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Common fixed points of two multivalued nonexpansive maps in Kohlenbach hyperbolic spaces
Fixed Point Theory and Applications volume 2014, Article number: 181 (2014)
In this paper, we introduce an iteration scheme for two multivalued maps in Kohlenbach hyperbolic spaces. This extends the single-valued iteration process due to Agarwal et al. (J. Nonlinear Convex Anal. 8(1):61-79, 2007). Using this new algorithm, we approximate common fixed points of two multivalued mappings through △-convergence and strong convergence under some weaker conditions. A necessary and sufficient condition is given for strong convergence.
MSC: 47A06, 47H09, 47H10, 49M05.
1 Introduction and preliminaries
A subset K of a metric space X is proximinal if for each , there exists an element such that
Let , and be the families of closed and bounded subsets, compact subsets and proximinal bounded subsets of K, respectively. Let H be the Hausdorff metric induced by the metric d of X, that is,
for every . A multivalued map is nonexpansive if
for all . A point is a fixed point of T if . Denote the set of all fixed points of T by and .
We consider the following definition of a hyperbolic space introduced by Kohlenbach .
Definition 1 A metric space is a hyperbolic space if there exists a map satisfying
for all and .
In the sequel, we shall use the term hyperbolic space instead of Kohlenbach hyperbolic space for the sake of simplicity.
A metric space is called a convex metric space introduced by Takahashi  if it satisfies only (i). A subset K of a hyperbolic space X is convex if for all and .
A hyperbolic space is uniformly convex  if for any , , and , there exists a such that whenever , and .
A map which provides such a for given and , is known as the modulus of uniform convexity. We call η monotone if it decreases with r (for a fixed ϵ).
Different definitions of hyperbolic space can be found in the literature (see for example [1, 4–6], for a comparison). The hyperbolic space introduced by Kohlenbach  is slightly restrictive than the space of hyperbolic type  but general than hyperbolic space of . spaces and Banach spaces are the examples of Kohlenbach hyperbolic spaces. Moreover, this class of hyperbolic spaces also contains Hadamard manifolds, Hilbert balls equipped with the hyperbolic metric , ℝ-trees and Cartesian products of Hilbert balls as special cases.
The study of fixed points for multivalued nonexpansive maps using Hausdorff metric was initiated by Markin  (see also ). The existence of fixed points for multivalued nonexpansive mappings in convex metric spaces has been shown by Shimizu and Takahashi . Actually, they obtained the following.
Theorem ST ()
Let be a bounded, complete and uniformly convex metric space. Then every multivalued map (the family of all compact subsets of X) has a fixed point.
Later, an interesting and rich fixed point theory for such maps was developed which has applications in control theory, convex optimization, differential inclusion and economics (see  and references cited therein). Since then many authors have published papers on the existence and convergence of fixed points for multivalued nonexpansive maps in convex metric spaces.
The theory of multivalued nonexpansive maps is harder than the corresponding theory of single valued nonexpansive maps. Different iterative algorithms have been used to approximate the fixed points of multivalued nonexpansive maps. Sastry and Babu  considered Mann and Ishikawa type iterative algorithms.
The following is a useful lemma due to Nadler .
Lemma Let and . If , then there exists such that .
Panyanak  proved some results using Ishikawa type iteration process without the condition on the mapping T. Based on the above lemma, Song and Wang  modified the iterative algorithm due to Panyanak  and improved the results presented therein. Song and Wang  showed that without this condition his process was not well defined. They reconstructed the process using the condition which made it well defined.
Recently, Shahzad and Zegeye  pointed out that the assumption for any is quite strong. In order to get rid of the condition for any , they used for a multivalued map and proved some strong convergence results using Mann and Ishikawa type iterative algorithms. Song and Cho  improved the results of  whereas Khan and Yildirim  used an iterative algorithm independent but faster than Ishikawa algorithm to further generalize the results of .
On the other hand Agarwal et al.  introduced the following iteration scheme for single valued mappings:
where . This scheme is independent of both Mann and Ishikawa schemes. They proved that this scheme converges at a rate faster than Picard and Mann iteration schemes for contractions. Following their method, it was observed in Example 3.7 of Khan and Kim  that this scheme also converges faster than Ishikawa iteration scheme. Two mappings case of the above scheme has also been considered by many authors including [20–22].
In this paper, we first give a two-mappings version of the algorithm (1.1) in hyperbolic spaces and use instead of a stronger condition for any to approximate common fixed points of two multivalued nonexpansive maps. We use the method of direct construction of Cauchy sequence as indicated by Song and Cho  (and opposed to ) but also used by many other authors including [17, 19, 23, 24]. Our algorithm in this paper is as follows:
Let K be a nonempty convex subset of a hyperbolic space X. Let be two multivalued maps and . Choose and define as
where , , and such that .
It follows from the definition of that for any x in K.
In order to verify that the algorithm (1.2) is well defined, define by
for some , and for some . Assume that and are nonexpansive multivalued mappings on K. For a given , the existence of is guaranteed if f has a fixed point. Now, for any , let , such that , and , such that .
On using (iv) of Definition 1, we have
Since , f is a contraction. By the Banach contraction principle, f has a unique fixed point. Thus the existence of is established. Continuing in this way, the existence of is guaranteed. Hence the above algorithm is well defined.
In 1976, Lim  introduced the concept △-convergence in metric spaces. In 2008, Kirk and Panyanak  specialized Lim’s concept to spaces and proved a number of results involving weak convergence in Banach spaces. Since then the notion of △-convergence has been widely studied and a number of articles have appeared e.g., [8, 9, 11, 12, 26, 27]. To reach the definition of △-convergence, we first recall the notions of asymptotic radius and asymptotic center as under:
Let be a bounded sequence in a metric space X. For , define a continuous functional by
of is called the asymptotic radius of with respect to ;
for any ,the set is called the asymptotic center of with respect to .
If the asymptotic radius and the asymptotic center are taken with respect to X, then these are simply denoted by and , respectively. In general, may be empty or may even contain infinitely many points. It is well known that a complete uniformly convex hyperbolic space with monotone modulus of uniform convexity enjoys the property that bounded sequences have unique asymptotic center with respect to closed convex subsets .
A sequence in X is said to △-converge to if x is the unique asymptotic center of for every subsequence of . In this case, we call x the △-limit of and write .
The following are the key results to be used in our main results.
Lemma 1.1 ()
Let K be a nonempty closed convex subset of a uniformly convex hyperbolic space and a bounded sequence in K with . If is another sequence in K such that , then .
Lemma 1.2 ()
Let be a uniformly convex hyperbolic space with monotone modulus of uniform convexity. Suppose that and are sequences in X and . Let be a sequence with . If , and for some , then .
2 Main results
The following lemma proved in  gives some properties of in metric (and hence hyperbolic) spaces.
Lemma 2.1 Let K be a nonempty subset of a metric space X and be a multivalued map. Then the following are equivalent:
, that is, ,
, that is, for each ,
, that is, .
In the sequel, denotes the set of all common fixed points of the multivalued maps S and T.
Lemma 2.2 Let K be a nonempty closed convex subset of a hyperbolic space X and let be two multivalued maps such that and are nonexpansive maps and . Then for the sequence in (1.2), exists for each .
Proof Let . Then and . Using (1.2), we have
Hence exists. □
Lemma 2.3 Let K be a nonempty closed convex subset of a uniformly convex hyperbolic space X and let be two multivalued maps such that and are nonexpansive and . Let and satisfy . Then for the sequence in (1.2), we have .
Proof By Lemma 2.2, exists for each . Assume that for some . For , the result is trivial. Suppose .
Now can be rewritten as
Since is nonexpansive, we have
Taking limsup, we have
Now (2.1) can be rewritten as
This implies that
and, in turn,
From , (2.2), (2.3), and Lemma 1.2, it follows that
Similarly we can show that
Since , therefore
We now prove △-convergence of the algorithm (1.2).
Theorem 2.4 Let K be a nonempty, closed, and convex subset of a uniformly convex hyperbolic space X with monotone modulus of uniform convexity η and S, T, , and be as in Lemma 2.3. Then △-converges to a common fixed point of S and T (or and ).
Proof By Lemma 2.2, is bounded, therefore has a unique asymptotic center. Thus . Let be any subsequence of such that . Then by Lemma 2.3. We now prove that u is a common fixed point of and . For this, take in . Then
This yields as . Lemma 1.1 gives . Note that being proximinal is closed, hence is closed. Moreover, is bounded. Consequently . Hence . Similarly, . Hence . Since exists (by Lemma 2.2), therefore by the uniqueness of asymptotic center, we have
a contradiction. Hence . Thus for every subsequence of . This proves that △-converges to a common fixed point of S and T (or and ). □
The following is a necessary and sufficient condition for the strong convergence of the algorithm (1.2).
Theorem 2.5 Let K be a nonempty, closed, and convex subset of a complete hyperbolic space X and S, T, , and be as in Lemma 2.2. Then the sequence converges strongly to if and only if .
Proof If converges to , then . Since , we have . To prove that the condition is also sufficient, assume that . By Lemma 2.2, we have
and so exists. By hypothesis , thus .
We now show that is a Cauchy sequence in K. Let and assume . Then it follows (along the lines similar to Lemma 2.2) that
Thus we have
Taking inf on the set F, we have . On letting , , the inequality shows that is a Cauchy sequence in K and hence converges, say to . Now it is left to show that . Indeed, by . So for each , there exists such that
This implies . From , it follows that
yields . Since ϵ is arbitrary, therefore . Similarly, we can show that . Since F is closed, as required.
Recall that a map is semi-compact if any bounded sequence satisfying as has a convergent subsequence.
We would also like to give here the definition of the so-called condition (A′). Let f be a nondecreasing selfmap on with and for all and let . Let be two multivalued maps with . Then the two maps are said to satisfy condition (A′) if
Applying Lemma 2.3, we can easily obtain the following.
Theorem 2.6 Let K be a nonempty closed convex subset of a complete and uniformly convex hyperbolic space X with monotone modulus of uniform convexity η and S, T, , and be as in Lemma 2.3. Suppose that a pair of maps and satisfies condition (A′), then the sequence defined in (1.2) converges strongly to .
Theorem 2.7 Let K be a nonempty closed convex subset of a uniformly convex hyperbolic space X with monotone modulus of uniform convexity η and S, T, , and be as in Lemma 2.3. Suppose that one of the map in and is semi-compact, then the sequence defined in (1.2) converges strongly to .
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The authors are greatly indebted to an anonymous referee for a very careful reading and pointing out necessary corrections.
The authors declare that they have no competing interests.
Both authors worked on the manuscript. Both read and approved the final manuscript.
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Khan, S.H., Abbas, M. Common fixed points of two multivalued nonexpansive maps in Kohlenbach hyperbolic spaces. Fixed Point Theory Appl 2014, 181 (2014). https://doi.org/10.1186/1687-1812-2014-181
- common fixed point
- hyperbolic space
- multivalued nonexpansive map
- strong convergence