Does Kirk's Theorem Hold for Multivalued Nonexpansive Mappings?
© T. Domínguez Benavides and B. Gavira. 2010
Received: 25 September 2009
Accepted: 29 December 2009
Published: 17 January 2010
Fixed Point Theory for multivalued mappings has many useful applications in Applied Sciences, in particular, in Game Theory and Mathematical Economics. Thus, it is natural to try of extending the known fixed point results for single-valued mappings to the setting of multivalued mappings. Some theorems of existence of fixed points of single-valued mappings have already been extended to the multivalued case. However, many other questions remain still open, for instance, the possibility of extending the well-known Kirk's Theorem, that is: do Banach spaces with weak normal structure have the fixed point property (FPP) for multivalued nonexpansive mappings? There are many properties of Banach spaces which imply weak normal structure and consequently the FPP for single-valued mappings (for example, uniform convexity, nearly uniform convexity, uniform smoothness,…). Thus, it is natural to consider the following problem: do these properties also imply the FPP for multivalued mappings? In this way, some partial answers to the problem of extending Kirk's Theorem have appeared, proving that those properties imply the existence of fixed point for multivalued nonexpansive mappings. Here we present the main known results and current research directions in this subject. This paper can be considered as a survey, but some new results are also shown.
The presence or absence of a fixed point (i.e., a point which remains invariant under a map) is an intrinsic property of a map. However, many necessary or sufficient conditions for the existence of such points involve a mixture of algebraic, topological, or metric properties of the mapping or its domain. By Metric Fixed Point Theory, we understand the branch of Fixed Point Theory concerning those results which depend on a metric and which are not preserved when this metric is replaced by another equivalent metric. The first metric fixed point theorem was given by Banach in 1922.
Theorem 1.1 (Banach Contraction Principle, ).
Banach Theorem is a basic tool in Functional Analysis, Nonlinear Analysis and Differential Equations. Thus, it is natural to look for some generalizations under weaker assumptions.
For many years Metric Fixed Point Theory just studied some extensions of Banach Theorem relaxing the contractiveness condition, and the extension of this result for multivalued mappings. In the 1960s, Metric Fixed Point Theory received a strong boost when Kirk  proved that every (singlevalued) nonexpansive mapping , defined from a convex closed bounded subset of a reflexive Banach space with normal structure, has a fixed point.
The celebrated Kirk's theorem had a profound impact in the development of Fixed Point Theory and iniciated the search of more general conditions for a Banach space and for a subset which assure the existence of fixed points.
The result obtained by Kirk is, in some sense, surprising because it uses geometric properties of Banach spaces (commonly used in Linear Functional Analysis, but rarely considered in Nonlinear Analysis until then). Thus, it is the starting point for a new mathematical field: the application of the Geometric Theory of Banach Spaces to Fixed Point Theory. From that moment on, many researchers have tried to exploit this connection, essentially considering some other geometric properties of Banach spaces which can be applied to prove the existence of fixed points for different types of nonlinear operators (e.g., uniform smoothness, Opial property, nearly uniform convexity, nearly uniform smoothness, etc.).
Fixed Point Theory for multivalued mappings has useful applications in Applied Sciences, in particular, in Game Theory and Mathematical Economics. Thus, it is natural to study the problem of the extension of the known fixed point results for singlevalued mappings to the setting of multivalued mappings.
Some theorems of existence of fixed points of single-valued mappings have already been extended to the multivalued case. For example, in 1969 Nadler  extended the Banach Contraction Principle to multivalued contractive mappings in complete metric spaces. However, many other questions remain open, for instance, the possibility of extending the well-known Kirk's Theorem , that is, do Banach spaces with weak normal structure have the fixed point property (FPP) for multivalued nonexpansive mappings?
There are many properties of Banach spaces which imply weak normal structure and consequently the FPP for singlevalued mappings (e.g., uniform convexity, nearly uniform convexity, uniform smoothness, …). Thus, it is natural to consider the following problem: Do these properties also imply the FPP for multivalued mappings? As a consequence, some partial answers to the problem of extending Kirk's Theorem have appeared, which are directed to prove that those properties imply the existence of fixed point for multivalued nonexpansive mappings.
Here we present the main known results and current research directions in this subject. This paper can be considered as a survey, but some new results are also included.
In this section we recall the notion of normal structure and some properties of Banach spaces which imply normal structure.
Normal structure plays an essential role in some problems of Metric Fixed Point Theory, especially those concerning nonexpansive mappings. The notion of normal structure was introduced by Brodskiĭ and Mil'man  in 1948 in order to study fixed points of isometries. Later, the notion of normal structure was generalized for the weak topology.
In 1965 Kirk  obtained a strong connection between normal structure and the FPP for nonexpansive mappings.
Let be a bounded closed (resp., weakly compact) convex subset of a Banach space and let be a nonexpansive mapping (i.e., for every ). If is a reflexive Banach space with normal structure (resp., a Banach space with -NS), then has a fixed point.
Bynum  defined two coefficients related to normal structure and weak normal structure.
We recall that is said to have uniform normal structure (UNS) (resp., weak uniform normal structure ( -UNS)) if (resp., ). Notice that this is not the common definition of weak uniform normal structure and is often known as Bynum's condition. It is known that if has uniform normal structure, then is reflexive .
In the latest fifty years, some geometrical properties implying normal structure have been studied. Here we are going to recall some of these properties and some results which prove that these properties imply the existence of fixed point for multivalued mappings.
First we consider the Opial property. Opial  was the first who studied such a property giving applications to Fixed Point Theory. The uniform Opial property was defined in  by Prus, and the Opial modulus was introduced in  by Lin et al.
under the same conditions.
There are some relationships between the notions of Opial property and normal structure. If is a Banach space which satisfies the Opial property, then has -NS . On the other hand, [9, Theorem ]. Consequently, has -UNS if .
The Clarkson modulus and the coefficient of normal structure are related by the following inequality: . Consequently, the condition implies that is reflexive and has uniform normal structure. In particular, notice that not only do uniformly convex spaces have normal structure, but so do all those spaces which do not have segments of length greater than or equal to near the unit sphere.
In 1980 Huff  initiated the study of nearly uniform convexity which is an infinite-dimensional generalization of uniform convexity. Independently of Huff, Goebel and Sękowski  also introduced a property which is equivalent to nearly uniform convexity under the name of noncompact uniform convexity. It is known that a Banach space is nearly uniformly convex (NUC) if and only if
When is a reflexive Banach space, is the separation measure and is the Hausdorff measure (for definitions see, for instance,  or ), we have the following relationships among the different moduli:
On the other hand, if (in particular, if is NUC), then is reflexive and has weak uniform normal structure (see [13, page 125]).
It is known that implies that is reflexive and has uniform normal structure [15–17]. However, the infinite-dimensional generalization of uniform smoothness, nearly uniform smoothness, does not imply normal structure [13, Example ].
3. Some Properties Implying Weak Normal Structure and the FPP for Multivalued Mappings
In this section we are going to show some results which prove that some properties implying weak normal structure also imply the existence of fixed point for multivalued nonexpansive mappings. As a consequence these results give some partial answers to the problem of extending Kirk's Theorem.
Throughout this section (resp., ) will denote the family of all nonempty compact (resp., compact convex) subsets of . We recall that a multivalued mapping is said to be nonexpansive if for every , where denotes the Hausdorff metric given by
In 1973 Lami Dozo gave the following result of existence of fixed point for those spaces which satisfy the Opial property.
Theorem 3.1 (Lami Dozo [18, Theorem ]).
Theorem 3.2 (Lim ).
In 1990 Kirk and Massa proved the following partial generalization of Lim's Theorem using asymptotic centers of sequences and nets. We recall that, given a bounded sequence in a Banach space and a subset of , the asymptotic center of with respect to is defined by
Theorem 3.3 (Kirk and Massa ).
We do not know a complete characterization of those spaces in which asymptotic centers of bounded sequences are compact. Nevertheless, there are some partial answers, for example, -uniformly convex Banach spaces satisfy that condition . However, an example given by Kuczumov and Prus  shows that in nearly uniformly convex spaces, the asymptotic center of a bounded sequence with respect to a closed bounded convex subset is not necessarily compact. Therefore, the problem of obtaining fixed point results in nearly uniformly convex spaces remained open. This question (together with the same question for uniformly smooth spaces) explicitly appeared in a survey about Metric Fixed Point Theory for multivalued mappings published by Xu  in 2000.
The analysis of the importance of the asymptotic center in Kirk-Massa Theorem led Domínguez Benavides and Lorenzo to study some connections between asymptotic centers and the geometry of certain spaces, including nearly uniformly convex spaces. Thus, in  Domínguez and Lorenzo obtained the following relationship between the Chebyshev radius of the asymptotic center of a bounded sequence and the modulus of noncompact convexity with respect to the measures and .
Theorem 3.4 (see [25, Theorem ]).
The previous inequalities give an iterative method which reduces at each step the value of the Chebyshev radius for a chain of asymptotic centers. Consequently, Domínguez and Lorenzo deduced in  the following partial extension of Kirk's Theorem which, in particular, assures that nearly uniformly convex spaces have the fixed point property for multivalued nonexpansive mappings.
Theorem 3.5 (see [26, Theorem ]).
This result guarantees, in particular, the existence of fixed points in nearly uniformly convex spaces (because if is NUC), giving a positive answer to one of the previous open problems proposed by Xu.
Dhompongsa et al.  observed that the main tool used in the proofs in [25, 26], in order to obtain fixed point results for multivalued nonexpansive mappings, is a relationship between the Chebyshev radius of the asymptotic center of a bounded sequence and the asymptotic radius of the sequence. This relationship also gives an iterative method which reduces at each step the value of the Chebyshev radius for a chain of asymptotic centers. Consequently, in [27, 28] they introduced the Domínguez-Lorenzo condition ((DL)-condition, in short) and property (D) in the following way.
From the definition it is easy to deduce that property (D) is weaker than the (DL)-condition. Dhompongsa et al. proved in [28, Theorem ] and [28, Theorem ] that property (D) implies -NS and the FPP for multivalued nonexpansive mappings.
Theorem 3.7 (see [28, Theorem ]).
Theorem 3.8 (see [28, Theorem ]).
From Theorem 3.5 every Banach space with satisfies the (DL)-condition. In this paper we present some other properties concerning geometrical constants of Banach spaces which also imply the (DL)-condition or property (D).
Since our goal is to study if properties implying -NS also imply the FPP for multivalued mappings, a possible approach to that problem is to study if these properties imply either the (DL)-condition or property (D). These results will give only partial answers to the problem of extending Kirk's Theorem for multivalued mappings because we know that uniform normal structure does not imply property (D) ([29, Proposition ]); therefore, the problem of extending Kirk's Theorem cannot be fully solved by this approach. In this setting the following results have been obtained.
Theorem 3.9 (Dhompongsa et al. [27, Theorem ]).
Theorem 3.10 (Dhompongsa et al. [28, Theorem ]).
Theorem 3.11 (Domínguez Benavides and Gavira [29, Corollary ]).
Theorem 3.12 (Domínguez Benavides and Gavira [29, Corollary ]).
Theorem 3.13 (Saejung [30, Theorem ]).
Theorem 3.14 (Kaewkhao [31, Corollary ]).
Theorem 3.16 (Kaewkhao [31, Theorem ]).
Theorem 3.17 (Gavira [32, Theorem ]).
Finally we show a new result which gives a property implying the (DL)-condition in terms of Clarkson modulus and the García-Falset coefficient.
In  it is proved that has normal structure under the slightly weaker condition
It is an open question if this condition implies the (DL)-condition.
4. Fixed Point Results for Multivalued Nonexpansive Mappings in Modular Function Spaces
The theory of modular spaces was initiated by Nakano  in 1950 in connection with the theory of order spaces and redefined and generalized by Musielak and Orlicz  in 1959. Even though a metric is not defined, many problems in metric fixed point theory can be reformulated and solved in modular spaces (see, for instance, [36–39]). In particular, Dhompongsa et al.  have obtained some fixed point results for multivalued mappings in modular functions spaces.
Let be a nonempty set and a nontrivial -algebra of subsets of . Let be a -ring of subsets of , such that for any and . Let us assume that there exists an increasing sequence of sets such that (for instance, can be the class of sets of finite measure in a -finite measure space). By we denote the linear space of all simple functions with supports from . By we will denote the space of all measurable functions, that is, all functions such that there exist a sequence , and for all .
Note that a countable union of -null sets is still -null. In the sequel we will identify sets and whose symmetric difference is -null, similarly we will identify measurable functions which differ only on a -null set.
Under the above conditions, we define the function by . We know from  that satisfies the following properties:
In addition, if the following property is satisfied
defines a norm which is frequently called the Luxemburg norm. The formula
We know by  that under the -condition the norm convergence and modular convergence are equivalent, which implies that the norm and modular convergence are also the same when we deal with the -type condition. In the sequel we will assume that the modular function is convex and satisfies the -type condition. Hence, the -convergence defines a topology which is identical to the norm topology.
In the same way as the Hausdorff distance defined on the family of bounded closed subsets of a metric space, we can define the analogue to the Hausdorff distance for modular function spaces. We will speak of -Hausdorff distance even though it is not a metric.
Dhompongsa et al.  stated the Banach Contraction Principle for multivalued mappings in modular function spaces.
Theorem 4.7 (see [40, Theorem ]).
Theorem 4.8 (see [40, Theorem ]).
They also applied the above theorem to obtain fixed point results in the Banach space (resp., ) for multivalued mappings whose domains are compact in the topology of the convergence locally in measure (resp., -topology).
Consider the space for a -finite measure with the usual norm. Let be a bounded closed convex subset of for and a multivalued nonexpansive mapping. Because of uniform convexity of , it is known that has a fixed point. For , can fail to have a fixed point even in the singlevalued case for a weakly compact convex set (see ). However, since is a modular space where for all , Theorem 4.8 implies the existence of a fixed point when we define mappings on a -a.e. compact -bounded convex subset of . Thus the following can be stated.
Corollary 4.9 (see [40, Corollary ]).
then is a fixed point free map. However, if we consider where , for all , then -a.e. convergence and -convergence are identical on bounded subsets of (see ). This fact leads to the following corollary.
Corollary 4.10 (see [40, Corollary ]).
Domínguez et al. introduced in  some compactness conditions concerning proximinal subsets called Property (P). Following this idea we will use the following similar notion for modular function spaces.
Lemma 4.12 (see [40, Lemma ]).
Lemma 4.13 (see [37, Lemma ]).
Let be a convex function modular satisfying the -type condition, a nonempty -closed -bounded convex subset of satisfying Property such that every sequence in has a -a.e. convergent subsequence in , and a -nonexpansive mapping. Then has a fixed point.
Now we define the mapping by . From [45, Proposition ] we know that the mapping is upper semicontinuous. Since is a nonempty -compact convex set and the -topology is a norm-topology, we can apply the Kakutani-Bohnenblust-Karlin Theorem (see ) to obtain a fixed point for and hence for .
If we apply the previous theorem in the particular case of the space for a -finite measure with the usual norm, we obtain the following result, which can be also deduced from [44, Theorem ].
Let be as above, a nonempty closed bounded convex set which satisfies Property (P). Suppose, in addition, that every sequence in has a convergent locally in measure subsequence in . If is a nonexpansive mapping, then has a fixed point.
Example 4.17 (see [44, Example ]).
Let be a bounded sequence of nonnegative real numbers and let be the standard Schauder basis of . It is clear that the set , where , is never weakly star compact. Nevertheless, by using [46, Example ] it is easy to show that has Property (P) if and only if is nonempty and finite.
The authors are very grateful to the anonymous referee for some useful suggestions to improve the presentation of this paper. This research was partially supported by DGES Grant no.BFM2006-13997-C02-01 and Junta de Andalucía Grant no.FQM-127. This research is dedicated to W. A. Kirk celebrating his wide and deep contribution in Metric Fixed Point Theory.
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