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Approximating Fixed Points of Nonexpansive Nonself Mappings in CAT(0) Spaces
Fixed Point Theory and Applications volume 2010, Article number: 367274 (2009)
Abstract
Suppose that is a nonempty closed convex subset of a complete CAT(0) space with the nearest point projection from onto . Let be a nonexpansive nonself mapping with . Suppose that is generated iteratively by , , , where and are real sequences in for some . Then converges to some point in . This is an analog of a result in Banach spaces of Shahzad (2005) and extends a result of Dhompongsa and Panyanak (2008) to the case of nonself mappings.
1. Introduction
A metric space is a CAT(0) space if it is geodesically connected and if every geodesic triangle in 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 space. Other examples include PreHilbert spaces, trees (see [1]), Euclidean buildings (see [2]), the complex Hilbert ball with a hyperbolic metric (see [3]), and many others. For a thorough discussion of these spaces and of the fundamental role they play in geometry see Bridson and Haefliger [1]. The work by Burago et al. [4] contains a somewhat more elementary treatment, and by Gromov [5] a deeper study.
Fixed point theory in a CAT(0) space was first studied by Kirk (see [6, 7]). He showed that every nonexpansive (singlevalued) mapping defined on a bounded closed convex subset of a complete CAT(0) space always has a fixed point. Since then the fixed point theory for singlevalued and multivalued mappings in CAT(0) spaces has been rapidly developed and much papers have appeared (see, e.g., [8–19]).
In 2008, Kirk and Panyanak [20] used the concept of convergence introduced by Lim [21] to prove the CAT(0) space analogs of some Banach space results which involve weak convergence, and Dhompongsa and Panyanak [22] obtained convergence theorems for the Picard, Mann and Ishikawa iterations in the CAT(0) space setting.
The purpose of this paper is to study the iterative scheme defined as follows. Let is a nonempty closed convex subset of a complete CAT(0) space with the nearest point projection from onto . If is a nonexpansive mapping with nonempty fixed point set, and if is generated iteratively by
where and are real sequences in for some we show that the sequence defined by (1.1) converges to a fixed point of This is an analog of a result in Banach spaces of Shahzad [23] and also extends a result of Dhompongsa and Panyanak [22] to the case of nonself mappings. It is worth mentioning that our result immediately applies to any CAT() space with since any CAT() space is a CAT() space for every (see [1, page 165]).
2. Preliminaries and Lemmas
Let be a metric space. A geodesic path joining to (or, more briefly, a geodesic from to ) is a map from a closed interval to such that and for all In particular, is an isometry and The image of is called a geodesic (or metric) segment joining and . When it is unique this geodesic segment is denoted by . The space is said to be a geodesic space if every two points of are joined by a geodesic, and is said to be uniquely geodesic if there is exactly one geodesic joining and for each A subset is said to be convex if includes every geodesic segment joining any two of its points.
A geodesic triangle in a geodesic metric space consists of three points in (thevertices of ) and a geodesic segment between each pair of vertices (the edges of ). A comparison triangle for the geodesic triangle in is a triangle in the Euclidean plane such that for
A geodesic space is said to be a CAT(0) space if all geodesic triangles of appropriate size satisfy the following comparison axiom.
CAT(0):Let be a geodesic triangle in and let be a comparison triangle for . Then is said to satisfy the CAT(0) inequality if for all and all comparison points
If are points in a CAT(0) space and if is the midpoint of the segment then the CAT(0) inequality implies
This is the (CN) inequality of Bruhat and Tits [24]. In fact (cf. [1, page 163]), a geodesic space is a CAT(0) space if and only if it satisfies the (CN) inequality.
We now collect some elementary facts about CAT(0) spaces which will be used frequently in the proofs of our main results.
Lemma 2.1.
Let be a CAT(0) space.

(i)
[1, Proposition 2.4] Let be a convex subset of which is complete in the induced metric. Then, for every there exists a unique point such that Moreover, the map is a nonexpansive retract from onto

(ii)
[22, Lemma 2.1(iv)] For and there exists a unique point such that
(2.2)
one uses the notation for the unique point satisfying (2.2).

(iii)
[22, Lemma 2.4] For and one has
(2.3)

(iv)
[22, Lemma 2.5] For and one has
(2.4)
Let be a nonempty subset of a CAT(0) space and let be a mapping. is called nonexpansive if for each
A point is called a fixed point of if . We shall denote by the set of fixed points of The existence of fixed points for nonexpansive nonself mappings in a CAT(0) space was proved by Kirk [6] as follows.
Theorem 2.2.
Let be a bounded closed convex subset of a complete CAT(0) space . Suppose that is a nonexpansive mapping for which
Then has a fixed point in
Let be a bounded sequence in a CAT(0) space . For we set
The asymptotic radius of is given by
and the asymptotic center of is the set
It is known (see, e.g., [12, Proposition ]) that in a CAT(0) space, consists of exactly one point.
We now give the definition of convergence.
Definition 2.3 (see [20, 21]).
A sequence in a CAT(0) space is said to converge to if is the unique asymptotic center of for every subsequence of . In this case one writes  and call the limit of
The following lemma was proved by Dhompongsa and Panyanak (see [22, Lemma ]).
Lemma 2.4.
Let be a closed convex subset of a complete CAT(0) space and let be a nonexpansive mapping. Suppose is a bounded sequence in such that and converges for all , then Here where the union is taken over all subsequences of Moreover, consists of exactly one point.
We now turn to a wider class of spaces, namely, the class of hyperbolic spaces, which contains the class of CAT(0) spaces (see Lemma 2.8).
Definition 2.5 (see [16]).
A hyperbolic space is a triple where is a metric space and is such that
(W1)
(W2)
(W3)
(W4)
for all
It follows from (W1) that for each and
In fact, we have
since if
we get
which is a contradiction. By comparing between (2.2) and (2.11), we can also use the notation for in a hyperbolic space
Definition 2.6 (see [16]).
The hyperbolic space is called uniformly convex if for any and there exists a such that for all
A mapping providing such a for given and is called a modulus of uniform convexity.
Lemma 2.7 (see [16, Lemma ]).
Let be a uniformly convex hyperbolic with modulus of uniform convexity For any , and
Lemma 2.8 (see [16, Proposition ]).
Assume that is a CAT(0) space. Then is uniformly convex, and
is a modulus of uniform convexity.
The following result is a characterization of uniformly convex hyperbolic spaces which is an analog of Lemma of Schu [25]. It can be applied to a CAT(0) space as well.
Lemma 2.9.
Let be a uniformly convex hyperbolic space with modulus of convexity , and let . Suppose that increases with (for a fixed ) and suppose that is a sequence in for some and , are sequences in such that , and for some Then
Proof.
The case is trivial. Now suppose . If it is not the case that as then there are subsequences, denoted by and , such that
Choose such that
Since and This implies Choose such that
Since
there are further subsequences again denoted by and , such that
Then by Lemma 2.7 and (2.20),
for all Taking we obtain
which contradicts to the hypothesis.
3. Main Results
In this section, we prove our main theorems.
Theorem 3.1.
Let be a nonempty closed convex subset of a complete CAT(0) space and let be a nonexpansive mapping with Let and be sequences in for some Starting from arbitrary define the sequence by the recursion (1.1). Then exists.
Proof.
By Lemma 2.1(i) the nearest point projection is nonexpansive. Then
Consequently, we have
This implies that is bounded and decreasing. Hence exists.
Theorem 3.2.
Let be a nonempty closed convex subset of a complete CAT(0) space and let be a nonexpansive mapping with Let and be sequences in for some From arbitrary define the sequence by the recursion (1.1). Then
Proof.
Let Then, by Theorem 3.1, exists. Let
If then by the nonexpansiveness of the conclusion follows. If , we let By Lemma 2.1(iv) we have
Therefore
It follows from (3.6) and Lemma 2.1(iv) that
Therefore
where Since
By (3.8), we have
This implies
Since is nonexpansive, we get that and hence
On the other hand, we can get from (3.6) that
Thus . This fact and (3.6) imply
Since is nonexpansive,
It follows from (3.4), (3.12), (3.13), and Lemma 2.9 that
This completes the proof.
The following theorem is an analog of [23, Theorem ] and extends [22, Theorem ] to nonself mappings.
Theorem 3.3.
Let be a nonempty closed convex subset of a complete CAT(0) space and let be a nonexpansive mapping with Let and be sequences in for some From arbitrary define the sequence by the recursion (1.1). Then converges to a fixed point of
Proof.
By Theorem 3.2, It follows from (3.2) that is bounded and decreasing for each and so it is convergent. By Lemma 2.4, consists of exactly one point and is contained in . This shows that the sequence converges to an element of
We now state two strong convergence theorems. Recall that a mapping is said to satisfy Condition I ([26]) if there exists a nondecreasing function with and for all such that
Theorem 3.4.
Let be a nonempty closed convex subset of a complete CAT(0) space and let be a nonexpansive mapping with Let and be sequences in for some From arbitrary define the sequence by the recursion (1.1). Suppose that satisfies condition I. Then converges strongly to a fixed point of
Theorem 3.5.
Let be a nonempty compact convex subset of a complete CAT(0) space and let be a nonexpansive mapping with Let and be sequences in for some From arbitrary define the sequence by the recursion (1.1). Then converges strongly to a fixed point of
Another result in [23] is that the author obtains a common fixed point theorem of two nonexpansive selfmappings. The proof is metric in nature and carries over to the present setting. Therefore, we can state the following result.
Theorem 3.6.
Let be a nonempty closed convex subset of a complete CAT(0) space and let be two nonexpansive mappings with Let and be sequences in for some From arbitrary define the sequence by the recursion
Then converges to a common fixed point of and
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Acknowledgments
The authors are grateful to Professor Sompong Dhompongsa for his suggestions and advices during the preparation of the article. The second author was supported by the Commission on Higher Education and Thailand Research Fund under Grant MRG5280025. This work is dedicated to Professor Wataru Takahashi.
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Laowang, W., Panyanak, B. Approximating Fixed Points of Nonexpansive Nonself Mappings in CAT(0) Spaces. Fixed Point Theory Appl 2010, 367274 (2009). https://doi.org/10.1155/2010/367274
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Keywords
 Nonexpansive Mapping
 Hyperbolic Space
 Common Fixed Point
 Nonempty Closed Convex Subset
 Uniform Convexity