Skip to main content
  • Research Article
  • Open access
  • Published:

Does Kirk's Theorem Hold for Multivalued Nonexpansive Mappings?

Abstract

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.

1. Introduction

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, [1]).

Let be a complete metric space and a contractive mapping, that is, there exists such that for every . Then has a (unique) fixed point . Moreover, for every .

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 [2] 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 [3] 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 [2], 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.

2. Preliminaries

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 [4] in 1948 in order to study fixed points of isometries. Later, the notion of normal structure was generalized for the weak topology.

Definition 2.1.

A Banach space is said to have normal structure (NS) (resp., weak normal structure (-NS)) if for every bounded closed (resp., weakly compact) convex subset of with there exists such that .

In 1965 Kirk [2] obtained a strong connection between normal structure and the FPP for nonexpansive mappings.

Theorem 2.2.

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 [5] defined two coefficients related to normal structure and weak normal structure.

Definition 2.3.

The normal structure coefficient of a Banach space is defined by

(2.1)

where denotes the eter of defined by and denotes the Chebyshev radius of defined by .

The weakly convergent sequence coefficient of is defined by

(2.2)

where the infimum is taken over all weakly convergent sequences which are not norm convergent, where,

(2.3)

denote the asymptotic eter and radius of respectively.

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 [6].

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 [7] was the first who studied such a property giving applications to Fixed Point Theory. The uniform Opial property was defined in [8] by Prus, and the Opial modulus was introduced in [9] by Lin et al.

Definition 2.4.

We will say that a Banach space satisfies the Opial property if for every weakly null sequence and every in we have

(2.4)

We will say that satisfies the nonstrict Opial property if

(2.5)

under the same conditions.

The Opial modulus of is defined for as

(2.6)

where the infimum is taken over all with and all weakly null sequences in with .

We will say that satisfies the uniform Opial property if for all .

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 [10]. On the other hand, [9, Theorem ]. Consequently, has -UNS if .

Next we study the uniform convexity of the space, which is another geometrical property related with normal structure. We recall that a Banach space is uniformly convex (UC) if and only if

(2.7)

for each , or equivalently

(2.8)

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 [11] initiated the study of nearly uniform convexity which is an infinite-dimensional generalization of uniform convexity. Independently of Huff, Goebel and Sękowski [12] 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

(2.9)

for each , or equivalently

(2.10)

where is a measure of noncompactness. Also we are going to use the following equivalent definition: is NUC if and only if is reflexive and

(2.11)

for each , or equivalently

(2.12)

When is a reflexive Banach space, is the separation measure and is the Hausdorff measure (for definitions see, for instance, [13] or [14]), we have the following relationships among the different moduli:

(2.13)

and consequently,

(2.14)

If the space satisfies the nonstrict Opial property, then coincides with .

On the other hand, if (in particular, if is NUC), then is reflexive and has weak uniform normal structure (see [13, page 125]).

The dual concept of uniform convexity is uniform smoothness which is also related to normal structure. A Banach space is said to be uniformly smooth (US) if

(2.15)

where is the modulus of smoothness of , defined by

(2.16)

for .

It is known that implies that is reflexive and has uniform normal structure [1517]. 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

(3.1)

for every bounded subsets and of .

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 ]).

Let be a Banach space which satisfies the Opial property, let be a weakly compact convex subset of and let be a nonexpansive mapping. Then has a fixed point, that is, there exists such that .

In 1974 Lim [19] gave a similar result for uniformly convex spaces using Edelstein's method of asymptotic centers [20].

Theorem 3.2 (Lim [19]).

Let be a uniformly convex Banach space, let be a closed bounded convex subset of and be a nonexpansive mapping. Then has a fixed point.

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

(3.2)

where denotes the asymptotic radius of with respect to defined by

(3.3)

Theorem 3.3 (Kirk and Massa [21]).

Let be a closed bounded convex subset of a Banach space and a nonexpansive mapping. If the asymptotic center in of each bounded sequence of is nonempty and compact, then has a fixed point.

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 [22]. However, an example given by Kuczumov and Prus [23] 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 [24] 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 [25] 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 ]).

Let be a closed convex subset of a reflexive Banach space and a bounded sequence in which is regular with respect to (i.e., the asymptotic radius is invariant for all subsequences of ). Then

(3.4)

where the Chebyshev radius of a bounded subset of relative to is defined by

(3.5)

Moreover, if satisfies the nonstrict Opial property, then

(3.6)

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 [26] 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 ]).

Let be a nonempty closed bounded convex subset of a Banach space such that . Let be a nonexpansive mapping. Then has a fixed point.

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. [27] 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.

We recall that a sequence is regular with respect to if for all subsequences of , and is asymptotically uniform with respect to if for all subsequences of .

Definition 3.6.

A Banach space is said to satisfy the (DL)-condition if there exists such that for every weakly compact convex subset of and for every bounded sequence in which is regular with respect to

(3.7)

A Banach space is said to satisfy property (D) if there exists such that for any nonempty weakly compact convex subset of , any bounded sequence in which is regular and asymptotically uniform with respect to , and any sequence which is regular and asymptotically uniform with respect to we have

(3.8)

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 ]).

Let be a Banach space satisfying property (D). Then has -NS.

Theorem 3.8 (see [28, Theorem ]).

Let be a nonempty weakly compact convex subset of a Banach space which satisfies property (D). Let be a nonexpansive mapping. Then has a fixed point.

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 ]).

Let be a uniformly nonsquare Banach space with property WORTH. Then satisfies the (DL)-condition.

We recall that a Banach space is uniformly nonsquare if there exists such that for every or equivalently where denotes the James constant of defined by

(3.9)

is said to satisfy property WORTH if

(3.10)

for any and any weakly null sequence in .

Theorem 3.10 (Dhompongsa et al. [28, Theorem ]).

Let be Banach space such that

(3.11)

where denotes the Jordan-von Neumann constant of defined by

(3.12)

Then satisfies property (D).

Theorem 3.11 (Domínguez Benavides and Gavira [29, Corollary ]).

Let be a Banach space such that

(3.13)

Then satisfies the (DL)-condition. In particular, uniformly smooth Banach spaces () satisfy the (DL)-condition.

Theorem 3.12 (Domínguez Benavides and Gavira [29, Corollary ]).

Let be a Banach space such that one of the following two equivalent conditions is satisfied

(1)

(2).

Then satisfies the (DL)-condition.

Theorem 3.13 (Saejung [30, Theorem ]).

A Banach space has property (D) whenever .

This result improves Theorem 3.10 because it is easy to see that .

Theorem 3.14 (Kaewkhao [31, Corollary ]).

Let be a Banach space such that

(3.14)

where denotes the James constant of defined by

(3.15)

and denotes the coefficient of worthwhileness of defined as the infimum of the set of real numbers such that

(3.16)

for all and all weakly null sequences in . Then satisfies the (DL)-condition.

Remark 3.15.

This result improves Theorem 3.9 because if is a uniformly nonsquare Banach space with property WORTH, then

(3.17)

Theorem 3.16 (Kaewkhao [31, Theorem ]).

Let be a Banach space such that

(3.18)

Then satisfies the (DL)-condition.

Theorem 3.17 (Gavira [32, Theorem ]).

Let be a Banach space such that

(3.19)

Then satisfies the (DL)-condition.

Remark 3.18 s.

  1. (i)

    This result is a strict generalization of Theorem 3.16 (see [32]).

  2. (ii)

    Theorem 3.17 applies to the Bynum space while Theorem 3.11 does not (see [32]). However, we do not know if implies .

Finally we show a new result which gives a property implying the (DL)-condition in terms of Clarkson modulus and the García-Falset coefficient.

Theorem 3.19.

Let be a Banach space such that

(3.20)

where denotes the García-Falset coefficient of defined by

(3.21)

Then satisfies the (DL)-condition.

Proof.

Let be a nonempty weakly compact convex subset of . Let be a bounded sequence in which is regular with respect to . Denote , and . By translation and multiplication we can assume that is weakly null and . Let , then . Denote by . By the definition of we have

(3.22)

For every there exists such that

(1)

(2)

(3)

(4)

Consider and . Using the above estimates we obtain

(3.23)

where tends to 0 as . Furthermore,

(3.24)

Also we have

(3.25)

Define and . Thus, .

Since for , we obtain

(3.26)

Consequently, we have

(3.27)

Since the last inequality is true for every and every , letting and using the continuity of , we obtain

(3.28)

In [33] it is proved that has normal structure under the slightly weaker condition

(3.29)

It is an open question if this condition implies the (DL)-condition.

Corollary 3.20.

Let be a uniformly nonsquare Banach space such that . Then satisfies the (DL)-condition.

4. Fixed Point Results for Multivalued Nonexpansive Mappings in Modular Function Spaces

The theory of modular spaces was initiated by Nakano [34] in 1950 in connection with the theory of order spaces and redefined and generalized by Musielak and Orlicz [35] 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, [3639]). In particular, Dhompongsa et al. [40] have obtained some fixed point results for multivalued mappings in modular functions spaces.

Let us recall some basic concepts about modular function spaces (for more details the reader is referred to [41, 42]).

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 .

Let us recall that a set function is called a -subadditive measure if , for any and for any sequence of sets . By we denote the characteristic function of the set .

Definition 4.1.

A functional is called a function modular if:

(1) for any ;

(2) whenever for any , and

(3) is a -subadditive measure for every

(4) as decreases to for every , where

(5)if there exists such that , then for every

(6)for any ,   is order continuous on , that is, if and decreases to .

A -subadditive measure is said to be additive if whenever such that and .

The definition of is then extended to by

(4.1)

Definition 4.2.

A set is said to be -null if for every . A property is said to hold -almost everywhere (-a.e.) if the set is -null. For example, we will say frequently -a.e.

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 [41] that satisfies the following properties:

(i) if and only if -a.e.

(ii) for every scalar with and .

(iii) if , and .

In addition, if the following property is satisfied

(iii) if , and ,

we say that is a convex modular.

A function modular is called -finite if there exists an increasing sequence of sets such that and .

The modular defines a corresponding modular space , which is given by

(4.2)

In general the modular is not subadditive and therefore does not behave as a norm or a distance. But one can associate to a modular an -norm. In fact, when is convex, the formula

(4.3)

defines a norm which is frequently called the Luxemburg norm. The formula

(4.4)

defines a different norm which is called the Amemiya norm. Moreover, and are equivalent norms. We can also consider the space

(4.5)

Definition 4.3.

A function modular is said to satisfy the -condition if

(4.6)

It is known that the -condition is equivalent to .

Definition 4.4.

A function modular is said to satisfy the -type condition if there exists such that for any we have .

In general, the -type condition and -condition are not equivalent, even though it is obvious that the -type condition implies the -condition.

Definition 4.5.

Let be a modular space.

(1)The sequence is said to be -convergent to if as .

(2)The sequence is said to be -a.e. convergent to if the set is -null.

(3)A subset of is called -a.e. closed if the -a.e. limit of a -a.e. convergent sequence of always belongs to .

(4)A subset of is called -a.e. compact if every sequence in has a -a.e. convergent subsequence in .

(5)A subset of is called -bounded if

(4.7)

We know by [41] 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.

Definition 4.6.

Let be a nonempty subset of . We will denote by the family of nonempty -closed subsets of and by the family of nonempty -compact subsets of . Let be the -Hausdorff distance on , that is,

(4.8)

where is the -distance between and . A multivalued mapping is said to be a -contraction if there exists a constant such that

(4.9)

If it is valid when , then is called -nonexpansive.

A function is called a fixed point for a multivalued mapping if .

Dhompongsa et al. [40] stated the Banach Contraction Principle for multivalued mappings in modular function spaces.

Theorem 4.7 (see [40, Theorem ]).

Let be a convex function modular satisfying the -type condition, a nonempty -bounded -closed subset of , and a -contraction mapping, that is, there exists a constant such that

(4.10)

Then has a fixed point.

By using that result, they proved the existence of fixed points for multivalued -nonexpansive mappings.

Theorem 4.8 (see [40, Theorem ]).

Let be a convex function modular satisfying the -type condition, a nonempty -a.e. compact -bounded convex subset of , and a -nonexpansive mapping. Then has a fixed point.

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 [43]). 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 ]).

Let be as above, a nonempty bounded convex set which is compact for the topology of the convergence locally in measure, and a nonexpansive mapping. Then has a fixed point.

In the case of the space we also can obtain a bounded closed convex set and a nonexpansive mapping which is fixed point free. Indeed, consider the following easy and well-known example.

Let

(4.11)

Define a nonexpansive mapping by

(4.12)

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 [36]). This fact leads to the following corollary.

Corollary 4.10 (see [40, Corollary ]).

Let be a nonempty -compact convex subset of and a nonexpansive mapping. Then has a fixed point.

Next we will give a property of closed convex bounded subsets of more general than weak star compactness which implies the fixed point property for nonexpansive mappings.

Domínguez et al. introduced in [44] some compactness conditions concerning proximinal subsets called Property (P). Following this idea we will use the following similar notion for modular function spaces.

Definition 4.11.

Let be a nonempty -closed convex -bounded subset of . It is said that has Property () if for every which is the -a.e. limit of a sequence in , the set is a nonempty and -compact subset of , where

Using that notion and the following two lemmas, we obtain a new fixed point result for multivalued -nonexpansive mappings.

Lemma 4.12 (see [40, Lemma ]).

Let be a convex function modular satisfying the -type condition, , and a nonempty -compact subset of . Then there exists such that

(4.13)

Lemma 4.13 (see [37, Lemma ]).

Let be a function modular satisfying the -type condition, and be a sequence in such that and there exists such that . Then,

(4.14)

Theorem 4.14.

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.

Proof.

Fix . For each , the -contraction is defined by

(4.15)

By Theorem 4.7, we can conclude that has a fixed point, say . It is easy to see that

(4.16)

By our assumptions, we can assume, by passing through a subsequence, that for some . By Lemma 4.12, for each there exists such that

(4.17)

Now we are going to show that for each . Taking any , from the -compactness of and Lemma 4.12, we can find such that

(4.18)

and we can assume, by passing through a subsequence, that for some . From above and using Lemma 4.13, it follows that

(4.19)

On the other hand, by Lemma 4.13 we also have

(4.20)

Thus, we deduce , which implies that and so .

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 [14]) 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 ].

Corollary 4.15.

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.

If we consider now the space , then the assumption of existence of a -convergent subsequence for every sequence in can be removed and we can state the following result.

Corollary 4.16.

Let be a nonempty closed bounded convex subset of which satisfies Property (P). If is a nonexpansive mapping, then has a fixed point.

Notice that in there exists a subset with Property (P) which is not -compact.

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.

References

  1. Banach S: Sur les opérations dans les ensembles abstraits et leurs applications. Fundamenta Mathematicae 1922, 3: 133–181.

    MATH  Google Scholar 

  2. Kirk WA: A fixed point theorem for mappings which do not increase distances. The American Mathematical Monthly 1965, 72: 1004–1006. 10.2307/2313345

    Article  MathSciNet  MATH  Google Scholar 

  3. Nadler SB Jr.: Multi-valued contraction mappings. Pacific Journal of Mathematics 1969, 30: 475–488.

    Article  MathSciNet  MATH  Google Scholar 

  4. Brodskiĭ MS, Mil'man DP: On the center of a convex set. Doklady Akademii Nauk SSSR 1948, 59: 837–840.

    MathSciNet  Google Scholar 

  5. Bynum WL: Normal structure coefficients for Banach spaces. Pacific Journal of Mathematics 1980,86(2):427–436.

    Article  MathSciNet  MATH  Google Scholar 

  6. Maluta E: Uniformly normal structure and related coefficients. Pacific Journal of Mathematics 1984,111(2):357–369.

    Article  MathSciNet  MATH  Google Scholar 

  7. Opial Z: Weak convergence of the sequence of successive approximations for nonexpansive mappings. Bulletin of the American Mathematical Society 1967, 73: 591–597. 10.1090/S0002-9904-1967-11761-0

    Article  MathSciNet  MATH  Google Scholar 

  8. Prus S: Banach spaces with the uniform Opial property. Nonlinear Analysis: Theory, Methods & Applications 1992,18(8):697–704. 10.1016/0362-546X(92)90165-B

    Article  MathSciNet  MATH  Google Scholar 

  9. Lin P-K, Tan K-K, Xu HK: Demiclosedness principle and asymptotic behavior for asymptotically nonexpansive mappings. Nonlinear Analysis: Theory, Methods & Applications 1995,24(6):929–946. 10.1016/0362-546X(94)00128-5

    Article  MathSciNet  MATH  Google Scholar 

  10. Gossez J-P, Lami Dozo E: Some geometric properties related to the fixed point theory for nonexpansive mappings. Pacific Journal of Mathematics 1972, 40: 565–573.

    Article  MathSciNet  MATH  Google Scholar 

  11. Huff R: Banach spaces which are nearly uniformly convex. The Rocky Mountain Journal of Mathematics 1980,10(4):743–749. 10.1216/RMJ-1980-10-4-743

    Article  MathSciNet  MATH  Google Scholar 

  12. Goebel K, Sękowski T: The modulus of noncompact convexity. Annales Universitatis Mariae Curie-Skłodowska. Sectio A 1984, 38: 41–48.

    MathSciNet  MATH  Google Scholar 

  13. Ayerbe Toledano JM, Domínguez Benavides T, López Acedo G: Measures of Noncompactness in Metric Fixed Point Theory, Operator Theory: Advances and Applications. Volume 99. Birkhäuser, Basel, Switzerland; 1997:viii+211.

    Book  MATH  Google Scholar 

  14. Goebel K, Kirk WA: Topics in Metric Fixed Point Theory, Cambridge Studies in Advanced Mathematics. Volume 28. Cambridge University Press, Cambridge, UK; 1990:viii+244.

    Book  MATH  Google Scholar 

  15. Turett B: A dual view of a theorem of Baillon. In Nonlinear Analysis and Applications (St. Johns, Nfld., 1981), Lecture Notes in Pure and Applied Mathematics. Volume 80. Marcel Dekker, New York, NY, USA; 1982:279–286.

    Google Scholar 

  16. Khamsi MA: On metric spaces with uniform normal structure. Proceedings of the American Mathematical Society 1989,106(3):723–726. 10.1090/S0002-9939-1989-0972234-4

    Article  MathSciNet  MATH  Google Scholar 

  17. Prus S: Some estimates for the normal structure coefficient in Banach spaces. Rendiconti del Circolo Matematico di Palermo 1991,40(1):128–135. 10.1007/BF02846365

    Article  MathSciNet  MATH  Google Scholar 

  18. Lami Dozo E: Multivalued nonexpansive mappings and Opial's condition. Proceedings of the American Mathematical Society 1973, 38: 286–292.

    Article  MathSciNet  MATH  Google Scholar 

  19. Lim TC: A fixed point theorem for multivalued nonexpansive mappings in a uniformly convex Banach space. Bulletin of the American Mathematical Society 1974, 80: 1123–1126. 10.1090/S0002-9904-1974-13640-2

    Article  MathSciNet  MATH  Google Scholar 

  20. Edelstein M: The construction of an asymptotic center with a fixed-point property. Bulletin of the American Mathematical Society 1972, 78: 206–208. 10.1090/S0002-9904-1972-12918-5

    Article  MathSciNet  MATH  Google Scholar 

  21. Kirk WA, Massa S: Remarks on asymptotic and Chebyshev centers. Houston Journal of Mathematics 1990,16(3):357–364.

    MathSciNet  MATH  Google Scholar 

  22. Kirk WA: Nonexpansive mappings in product spaces, set-valued mappings and -uniform rotundity. In Nonlinear Functional Analysis and Its Applications, Part 2 (Berkeley, Calif., 1983), Proc. Sympos. Pure Math.. Volume 45. American Mathematical Society, Providence, RI, USA; 1986:51–64.

    Chapter  Google Scholar 

  23. Kuczumow T, Prus S: Compact asymptotic centers and fixed points of multivalued nonexpansive mappings. Houston Journal of Mathematics 1990,16(4):465–468.

    MathSciNet  MATH  Google Scholar 

  24. Xu H-K: Metric fixed point theory for multivalued mappings. Dissertationes Mathematicae 2000, 389: 39.

    Article  MathSciNet  MATH  Google Scholar 

  25. Domínguez Benavides T, Lorenzo Ramírez P: Fixed-point theorems for multivalued non-expansive mappings without uniform convexity. Abstract and Applied Analysis 2003,2003(6):375–386. 10.1155/S1085337503203080

    Article  MathSciNet  MATH  Google Scholar 

  26. Domínguez Benavides T, Lorenzo Ramírez P: Asymptotic centers and fixed points for multivalued nonexpansive mappings. Annales Universitatis Mariae Curie-Skłodowska. Sectio A 2004, 58: 37–45.

    MathSciNet  MATH  Google Scholar 

  27. Dhompongsa S, Kaewcharoen A, Kaewkhao A: The Domínguez-Lorenzo condition and multivalued nonexpansive mappings. Nonlinear Analysis: Theory, Methods & Applications and Methods 2006,64(5):958–970. 10.1016/j.na.2005.05.051

    Article  MathSciNet  MATH  Google Scholar 

  28. Dhompongsa S, Domínguez Benavides T, Kaewcharoen A, Kaewkhao A, Panyanak B: The Jordan-von Neumann constants and fixed points for multivalued nonexpansive mappings. Journal of Mathematical Analysis and Applications 2006,320(2):916–927. 10.1016/j.jmaa.2005.07.063

    Article  MathSciNet  MATH  Google Scholar 

  29. Domínguez Benavides T, Gavira B: The fixed point property for multivalued nonexpansive mappings. Journal of Mathematical Analysis and Applications 2007,328(2):1471–1483. 10.1016/j.jmaa.2006.06.059

    Article  MathSciNet  MATH  Google Scholar 

  30. Saejung S: Remarks on sufficient conditions for fixed points of multivalued nonexpansive mappings. Nonlinear Analysis: Theory, Methods & Applications 2007,67(5):1649–1653. 10.1016/j.na.2006.07.037

    Article  MathSciNet  MATH  Google Scholar 

  31. Kaewkhao A: The James constant, the Jordan-von Neumann constant, weak orthogonality, and fixed points for multivalued mappings. Journal of Mathematical Analysis and Applications 2007,333(2):950–958. 10.1016/j.jmaa.2006.12.001

    Article  MathSciNet  MATH  Google Scholar 

  32. Gavira B: Some geometric conditions which imply the fixed point property for multivalued nonexpansive mappings. Journal of Mathematical Analysis and Applications 2008,339(1):680–690. 10.1016/j.jmaa.2007.07.015

    Article  MathSciNet  MATH  Google Scholar 

  33. Jiao H, Guo Y, Wang F: Modulus of convexity, the coefficient , and normal structure in Banach spaces. Abstract and Applied Analysis 2008, 2008:-5.

    Google Scholar 

  34. Nakano H: Modulared Semi-Ordered Linear Spaces. Maruzen, Tokyo, Japan; 1950.

    MATH  Google Scholar 

  35. Musielak J, Orlicz W: On modular spaces. Studia Mathematica 1959, 18: 49–65.

    MathSciNet  MATH  Google Scholar 

  36. Domínguez Benavides T, Khamsi MA, Samadi S: Asymptotically regular mappings in modular function spaces. Scientiae Mathematicae Japonicae 2001,53(2):295–304.

    MathSciNet  MATH  Google Scholar 

  37. Domínguez Benavides T, Khamsi MA, Samadi S: Asymptotically nonexpansive mappings in modular function spaces. Journal of Mathematical Analysis and Applications 2002,265(2):249–263. 10.1006/jmaa.2000.7275

    Article  MathSciNet  MATH  Google Scholar 

  38. Khamsi MA: Fixed point theory in modular function spaces. In Recent Advances on Metric Fixed Point Theory (Seville, 1995), Ciencias. Volume 48. Universidad de Sevilla, Seville, Spain; 1996:31–57.

    Google Scholar 

  39. Khamsi MA, Kozłowski WM, Reich S: Fixed point theory in modular function spaces. Nonlinear Analysis: Theory, Methods & Applications 1990,14(11):935–953. 10.1016/0362-546X(90)90111-S

    Article  MathSciNet  MATH  Google Scholar 

  40. Dhompongsa S, Domínguez Benavides T, Kaewcharoen A, Panyanak B: Fixed point theorems for multivalued mappings in modular function spaces. Scientiae Mathematicae Japonicae 2006,63(2):161–169.

    MathSciNet  MATH  Google Scholar 

  41. Kozlowski WM: Modular Function Spaces, Monographs and Textbooks in Pure and Applied Mathematics. Volume 122. Marcel Dekker, New York, NY, USA; 1988:x+252.

    Google Scholar 

  42. Musielak J: Orlicz Spaces and Modular Spaces, Lecture Notes in Mathematics. Volume 1034. Springer, Berlin, Germany; 1983:iii+222.

    Google Scholar 

  43. Alspach DE: A fixed point free nonexpansive map. Proceedings of the American Mathematical Society 1981,82(3):423–424. 10.1090/S0002-9939-1981-0612733-0

    Article  MathSciNet  MATH  Google Scholar 

  44. Domínguez Benavides T, García Falset J, Llorens-Fuster E, Lorenzo Ramírez P: Fixed point properties and proximinality in Banach spaces. Nonlinear Analysis: Theory, Methods & Applications 2009,71(5–6):1562–1571. 10.1016/j.na.2008.12.048

    Article  MathSciNet  MATH  Google Scholar 

  45. Hu S, Papageorgiou NS: Handbook of Multivalued Analysis. Vol. I. Theory, Mathematics and Its Applications. Volume 419. Kluwer Academic Publishers, Dordrecht, The Netherlands; 1997:xvi+964.

    Google Scholar 

  46. Goebel K, Kuczumov T: Irregular convex sets with fixed point property for nonexpansive mappings. Colloqium Mathematicum 1979, 259–264.

    Google Scholar 

Download references

Acknowledgments

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.

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to T Domínguez Benavides.

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Reprints and permissions

About this article

Cite this article

Domínguez Benavides, T., Gavira, B. Does Kirk's Theorem Hold for Multivalued Nonexpansive Mappings?. Fixed Point Theory Appl 2010, 546761 (2010). https://doi.org/10.1155/2010/546761

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1155/2010/546761

Keywords