- Research Article
- Open Access

# Some Maximal Elements' Theorems in -Spaces

- Rong-Hua He
^{1, 2}Email author and - Yong Zhang
^{1}

**2009**:905605

https://doi.org/10.1155/2009/905605

© R.-H. He and Y. Zhang. 2009

**Received:**30 March 2009**Accepted:**1 September 2009**Published:**28 September 2009

## Abstract

Let be a finite or infinite index set, let be a topological space, and let be a family of -spaces. For each , let be a set-valued mapping. Some new existence theorems of maximal elements for a set-valued mapping and a family of set-valued mappings involving a better admissible set-valued mapping are established under noncompact setting of -spaces. Our results improve and generalize some recent results.

## Keywords

- Topological Space
- Existence Theorem
- Linear Structure
- Maximal Element
- Topological Vector Space

## 1. Introduction

It is well known that many existence theorems of maximal elements for various classes of set-valued mappings have been established in different spaces. For their applications to mathematical economies, generalized games, and other branches of mathematics, the reader may consult [1–12] and the references therein.

In most of the known existence results of maximal elements, the convexity assumptions play a crucial role which strictly restrict the applicable area of these results. In this paper, we will continue to study existence theorems of maximal elements in general topological spaces without convexity structure. We introduce a new class of generalized -majorized mappings for each which involve a set-valued mapping . The notion of generalized -majorized mappings unifies and generalizes the corresponding notions of -majorized mappings in [4]; -majorized mappings in [2, 13]; -majorized mappings in [14]. Some new existence theorems of maximal elements for generalized -majorized mappings are proved under noncompact setting of -spaces. Our results improve and generalize the corresponding results in [2, 4, 14–16].

## 2. Preliminaries

Let and be two nonempty sets. We denote by and the family of all subsets of and the family of all nonempty finite subsets of , respectively. For each , we denote by the cardinality of . Let denote the standard -dimensional simplex with the vertices . If is a nonempty subset of we will denote by the convex hull of the vertices .

Let and be two sets, and let be a set-valued mapping. We will use the following notations in the sequel:

(i) ,

(ii) ,

(iii) .

For topological spaces and , a subset of is said to be compactly open (resp., compactly closed) if for each nonempty compact subset of , is open (resp., closed) in . The compact closure of and the compact interior of (see [17]) are defined, respectively, by

It is easy to see that , is compactly open (resp., compactly closed) in if and only if (resp., ). For each nonempty compact subset of , and , where (resp., )) denotes the closure (resp., interior) of in . A set-valued mapping is transfer compactly open valued on (see [17]) if for each and , there exists such that . Let be transfer compactly open valued, then . It is clear that each transfer open valued correspondence is transfer compactly open valued. The inverse is not true in general.

The definition of -space and the class of better admissible mapping were introduced by Ding in [8]. Note that the class of better admissible mapping includes many important classes of mappings, for example, the class in [18], in [19] and so on as proper subclasses. Now we introduce the following definition.

Definition 2.1.

An -space is said to be an -space if for each , there exists a compact -subspace of containing .

Lemma 2.2 ([8]).

Let be any index set. For each , let be an -space, and . Then is also an -space.

Let be a topological space, and let be any index set. For each , let be an -space, and let such that is an -space defined as in Lemma 2.2. Let and for each , let be a set-valued mapping. For each ,

(1) is said to be a generalized -mapping if

(a)for each and , , where is the projection of onto and ;

(b) is transfer compactly open in for each ;

(2) is said to be a generalized -majorant of at if is a generalized -mapping and there exists an open neighborhood of in such that for all ;

(3) is said to be a generalized -majorized if for each with , there exists a generalized -majorant of at , and for any , the mapping is transfer compactly open in ;

(4) is said to be a generalized -majorized if for each , there exists a generalized -majorant of at .

Then is said to be a family of generalized -mappings (resp., -majorant mappings) if for each is a generalized -mapping (resp., -majorant mapping).

If for each , let be a -convex space, a family of -mappings (resp., -majorant mappings) were introduced by Ding in [4]. Clearly, each family of generalized -mappings must be a family of generalized -majorant mappings. If is a single-valued mapping and is an -subspace of for each , then condition for each implies that condition (a) in (1) holds. Indeed, if (a) is false, then there exist , , and such that and hence for each . It follows from that where . It follows from being an -subspace of that which contradicts condition for each . Hence each -mapping (resp., -majorant mapping) introduced by Deguire et al. (see [2, page 934]) must be a generalized -mapping (resp., -majorant mapping). The inverse is not true in general.

## 3. Maximal Elements

In order to obtain our main results, we need the following lemmas.

Lemma 3.1 ([3]).

Let and be topological spaces, let be a nonempty compact subset of and let be a set-valued mapping such that for each . Then the following conditions are equivalent:

(1) have the compactly local intersection property;

(2)for each , there exists an open subset of (which may be empty) such that and ;

(3)there exists a set-valued mapping such that for each is open or empty in , and ;

(4)for each , there exists such that and ;

(5) is transfer compactly open valued on .

Lemma 3.2 ([8]).

Let be a topological space, and let be an -space, and such that

(ii) is transfer compactly open valued;

(iii)there exists a nonempty set and for each , there exists a compact -subspace of containing such that is empty or compact in , where denotes the complement of .

Then there exists a point such that .

Theorem 3.3.

Let be a topological space, let be a nonempty compact subset of , and let be an -space, and be a generalized -mapping such that

(i)for each , there exists a compact -subspace of containing such that for each .

Then there exists a point such that .

Proof.

By using similar argument as in the proof of Lemma 3.2, we can show that there exists such that . Condition (i) implies that must be in . This completes the proof.

Remark 3.4.

Theorem 3.3 generalizes in [4, Theorem 2.2] in the following several aspects: (a) from -convex space to -space without linear structure; (b) from -mappings to generalized -mappings.

Theorem 3.5.

Let be a topological space, and let be an -space. Let and be a generalized -majorized mapping such that

(i)there exists a paracompact subset of such that ;

(ii)there exists a nonempty set and for each , there exists a compact -subspace of containing such that the set is empty or compact.

Then there exists a point such that .

Proof.

Suppose that for each . Since is a generalized -majorized, for each , there exists an open neighborhood of in and a generalized -mapping such that

(a) for each ,

(b)for each and ,

(c) is transfer compactly open in ,

(d)for any , the mapping is transfer compactly open in .

Hence is transfer compactly open in by (c).

Now define a mapping by

This proves that is transfer compactly open valued in .

On the other hand, for each and , if , then . Since there exists such that and , we have , and hence by (b). Hence we have

for each and . This shows that is a generalized -mapping.

For each , if , then there exists an such that and . It follows from (a) that . Hence we have for each . By condition (ii), there exists a nonempty set and for each , there exists a compact -subspace of containing such that the set is empty or compact. Note that for each implies for each . Hence and is empty or compact. By Lemma 3.2, there exists a point such that , and hence which contradicts the assumption that for each . Therefore, there exists such that .

Theorem 3.6.

Let be a topological space, let be a nonempty compact subset of and be an -space. Let and be a generalized -majorized mapping such that

(i)there exists a paracompact subset of such that ;

(ii)for each , there exists a compact -subspace of containing such that for each .

Then there exists such that .

Proof.

Suppose that for each . By using similar argument as in the proof of Theorem 3.5, we can show that there exists a generalized -mapping such that for each . It follows from condition (ii) that for each . By Theorem 3.3, there exists such that , and hence which contradicts the assumption that for each . Therefore, there exists such that . Condition (ii) implies . This completes the proof.

Remark 3.7.

Theorem 3.5 generalizes [4, Theorem 2.3] in several aspects: Section 1(1) from -convex space to -space without linear structure; Section 1(2) from a -majorized mapping to a generalized -majorized mapping; Section 1(3) condition (ii) of Theorem 3.5 is weaker than condition (ii) of [4, Theorem 2.3]. If is compact, condition (i) is satisfied trivially. If is a compact -space, then by letting for all , conditions (i) and (ii) are satisfied automatically. Theorem 3.6 unifies and generalizes Shen's [14, Theorem 2.1, Corollary 2.2 and Theorem 2.3] in the following ways: Section 2(1) from -convex space to -space without linear structure; Section 2(2) from -majorized correspondences to generalized -majorized mapping; Section 2(3) condition (ii) of Theorem 3.6 is weaker than that in the corresponding results of Shen in [14]. Theorem 3.6 also generalizes in [4, Theorem 2.4], Ding in [15, Theorem 5.3], and Ding and Yuan in [16, Theorem 2.3] in several aspects.

Corollary 3.8.

Let be a compact topological space, and let be an -space. Let and be a generalized -majorized mapping. Then there exists a point such that .

Proof.

The conclusion of Corollary 3.8 follows from Theorem 3.6 with .

Corollary 3.9.

Let be a topological space, and let be an -space. Let be a compact mapping and be a generalized -majorized mapping. Then there exists a point such that .

Proof.

Since is a compact mapping, there exists a compact subset of such that . The mapping be the restriction of to . It is easy to see that is also generalized -majorized. By Corollary 3.8, there exists such that .

Remark 3.10.

Corollary 3.8 generalizes Deguire et al. [2, Theorem 1] in the following ways: (1.1) from a convex subset of Hausdorff topological vector space to an -space without linear structure; (1.2) from a -majorized mapping to a generalized -majorized mapping. Corollary 3.8 also generalizes [4, Corollary 2.3] from -convex space to -space and from a -majorized mapping to a generalized -majorized mapping. Corollary 3.9 generalizes [2, Theorem 2] and [4, Corollary 2.4] in several aspects.

Theorem 3.11.

Let be a topological space, and let be any index set. For each , let be an -space, and let such that is an -space defined as in Lemma 2.2. Let such that for each ,

(i)let be a generalized -majorized mapping;

(ii) ;

(iii)there exists a paracompact subset of such that ;

(iv)there exists a nonempty set and for each , there exists a compact -subspace of containing such that the set is empty or compact, where .

Then there exists such that for each .

Proof.

Then for each , if and only if . Let with , then there exists such that . By condition (ii), there exists such that . Since is generalized -majorized, there exist an open neighborhood of in and a generalized -majorant of at such that

(a) for all ,

(c) is transfer compactly open in ,

(d)for each , the mapping is transfer compactly open in .

We claim that is a generalized -majorant of at . Indeed, we have

()for each , ,

is transfer compactly open in by (c).

Hence is a generalized -majorant of at .

For each and , by (3.15), we have

It follows from (d) that is transfer compactly open in .

Hence is generalized -majorized. By condition (iii), we have

It follows from condition (iv) that is empty or compact and hence all conditions of Theorem 3.5 are satisfied. By Theorem 3.5, there exists such that which implies , that is, for each .

Theorem 3.12.

Let be a topological space, and let be any index set. For each , let be an -space, and let . Let be a compact mapping such that for each ,

(i)let be a generalized -majorized mapping;

(ii) .

Then there exists such that for each .

Proof.

Since for each , let be an -space, then for each , there exists a compact -subspace of containing . Let and , then is a compact -subspace of for each , is a compact -subspace of containing . Hence is also an -space.

For each , . Define

Then for each , if and only if . By using similar argument as in the proof of Theorem 3.11, we can show that is a generalized -majorized mapping. By Corollary 3.9, there exists such that , and so . Hence, we have for each .

Theorem 3.13.

Let be a topological space, let be a nonempty compact subset of and let be any index set. For each , let be an -space, and let such that is an -space defined as in Lemma 2.2. Let such that for each , be a generalized -mapping such that

(i)for each and , there exists a compact -subspace of containing and for each , there exists satisfying .

Then there exists such that for each .

Proof.

where .

Hence for each , is a generalized -mapping and hence it is also a generalized -majorized mapping. All conditions of Corollary 3.8 are satisfied. By Corollary 3.8, there exists such that for each , so we have which contradicts the fact that for each there exists such that . Therefore, there exists such that for each .

Remark 3.14.

Theorem 3.11 generalizes [4, Theorem 2.5] in several aspects. Theorem 3.12 improves [2, Theorem 3] from convex subsets of topological vector spaces to -spaces without linear structure and from a family of -majorized mappings to the family of generalized -majorized mappings. Theorem 3.13 generalizes [4, Theorem 2.6] in several aspects: (1.1) from -convex spaces to -spaces without linear structure; (1.2) from a -mapping to a generalized -mapping; (1.3) condition (i) of Theorem 3.13 is weaker than condition (i) of [4, Theorem 2.6]. Theorem 3.13 improves and generalizes [2, Theorem 7] in the following ways: (2.1) from nonempty convex subsets of Hausdorff topological vector spaces to -space without linear structure; (2.2) from the family of -majorized mappings to the family of generalized -majorized mappings.

## Declarations

### Acknowledgment

This work is supported by a Grant of the Natural Science Development Foundation of CUIT of China (no. CSRF200709).

## Authors’ Affiliations

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