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Convergence theorems for total asymptotically nonexpansive non-self mappings in CAT(0) spaces

Abstract

In this paper, we introduce the concept of total asymptotically nonexpansive nonself mappings and prove the demiclosed principle for this kind of mappings in CAT(0) spaces. As a consequence, we obtain a Δ-convergence theorem of total asymptotically nonexpansive nonself mappings in CAT(0) spaces. Our results extend and improve the corresponding recent results announced by many authors.

MSC:47H09, 47J25.

1 Introduction

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. That is to say, let (X,d) be a metric space, and let x,yX with d(x,y)=l. A geodesic path from x to y is an isometry c:[0,l]X such that c(0)=x and c(l)=y. The image of a geodesic path is called a geodesic segment. A metric space X is a (uniquely) geodesic space if every two points of X are joined by only one geodesic segment. A geodesic triangle Δ( x 1 , x 2 , x 3 ) in a geodesic space X consists of three points x 1 , x 2 , x 3 of X and three geodesic segments joining each pair of vertices. A comparison triangle of the geodesic triangle Δ( x 1 , x 2 , x 3 ) is the triangle Δ ¯ ( x 1 , x 2 , x 3 ):=Δ( x 1 ¯ , x 2 ¯ , x 3 ¯ ) in the Euclidean space R 2 such that

d( x i , x j )= d R 2 ( x i ¯ , x j ¯ ),i,j=1,2,3.

A geodesic space X is a CAT(0) space if for each geodesic triangle Δ( x 1 , x 2 , x 3 ) in X and its comparison triangle Δ ¯ :=Δ( x 1 ¯ , x 2 ¯ , x 3 ¯ ) in R 2 , the CAT(0) inequality

d(x,y) d R 2 ( x ¯ , y ¯ )
(1.1)

is satisfied for all x,yΔ and x ¯ , y ¯ Δ ¯ .

A thorough discussion on these spaces and their important role in various branches of mathematics is given in [15].

In 1976, Lim [6] introduced the concept of Δ-convergence in a general metric space. Fixed point theory in a CAT(0) space was first studied by Kirk [7, 8]. He showed that every nonexpansive (single-valued) mapping defined on a bounded closed convex subset of a complete CAT(0) space always has a fixed point. In 2008, Kirk and Panyanak [9] specialized Lim’s concept to CAT(0) spaces and proved that it is very similar to the weak convergence in the Banach space setting. So, the fixed point and Δ-convergence theorems for single-valued and multivalued mappings in CAT(0) spaces have been rapidly developed and many papers have appeared [1025].

Let (X,d) be a metric space. Recall that a mapping T:XX is said to be nonexpansive if

d(Tx,Ty)d(x,y),x,yX.
(1.2)

T is said to be asymptotically nonexpansive, if there is a sequence { k n }[1,) with k n 1 such that

d ( T n x , T n y ) k n d(x,y),n1,x,yX.
(1.3)

T is said to be ({ μ n },{ ν n },ζ)-total asymptotically nonexpansive, if there exist nonnegative sequences { μ n }, { ν n } with μ n 0, ν n 0 and a strictly increasing continuous function ζ:[0,)[0,) with ζ(0)=0 such that

d ( T n x , T n y ) d(x,y)+ ν n ζ ( d ( x , y ) ) + μ n ,n1,x,yX.
(1.4)

Let (X,d) be a metric space, and let C be a nonempty and closed subset of X. Recall that C is said to be a retract of X if there exists a continuous map P:XC such that Px=x, xC. A map P:XC is said to be a retraction if P 2 =P. If P is a retraction, then Py=y for all y in the range of P.

Definition 1.1 Let X and C be the same as above. A mapping T:CX is said to be ({ μ n },{ ν n },ζ)-total asymptotically nonexpansive nonself mapping if there exist nonnegative sequences { μ n }, { ν n } with μ n 0, ν n 0 and a strictly increasing continuous function ζ:[0,)[0,) with ζ(0)=0 such that

d ( T ( P T ) n 1 x , T ( P T ) n 1 y ) d(x,y)+ ν n ζ ( d ( x , y ) ) + μ n ,n1,x,yC,
(1.5)

where P is a nonexpansive retraction of X onto C.

Remark 1.2 From the definitions, it is to know that each nonexpansive nonself mapping is an asymptotically nonexpansive nonself mapping with a sequence { k n =1}, and each asymptotically nonexpansive mapping is a ({ μ n },{ ν n },ζ)-total asymptotically nonexpansive mapping with μ n =0, ν n = k n 1, n1 and ζ(t)=t, t0.

Definition 1.3 A nonself mapping T:CX is said to be uniformly L-Lipschitzian if there exists a constant L>0 such that

d ( T ( P T ) n 1 x , T ( P T ) n 1 y ) Ld(x,y),n1,x,yC.
(1.6)

Recently, Chang et al. [26] introduced the following Krasnoselskii-Mann type iteration for finding a fixed point of a total asymptotically nonexpansive mappings in CAT(0) spaces.

x n + 1 =(1 α n ) x n α n T n x n ,n1.
(1.7)

Under some limit conditions, they proved that the sequence { x n } Δ-converges to a fixed point of T.

Inspired and motivated by the recent work of Chang et al. [26], Tang et al. [27], Laowang et al. [19] and so on, the purpose of this paper is to introduce the concept of total asymptotically nonexpansive nonself mappings and prove the demiclosed principle for this kind of mappings in CAT(0) spaces. As a consequence, we obtain a Δ-convergence theorem of total asymptotically nonexpansive nonself mappings in CAT(0) spaces. The results presented in this paper improve and extend the corresponding recent results in [19, 26, 27].

2 Preliminaries

The following lemma plays an important role in our paper.

In this paper, we write (1t)xty for the unique point z in the geodesic segment joining from x to y such that

d(z,x)=td(x,y),d(z,y)=(1t)d(x,y).
(2.1)

We also denote by [x,y] the geodesic segment joining from x to y, that is, [x,y]={(1t)xty:t[0,1]}.

A subset C of a CAT(0) space is convex if [x,y]C for all x,yC.

Lemma 2.1 [18]

A geodesic space X is a CAT(0) space if and only if the following inequality holds:

d 2 ( ( 1 t ) x t y , z ) (1t) d 2 (x,z)+t d 2 (y,z)t(1t) d 2 (x,y)
(2.2)

for all x,y,zX and all t[0,1]. In particular, if x, y, z are points in a CAT(0) space and t[0,1], then

d ( ( 1 t ) x t y , z ) (1t)d(x,z)+td(y,z).
(2.3)

Let { x n } be a bounded sequence in a CAT(0) space X. For xX, we set

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

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

r ( { x n } ) =inf { r ( x , { x n } ) : x X } .
(2.4)

The asymptotic radius r C ({ x n }) of { x n } with respect to CX is given by

r C ( { x n } ) =inf { r ( x , { x n } ) : x C } .
(2.5)

The asymptotic center A({ x n }) of { x n } is the set

A ( { x n } ) = { x X : r ( x , { x n } ) = r ( { x n } ) } .
(2.6)

And the asymptotic center A C ({ x n }) of { x n } with respect to CX is the set

A C ( { x n } ) = { x C : r ( x , { x n } ) = r C ( { x n } ) } .
(2.7)

Recall that a bounded sequence { x n } in X is said to be regular if r({ x n })=r({ u n }) for every subsequence { u n } of { x n }.

Proposition 2.2 [14]

Let X be a complete CAT(0) space, let { x n } be a bounded sequence in X, and let C be a closed convex subset of X. Then

  1. (1)

    there exists a unique point uC such that

    r ( u , { x n } ) = inf x C r ( x , { x n } ) ;
  2. (2)
    A({ x n })

    and A C ({ x n }) both are singleton.

Definition 2.3 [6, 9]

Let X be a CAT(0) space. A sequence { x n } in X is said to Δ-converge to pX if p is the unique asymptotic center of { u n } for each subsequence { u n } of { x n }. In this case we write Δ- lim n x n =p and call p the Δ-limit of { x n }.

Lemma 2.4 [9]

Every bounded sequence in a complete CAT(0) space always has a Δ-convergent subsequence.

Lemma 2.5 [11]

Let X be a complete CAT(0) space, and let C be a closed convex subset of X. If { x n } is a bounded sequence in C, then the asymptotic center of { x n } is in C.

Remark 2.6 Let X be a CAT(0) space, and let C be a closed convex subset of X. Let { x n } be a bounded sequence in C. In what follows, we denote

{ x n }wΦ(w)= inf x C Φ(x),

where Φ(x):= lim sup n d( x n ,x).

Now we give a connection between the ‘’ convergence and Δ-convergence.

Proposition 2.7 [26]

Let X be a CAT(0) space, let C be a closed convex subset of X, and let { x n } be a bounded sequence in C. Then Δ- lim n x n =p implies that { x n }p.

Lemma 2.8 Let C be a closed and convex subset of a complete CAT(0) space X, and let T:CX be a uniformly L-Lipschitzian and ({ μ n },{ ν n },ζ)-total asymptotically nonexpansive nonself mapping. Let { x n } be a bounded sequence in C such that { x n }p and lim n d( x n ,T x n )=0. Then Tp=p.

Proof By the definition, { x n }p if and only if A C ({ x n })={p}. By Lemma 2.5, we have A({ x n })={p}.

Since lim n d( x n ,T x n )=0, by induction we can prove that

lim n d ( x n , T ( P T ) m x n ) =0for each m0.
(2.8)

In fact, it is obvious that the conclusion is true for m=0. Suppose that the conclusion holds for m1, now we prove that the conclusion is also true for m+1.

Indeed, since T is uniformly L-Lipschitzian, we have

d ( x n , T ( P T ) m x n ) d ( x n , T ( P T ) m 1 x n ) + d ( T ( P T ) m 1 x n , T ( P T ) m x n ) d ( x n , T ( P T ) m 1 x n ) + L d ( x n , P T x n ) = d ( x n , T ( P T ) m 1 x n ) + L d ( P x n , P T x n ) d ( x n , T ( P T ) m 1 x n ) + L d ( x n , T x n ) 0 ( as  n ) .

(2.8) is proved. Hence for each xC and m1, from (2.8), we have

Φ(x):= lim sup n d( x n ,x)= lim sup n d ( T ( P T ) m 1 ( x n ) , x ) .
(2.9)

In (2.9) taking x=T ( P T ) m 1 p, m1, we have

Φ ( T ( P T ) m 1 p ) = lim sup n d ( T ( P T ) m 1 ( x n ) , T ( P T ) m 1 p ) lim sup n { d ( x n , p ) + ν m ζ ( d ( x n , p ) ) + μ m } .

Letting m and taking the superior limit on the both sides, we get that

lim sup m Φ ( T ( P T ) m 1 p ) Φ(p).
(2.10)

Furthermore, for any n,m1, it follows from inequality (2.2) with t= 1 2 that

d 2 ( x n , p T ( P T ) m 1 ( p ) 2 ) 1 2 d 2 ( x n , p ) + 1 2 d 2 ( x n , T ( P T ) m 1 ( p ) ) 1 4 d 2 ( p , T ( P T ) m 1 ( p ) ) .
(2.11)

Letting n and taking the superior limit on the both sides of the above inequality, for any m1, we get

Φ ( p T ( P T ) m 1 ( p ) 2 ) 2 1 2 Φ ( p ) 2 + 1 2 Φ ( T ( P T ) m 1 ( p ) ) 2 1 4 d 2 ( p , T ( P T ) m 1 ( p ) ) .
(2.12)

Since A({ x n })={p}, for any m1, we have

Φ ( p ) 2 Φ ( p T ( P T ) m 1 ( p ) 2 ) 2 1 2 Φ ( p ) 2 + 1 2 Φ ( T ( P T ) m 1 ( p ) ) 2 1 4 d 2 ( p , T ( P T ) m 1 ( p ) ) ,
(2.13)

which implies that

d 2 ( p , T ( P T ) m 1 ( p ) ) 2Φ ( T ( P T ) m 1 ( p ) ) 2 2Φ ( p ) 2 .
(2.14)

By (2.10) and (2.14), we have lim m d(p,T ( P T ) m 1 p)=0. Hence we have

d ( T p , p ) d ( T p , T ( P T ) m p ) + d ( T ( P T ) m p , p ) L d ( p , T ( P T ) m 1 p ) + d ( T ( P T ) m p , p ) 0 ( as  m ) ,

i.e., p=Tp, as desired. □

The following result can be obtained from Lemma 2.8 immediately.

Lemma 2.9 Let C be a closed and convex subset of a complete CAT(0) space X, and let T:CX be an asymptotically nonexpansive nonself mapping with a sequence { k n }[1,), k n 1. Let { x n } be a bounded sequence in C such that lim n d( x n ,T x n )=0 and Δ- lim n x n =p. Then Tp=p.

Lemma 2.10 [26]

Let X be a CAT(0) space, let xX be a given point, and let { t n } be a sequence in [b,c] with b,c(0,1) and 0<b(1c) 1 2 . Let { x n } and { y n } be any sequences in X such that

lim sup n d ( x n , x ) r , lim sup n d ( y n , x ) r and lim n d ( ( 1 t n ) x n t n y n , x ) = r ,

for some r0. Then

lim n d( x n , y n )=0.
(2.15)

Lemma 2.11 [26]

Let { a n }, { λ n } and { c n } be the sequences of nonnegative numbers such that

a n + 1 (1+ λ n ) a n + c n ,n1.

If n = 1 λ n < and n = 1 c n <, then lim n a n exists. If there exists a subsequence of { a n } which converges to zero, then lim n a n =0.

Lemma 2.12 [18]

Let X be a complete CAT(0) space, and let { x n } be a bounded sequence in X with A({ x n })={p}; { u n } is a subsequence of { x n } with A({ u n })={u}, and the sequence {d( x n ,u)} converges, then p=u.

3 Main results

Theorem 3.1 Let C be a nonempty, closed and convex subset of a complete CAT(0) space E. Let T i :CE be a uniformly L-Lipschitzian and total asymptotically nonexpansive nonself mapping with sequences { μ n ( i ) } and { υ n ( i ) } satisfying lim n μ n ( i ) =0 and lim n υ n ( i ) =0, and strictly increasing function ξ ( i ) :[0,)[0,) with ξ ( i ) (0)=0, i=1,2. For arbitrarily chosen x 1 K, { x n } is defined as follows:

{ y n = P ( ( 1 β n ) x n β n T 2 ( P T 2 ) n 1 x n ) , x n + 1 = P ( ( 1 α n ) x n α n T 1 ( P T 1 ) n 1 y n ) ,
(3.1)

where { μ n ( 1 ) }, { μ n ( 2 ) }, { υ n ( 1 ) }, { υ n ( 2 ) }, ξ ( 1 ) , ξ ( 2 ) , { α n } and { β n } satisfy the following conditions:

  1. (1)
    n = 1 μ n ( i ) <

    , n = 1 υ n ( i ) <, i=1,2;

  2. (2)

    there exist constants a,b(0,1) with 0<b(1a) 1 2 such that { α n }[a,b] and { β n }[a,b];

  3. (3)

    there exists a constant M >0 such that ξ ( i ) (r) M r, r0, i=1,2.

Then the sequence { x n } defined in (3.1) Δ-converges to a common fixed point of T 1 and T 2 .

Proof We divide the proof into three steps.

Step 1. We first show that lim n d( x n ,q) exists for each qF( T 1 )F( T 2 ).

Set μ n =max{ μ n ( 1 ) , μ n ( 2 ) } and υ n ={ υ n ( 1 ) , υ n ( 2 ) }, n=1,2,,. Since n = 1 μ n ( i ) <, n = 1 υ n ( i ) <, i=1,2, we know that n = 1 μ n < and n = 1 υ n <. For any qF( T 1 )F( T 2 ), we have

d ( x n + 1 , q ) = d ( P ( ( 1 α n ) x n α n T 1 ( P T 1 ) n 1 y n ) , q ) d ( ( 1 α n ) x n α n T 1 ( P T 1 ) n 1 y n , q ) ( 1 α n ) d ( x n , q ) + α n d ( T 1 ( P T 1 ) n 1 y n , q ) ( 1 α n ) d ( x n , q ) + α n [ d ( y n , q ) + υ n ξ ( 1 ) ( d ( y n , q ) ) + μ n ] ( 1 α n ) d ( x n , q ) + α n [ ( 1 + υ n M ) d ( y n , q ) + μ n ] ,
(3.2)

where

d ( y n , q ) = d ( P ( ( 1 β n ) x n β n T 2 ( P T 2 ) n 1 x n ) , q ) d ( ( 1 β n ) x n β n T 2 ( P T 2 ) n 1 x n , q ) ( 1 β n ) d ( x n , q ) + β n d ( T 2 ( P T 2 ) n 1 x n , q ) ( 1 β n ) d ( x n , q ) + β n [ d ( x n , q ) + υ n ξ ( 2 ) ( d ( x n , q ) ) + μ n ] ( 1 + β n υ n M ) d ( x n , q ) + β n μ n .
(3.3)

Substituting (3.3) into (3.2), we have

d ( x n + 1 , q ) ( 1 α n ) d ( x n , q ) + α n [ ( 1 + υ n M ) ( ( 1 + β n υ n M ) d ( x n , q ) + β n μ n ) + μ n ] [ 1 + ( 1 + β n + β n υ n M ) α n M υ n ] d ( x n , q ) + ( 1 + β n ) ( 1 + υ n M ) α n μ n .
(3.4)

Since n = 1 μ n < and n = 1 υ n <, it follows from Lemma 2.11 that lim n d( x n ,q) exists for each qF( T 1 )F( T 2 ).

Step 2. We show that lim n d( x n , T 1 x n )= lim n d( x n , T 2 x n )=0.

For each qF( T 1 )F( T 2 ), from the proof of Step 1, we know that lim n d( x n ,q) exists. We may assume that lim n d( x n ,q)=k. From (3.3), we have

d( y n ,q) ( 1 + β n υ n M ) d( x n ,q)+ β n μ n .
(3.5)

Taking lim sup on both sides in (3.5), we have

lim sup n d( y n ,q)k.
(3.6)

In addition, since

d ( T 1 ( P T 1 ) n 1 y n , q ) d ( y n , q ) + υ n ξ ( 2 ) ( d ( y n , q ) ) + μ n ( 1 + υ n M ) d ( y n , q ) + μ n ,

we have

lim sup n d ( T 1 ( P T 1 ) ( n 1 ) y n , q ) k.
(3.7)

Since lim n d( x n + 1 ,q)=k, it is easy to prove that

lim n d ( ( 1 α n ) x n α n T 1 ( P T 1 ) ( n 1 ) y n , q ) =k.
(3.8)

It follows from Lemma 2.10 that

lim n d ( x n , T 1 ( P T 1 ) n 1 y n ) =0.
(3.9)

On the other hand, since

d ( x n , q ) d ( x n , T 1 ( P T 1 ) n 1 y n ) + d ( T 1 ( P T 1 ) n 1 y n , q ) d ( x n , T 1 ( P T 1 ) n 1 y n ) + d ( y n , q ) + υ n M d ( y n , q ) + μ n = d ( x n , T 1 ( P T 1 ) n 1 y n ) + ( 1 + υ n M ) d ( y n , q ) + μ n ,

we have lim inf n d( y n ,q)k. Combined with (3.6), it yields that

lim n d( y n ,q)=k.
(3.10)

This implies that

lim n d ( ( 1 β n ) x n β n T 2 ( P T 2 ) n 1 x n , q ) =k.
(3.11)

It is easy to show that

lim sup n d ( T 2 ( P T 2 ) n 1 x n , q ) k.
(3.12)

So, it follows from (3.12) and Lemma 2.10 that

lim n d ( x n , T 2 ( P T 2 ) n 1 x n ) =0.
(3.13)

Observe that

d ( x n , T 1 ( P T 1 ) n 1 x n ) d ( x n , T 1 ( P T 1 ) n 1 y n ) + d ( T 1 ( P T 1 ) n 1 y n , T 1 ( P T 1 ) n 1 x n ) d ( x n , T 1 ( P T 1 ) n 1 y n ) + d ( x n , y n ) + υ n ξ ( 1 ) ( d ( x n , y n ) ) + μ n = d ( x n , T 1 ( P T 1 ) n 1 y n ) + ( 1 + υ n M ) d ( x n , y n ) + μ n ,
(3.14)

where

d ( x n , y n ) = d ( P ( ( 1 β n ) x n β n T 2 ( P T 2 ) n 1 x n ) , x n ) β n d ( x n , T 2 ( P T 2 ) n 1 x n ) .
(3.15)

It follows from (3.13) that

lim n d( x n , y n )=0.
(3.16)

Thus, from (3.9), (3.14) and (3.16), we have

lim n d ( x n , T 1 ( P T 1 ) n 1 x n ) =0.
(3.17)

In addition, since

d ( x n + 1 , x n ) = d ( P ( ( 1 α n ) x n α n T 1 ( P T 1 ) n 1 y n ) , x n ) d ( ( 1 α n ) x n α n T 1 ( P T 1 ) n 1 y n , x n ) α n d ( T 1 ( P T 1 ) n 1 y n , x n ) ,

from (3.9), we have

lim n d( x n + 1 , x n )=0.
(3.18)

Finally, since

d ( x n , T 1 x n ) d ( x n , x n + 1 ) + d ( x n + 1 , T 1 ( P T 1 ) n x n + 1 ) + d ( T 1 ( P T 1 ) n x n + 1 , T 1 ( P T 1 ) n x n ) + d ( T 1 ( P T 1 ) n x n , T 1 x n ) ( 1 + L ) d ( x n , x n + 1 ) + d ( x n + 1 , T 1 ( P T 1 ) n x n + 1 ) + L d ( T 1 ( P T 1 ) n 1 x n , x n ) ,

it follows from (3.17) and (3.18) that lim n d( x n , T 1 x n )=0. Similarly, we also can show that lim n d( x n , T 2 x n )=0.

Step 3. We show that { x n } Δ-converges to a common fixed point of T 1 and T 2 .

Let W ω ( x n )= { u n } { x n } A({ u n }). Firstly, we show that W ω F( T 1 )F( T 2 ). Let u W ω , then there exists a subsequence { u n } of { x n } such that A({ u n })={u}. By Lemma 2.4 and Lemma 2.5, there exists a subsequence { u n i } of { u n } such that Δ- lim i u n i =pK. Since lim n d( x n , T 1 x n )= lim n d( x n , T 2 x n )=0, it follows from Lemma 2.8 that pF( T 1 )F( T 2 ). So, lim n d( x n ,p) exists. By Lemma 2.12, we know that p=uF( T 1 )F( T 2 ). This implies that W ω ( x n )F( T 1 )F( T 2 ). Next, let { u n } be a subsequence of { x n } with A({ u n })={u} and A{ x n }={v}. Since u W ω ( x n )F( T 1 )F( T 2 ), lim n d( x n ,u) exists. By Lemma 2.12, we know that v=u. This implies that W ω ( x n ) contains only one point. Thus, since W ω ( x n )F( T 1 )F( T 2 ), W ω ( x n ) contains only one point and lim n d( x n ,q) exists for each qF( T 1 )F( T 2 ), we know that { x n } Δ-converges to a common fixed point of T 1 and T 2 . The proof is completed. □

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Acknowledgements

The authors would like to express their thanks to the referees for their helpful comments and suggestions. This work was supported by the Natural Scientific Research Foundation of Yunnan Province (Grant No. 2011FB074).

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Correspondence to Shih-sen Chang.

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Wang, L., Chang, S. & Ma, Z. Convergence theorems for total asymptotically nonexpansive non-self mappings in CAT(0) spaces. J Inequal Appl 2013, 135 (2013). https://doi.org/10.1186/1029-242X-2013-135

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Keywords

  • total asymptotically nonexpansive nonself mappings
  • CAT(0) space
  • Δ-convergence
  • demiclosed principle