = 1 m F( T i )and the setJ={kN: n k + 1 =1+ n k }is quasi-periodic. Then{ x k }converges strongly to a common fixed point of the family{ T i :i=1,2,,m}.

Proof By Lemma 3.2, we have

lim k x k T i x k =0,for alli=1,,m.
(27)

Let i{1,,m} be such that T i l is semi-compact. Thus, by Lemma 2.8,

lim k x k T i l x k =0.

We can also find a subsequence { x n j } of { x k } such that lim j x k j =qC. Hence, from (27), we have

q T i q= lim j x k j T i x k j =0,for alli=1,,m.

Thus qF. Therefore, { x k j } converges strongly to qF. But since lim k x k q exists, { x k } must itself converges to q. This completes the proof. □

Theorem 3.6 Let X be a uniformly convex Banach space and C be a nonempty bounded closed convex subset of X. Let{ T 1 ,, T m } T r (C)be satisfy Condition ( A ). Let { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (9) is well defined. Suppose thatF:= i = 1 m F( T i )and the setJ={kN: n k + 1 =1+ n k }is quasi-periodic. Then{ x k }converges strongly to a common fixed point of the family{ T i :i=1,2,,m}.

Proof By Lemma 3.2, lim k x k T i x k =0, for all i=1,2,,m. By using Condition ( A ), there exists a nondecreasing function f:[0,)[0,) with f(0)=0, f(r)>0 for r(0,) such that

lim k f ( dist ( x k , F ) ) lim k x k T j x k =0for somej=1,,m.

This implies that lim k dist( x k ,F)=0. The conclusion follows from Lemma 3.3. □

3.2 Results for unbounded domains

To relax the boundedness of the domains we have to add some condition on the sequence { b n k }.

Lemma 3.7 Let C be a nonempty closed convex subset of a Banach space X and T 1 ,, T m T r (C)be such thatF:= i = 1 m F( T i ). Let { t i k } k = 1 [0,1]and{ n k }Nbe such that{ x k }in (9) is well defined. Assume that k = 1 sup x C b n k (x)<. Then forpF, we have lim k x k pexists.

Proof Similar to the proof of Lemma 3.1, we can show that x k + 1 p(1+ η k ) x k p for all kN, where η k = ( m 1 ) s k + ( m 2 ) s k 2 ++ ( m m ) s k m and s k = sup x C b n k (x). By assumption, we have k = 1 s k i < for all i=1,,m. It follows that k = 1 η k <. By Lemma 2.5, we get that lim k x k p exists. □

By using Lemma 3.7 and the argument in Section 3.1 we can obtain the following results.

Lemma 3.8 Let C be a nonempty closed convex subset of a Banach space X and T 1 ,, T m T r (C)be such thatF:= i = 1 m F( T i ). Let { t i k } k = 1 [0,1]and{ n k }Nbe such that{ x k }in (9) is well defined. Assume that k = 1 sup x C b n k (x)<. Then

  1. (i)

    lim k y ( i 1 ) k T i n k y ( i 1 ) k =0, for all i=1,2,,m;

  2. (ii)

    lim k x k T i n k y ( i 1 ) k =0, for all i=1,2,,m;

  3. (iii)

    If the set J={kN: n k + 1 =1+ n k } is quasi-periodic, then lim k x k T i x k =0, for all i=1,2,,m.

Lemma 3.9 Let C be a nonempty closed convex subset of a Banach space X and T 1 ,, T m T r (C)be such thatF:= i = 1 m F( T i ). Let { t i k } k = 1 [0,1]and{ n k }Nbe such that{ x k }in (9) is well defined. Assume that k = 1 sup x C b n k (x)<. Then{ x k }converges strongly to a point in F if and only if lim inf k dist( x k ,F)=0.

Theorem 3.10 Let X be a uniformly convex Banach space with the Opial property and C be a nonempty closed convex subset of X. Let T 1 ,, T m T r (C)be such thatF:= i = 1 m F( T i ). { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (9) is well defined. Assume that k = 1 sup x C b n k (x)<and the setJ={kN: n k + 1 =1+ n k }is quasi-periodic. Then the sequence{ x k }converges weakly to a common fixed point of the family{ T i :i=1,,m}.

Theorem 3.11 Let X be a uniformly convex Banach space and C be a nonempty closed convex subset of X. Let T 1 ,, T m T r (C)be such that T i l is semi-compact for somei{1,,m}andlN, { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (9) is well defined. Suppose that k = 1 sup x C b n k (x)<, F:= i = 1 m F( T i )and the setJ={kN: n k + 1 =1+ n k }is quasi-periodic. Then{ x k }converges strongly to a common fixed point of the family{ T i :i=1,2,,m}.

Theorem 3.12 Let X be a uniformly convex Banach space and C be a nonempty closed convex subset of X. Let{ T 1 ,, T m } T r (C)be satisfy Condition ( A ). Let { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (9) is well defined. Suppose that k = 1 sup x C b n k (x)<, F:= i = 1 m F( T i )and the setJ={kN: n k + 1 =1+ n k }is quasi-periodic. Then{ x k }converges strongly to a common fixed point of the family{ T i :i=1,2,,m}.

4 Results in CAT(0) spaces

A metric space X is a CAT(0) space if it is geodesically connected, and if every geodesic triangle in X is at least as thin as its comparison triangle in the Euclidean plane. It is well known that any complete, simply connected Riemannian manifold having nonpositive sectional curvature is a CAT(0) space. Other examples include Pre-Hilbert spaces (see [32]), R-trees (see [33]), Euclidean buildings (see [34]), the complex Hilbert ball with a hyperbolic metric (see [35]), and many others. For a thorough discussion of these spaces and of the fundamental role they play in geometry, we refer the reader to Bridson and Haefliger [32].

Let x,yX, by Lemma 2.1(iv) of [36] for each t[0,1], there exists a unique point z[x,y] such that

d(x,z)=td(x,y)andd(y,z)=(1t)d(x,y).
(28)

From now on, we will use the notation (1t)xty for the unique point z satisfying (28).

Let { x n } be a bounded sequence in a metric space (X,d). For xX, we set

r ( x , { x n } ) = lim sup n d(x, x n ).

The asymptotic radiusr({ x n }) of { x n } is given by

r ( { x n } ) =inf { r ( x , { x n } ) : x X } ,

and the asymptotic centerA({ x n }) of { x n } is the set

A ( { x n } ) = { x X : r ( x , { x n } ) = r ( { x n } ) } .

It is known from Proposition 7 of [37] that in a CAT(0) space, A({ x n }) consists of exactly one point. We now give the definition of Δ-convergence.

Definition 4.1[38, 39]

A sequence { x n } in a metric space X is said to Δ-converge to xX if x is the unique asymptotic center of { u n } for every subsequence { u n } of { x n }. In this case we write Δ lim n x n =x and call x the Δ-limit of { x n }.

Let C be a nonempty closed convex subset of a CAT(0) space X and fix x 1 C. Define a sequence { x k } in C as

(29)

where T 1 ,, T m T(C), { t i k } k = 1 are sequences in [0,1] for all i=1,2,,m, and { n k } be an increasing sequence of natural numbers.

By using the argument in Section 3 together with the results in [25, 36, 40, 41], we can also obtain the analogous results for CAT(0) spaces.

Theorem 4.2 Let C be a nonempty closed convex subset of a complete CAT(0) space X. Let T 1 ,, T m T r (C)be such thatF:= i = 1 m F( T i ), { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (29) is well defined. Suppose that either C is bounded or k = 1 sup x C b n k (x)<. If the setJ={kN: n k + 1 =1+ n k }is quasi-periodic, then the sequence{ x k } Δ-converges to a common fixed point of the family{ T i :i=1,,m}.

Theorem 4.3 Let C be a nonempty closed convex subset of a complete CAT(0) space X. Let T 1 ,, T m T r (C)be such thatF:= i = 1 m F( T i )and T i l is semi-compact for somei{1,,m}andlN. Let { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (29) is well defined. Suppose that either C is bounded or k = 1 sup x C b n k (x)<. If the setJ={kN: n k + 1 =1+ n k }is quasi-periodic, then{ x k }converges strongly to a common fixed point of the family{ T i :i=1,2,,m}.

Theorem 4.4 Let C be a nonempty closed convex subset of a complete CAT(0) space X. Let{ T 1 ,, T m } T r (C)be satisfy Condition ( A ) andF:= i = 1 m F( T i ). Let { t i k } k = 1 [a,b](0,1)and{ n k }Nbe such that{ x k }in (29) is well defined. Suppose that either C is bounded or k = 1 sup x C b n k (x)<. If the setJ={kN: n k + 1 =1+ n k }is quasi-periodic, then{ x k }converges strongly to a common fixed point of the family{ T i :i=1,2,,m}.

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Acknowledgements

This article is dedicated to Professor Anthony To-Ming Lau for celebrating his great achievements in the development of fixed point theory and applications. It was supported by the Centre of Excellence in Mathematics, the Commission on Higher Education, Thailand. Bancha Nanjaras also thanks the Graduate School of Chiang Mai University, Thailand.

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Correspondence to Bancha Panyanak.

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Both the authors contributed equally and significantly in writing this paper. All authors read and approved the final manuscript.

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Keywords

  • common fixed point
  • asymptotic pointwise nonexpansive mapping
  • weak convergence
  • strong convergence
  • Banach space
  • CAT(0) space