#### Abstract

Authors define nearly dentable Banach space. Authors study Radon-Nikodym property, approximative compactness and continuity metric projector operator in nearly dentable space. Moreover, authors obtain some examples of nearly dentable space in Orlicz function spaces. Finally, by the method of geometry of Banach spaces, authors give important applications of nearly dentability in generalized inverse theory of Banach space.

#### 1. Introduction

Let be a real Banach space. and denote the unit sphere and the unit ball, respectively. By we denote the dual space of . Let , and denote the set of natural numbers, reals, and nonnegative reals, respectively. and . By denote that is weakly convergent to . denote that is convergent to . The weak topology of is denoted by or . The topology of is denoted by or . denote closed hull of (weak closed hull, closed hull closed hull). dist denote distance of and .

The study of geometric and topological properties of unit balls of Banach spaces plays a central rule in the geometry of Banach spaces. Almost all properties of Banach spaces, such as convexity, smoothness, nonsquareness, and the Radon-Nikodym property, can be viewed as properties of the unit ball. We should mention here that there are many topics studying behavior of ball collections. For example, the Mazur intersection property, the packing sphere problem of unit balls, the measure of noncompactness with respect to topological degree, and the ball topology have also brought great attention of many mathematicians. In 2006, Cheng defined ball-covering property of Banach space (see [1]). In [2], Chen and Cheng proved analytical characterizations of Mazur intersection property.

Starting with a different viewpoint, a notion of nearly dentable space is introduced by Shang et al. in [3].

*Definition 1. *Banach space is called nearly dentable if, for any and any open set , we have and , where .

By James theorem, it is easy to see that nearly dentable space is reflexive.

*Definition 2. *A nonempty subset of is said to be approximatively compact if for any and any satisfying , then has a subsequence converging to an element in . is called approximatively compact if every nonempty closed convex subset of is approximatively compact.

Let us present the history of the approximative compactness and related notions. This notion has been introduced by Jefimow and Stechkin in [4] as a property of Banach spaces, which guarantees the existence of the best approximation element in a nonempty closed convex set for any . In [3] it was established (without proof) that the approximative compactness of in a rotund Banach space gives also the continuity of the projector operator . Fully rotund Banach spaces were defined by Fan and Glicksberg in [5]. In [6] it has been proved that fully rotund Banach spaces are approximately compact. In [7] it has been proved that approximative compactness of a Banach space means that is reflexive and it has the Kadec-Klee property (called also the property ). In 2007, Chen et al. (see [8]) proved that a nonempty closed convex of a midpoint locally uniformly rotund space is approximately compact if and only if is a Chebyshev set and the metric projection operator is continuous. In 1972, Oshman (see [9]) proved that the metric projection operator is upper semicontinuous if is approximatively compact subset of a Banach space .

Let be a nonempty open convex subset of and a real-valued continuous convex function on . is said to be Gateaux differentiable at the point in if the limit exists for all . When this is the case, the limit is a continuous linear function of , denoted by . If the difference quotient in converges to uniformly for in the unit ball, then is said to be Fréchet differentiable at . is called a weak Asplund space (Asplund space) or said to have the weak Asplund property if, for every and as above, is “generically” Gateaux (Frechet) differentiable; that is, there exists a dense subset of such that is Gateaux (Frechet) differentiable at each point of .

The paper is organized as follows. In Section 1, authors define nearly dentable Banach space and some necessary definitions and notations are collected. In Section 2, authors prove that if is a nearly dentable space, then is an Asplund space if and only if is a separable subset of , where is a separable closed subspace of . In Section 3, authors prove that the following statements are equivalent: every hyperplane of is approximatively compact; if is a open convex set, then is approximatively compact; is nearly dentable space and is compact for any . Moreover, authors also prove that if is nearly dentable space, then for any open convex set metric projector operator is upper semicontinuous; For any hyperplane metric projector operator is upper semicontinuous. In Section 4, authors prove that there exists a Banach space such that is not nearly dentable Banach space. Moreover, authors also prove that there exists a Banach space such that is nearly dentable Banach space and not nearly dentable Banach space. The main results in this paper have important applications to the ill-posed equation theory and the generalized inverse theory. In Section 5, authors give an important application of nearly dentability in generalized inverse theory of Banach space.

*Definition 3. * is called nearly dentable space, if, for any and open set , we have .

Let us recall some definitions and some lemmas which will be used in the further part of the paper.

*Definition 4 (see [8]). *A subset is said to be proximinal if for all . is said to be semi-Chebyshev if is at most a singleton for all . is said to be Chebyshev if it is proximinal and semi-Chebyshev.

*Definition 5. *A Banach space is said to have the Radon-Nikodym property whenever if is a nonatomic measure space and is a vector measure on with values in which is absolutely continuous with respect to and has bounded variation; then there exists such that, for any , It is well known that is reflexive if and only if each closed convex subset of is proximinal. It is well known that is Asplund space if and only if has the Radon-Nikodym property.

*Definition 6 (see [10]). *Set-valued mapping is called upper semicontinuous at , if, for each norm open set with , there exists a norm neighborhood of such that for all in . is called lower continuous at , if, for any and any in with , there exists such that as . is called continuous at , if is upper semicontinuous and is lower continuous at .

*Definition 7. * is called hyperplane, if there exist and such that .

Lemma 8 (see [11]). *Suppose that is a Banach space. Then the following statements are equivalent:*(1)*is Asplund space.*(2)*If is a separable closed subspace of , then is a separable space.*(3)* has the Radon-Nikodym property.*

Lemma 9 (see [12]). *Let and be not an emptyset for all . Assume that for some countable set and some . Then is the closed linear span of .*

#### 2. Nearly Dentability and Radon-Nikodym Property in Banach Space

Theorem 10. *Suppose that is a nearly dentable space. Then the following statements are equivalent:*(1)* is a Asplund space.*(2)* has the Radon-Nikodym property.*(3)*If is a separable closed subspace of , then is a separable space.*(4)*If is a separable closed subspace of , then is a separable subset of .*

*Proof. * Let be a separable closed subspace of and be a dense subset of . Then, for any , we have In fact, suppose that . Then, for any , there exist and such that , where . This implies thatSince is arbitrary, we obtain thatThis implies that there exists such that . Hence we define an open set Since is a nearly dentable space, we have . Since is a compact set, by the separation theorem of locally convex space, there exists such that Since the sequence is a dense subset of , we may assume without loss of generality that as . Pick . Then Moreover, we may assume without loss of generality that if , then . Since is compact, there exists such that is a accumulation point of . This implies that . Then . Therefore, by formula (6), we obtain that Since and , by formula (6), we have , which contradicts . Hence formula (2) is true. Since is a separable subset of , we obtain that there exists a countable dense subset of for each . Then is a countable set. Pick . Therefore, by formula (2), we obtain that there exist and such that for any . Hence, for every , we obtain that for any . Since and is a countable dense subset of , we obtain thatfor any . This implies that for any . Noticing that , we have By Lemma 9, we obtain that , and so is separable.

This means that if is a separable closed subspace of , then is a separable space. By Lemma 15, we obtain that is Asplund space and has the Radon-Nikodym property.

. Since has the Radon-Nikodym property, we obtain that if is a separable closed subspace of , then is a separable space. Hence is a separable subset of .

Therefore, by Lemma 8, we obtain that Theorem 10 is true, which completes the proof.

#### 3. Nearly Dentability and Approximative Compactness and Continuity of Metric Projector Operator in Banach Spaces

Theorem 11. *Suppose that is a Banach space. Then the following statements are equivalent:*(1)*Every hyperplane of is approximatively compact.*(2)*If is a open convex set, then is approximatively compact.*(3)* is nearly dentable space and is compact for any .*

*Proof. *. (a) Let as , where and . Since is compact, there exists such that is a accumulation point of . Hence there exists a net such that and . We next will prove that the sequence is relatively compact. The proof requires considering few cases separately.*Case **I *. Suppose that is not relatively compact. Then there exists a subsequence denoted again by such that does not contain convergent subsequence. This implies that every point of is not accumulation point of . It is easy to see that for any , there exists such that . Put Then, it is easy to see that . Moreover, since is a compact convex set, we obtain that is a compact convex set. Since is a nearly dentable space, we obtain that . By separation theorem, there exist and such that From the previous proof, there exists such that . By , we have Therefore, by formulas (14) and (15), we obtain that . This implies that is impossible, a contradiction. Hence is relatively compact.*Case **II *. From the previous proof, is relatively compact. Since is compact, we obtain that is relatively compact. Noticing that , we obtain that is relatively compact.

Let be a open convex set and . Then, for any . we may assume without loss of generality that . We next will prove that if as , then the sequence has a subsequence converging to an element in , where . Since is a closed set, by proof of theorem 2.1 of [13], we obtain that is a proximinal set and . Pick . Then . In fact, suppose that . Then there exist and such that and . Since we obtain that a contradiction. HenceNoticing that and separation theorem of locally convex space, there exists such that This implies that Since , we obtain that This means that the inequality holds. ThereforeHence and as . Furthermore, we havewhich shows that . From the previous proof, we obtain that the sequence is relatively compact. Noticing that as , we obtain that the sequence is relatively compact. This implies that the sequence has a subsequence converging to an element in . Hence we obtain that is approximatively compact.

. Let be a hyperplane. Then it is easy to see that and are closed convex sets. Moreover, it is easy to see that if , then and if , then . Since and are approximatively compact, we obtain that is approximatively compact.

. For any , we define a hyperplane . Let . Then . Hence has a subsequence converging to . Moreover, it is easy to see that is a closed set. Hence is compact. We claim thatfor any open set . In fact, suppose that . Then there exists such that dist as . Since is compact, there exists such that . Since is compact, without loss of generality, we may assume that is a Cauchy sequence. Let as . Then . Hence there exists such that . Moreover, we have Therefore, , when is large enough, which contradicts . Hence, for any open set , we have . We next will prove that there exists such that whenever . Otherwise, there exists such that . It is easy to see that . Let . Then it is easy to see that . Hence we define a hyperplane . Then it is easy to see that dist. Hence, for any , there exists a net such that . Moreover, it is easy to see that . This implies that . Hence we obtain that is a closed set. By the proof of Theorem 2.1 of [13], we know that if is a closed set, then is a proximinal set. Hence there exists such that . Pick . Then . Sincewe obtain that Therefore, by and formula (27), we obtain that Since every hyperplane of is approximatively compact, we obtain that has a subsequence converging to an element in . By formula (26), we obtain that has a subsequence converging to a element in . Hence we may assume without loss of generality that . Then . By and , we have . This implies that . Then dist as . By and , we obtain that dist as . Moreover, we have proved that for any open set , a contradiction. This implies that there exists such that whenever . Hence, for any , we have This implies that . Therefore, by the arbitrariness of , we have . Hence is nearly dentable space. This completes the proof.

Theorem 12. *Suppose that is nearly dentable space. Then, *(1)*for any open convex set , metric projector operator is upper semicontinuous;*(2)*for any hyperplane , metric projector operator is upper semicontinuous.*

*Proof. * Let be a open convex set and . Suppose that metric projector operator is not upper semicontinuous. Then we without loss of generality may assume that metric projector operator is not upper semicontinuous in orgin. Hence there exist a sequence and an open set such that , and as . This implies that there exists such that . We will derive a contradiction for each of the following two cases.*Case **I*. Let . Then, by , we have dist as . Moreover, by , we have as . Therefore, This implies that as . Hence there exists such that , a contradiction.*Case **II*. Let . Then , , and . Therefore, by and the separation theorem of locally convex space, there exists such that . This implies that Therefore, by , we obtain that for any . Since , we obtain that . Since the distance function is continuous, we obtain thatMoreover, we have Therefore, by formulas (32) and (33), we have . By , we get that every subsequence does not converge to point of . Moreover, by , we have . Hence, for any , there exists such that . Since is closed convex set and , we obtain that for any , there exists such that dist. This implies that . Noticing that we obtain that, for any , there exists such thatTherefore, by and , we have Let us define sequence , where Then Noticing that , we obtain that . This implies that . Since , we have Hence there exists such that whenever . Therefore, by formula (35), we have . Hence if , then Since and , we have . PutThen, by formulas (40) and (41), we have . Moreover, by the arbitrariness of , we havePut Then, by formulas (42) and (43), we have .

Since the distance function is continuous, we have as . Moreover, we have as . This implies that as . Therefore, by as , we obtain that Since , we obtain that as . By we have as . Since is compact, there exists such that is a accumulation point of . Hence there exists a net such that and . Since , we obtain that Let . Then, by and , we have . This implies that and . Therefore, This implies that . Since is a nearly dentable space, we obtain that . Since is compact, it is easy to see that is compact. Moreover, since is closed convex set, by the separation theorem of locally convex space, there exist and such that Therefore, by , we obtain that . Noticing that , we obtain that Therefore, by formulas (48)-(49) and , we have , which contradicts . Hence the metric projector operator is upper semicontinuous.

By the proof of Theorem 11, it is easy to see that is true. This completes the proof.

#### 4. Some Examples of Nearly Dentable Space in Orlicz Function Space

In the section, authors prove that there exists a Banach space such that is not a nearly dentable Banach space. Moreover, authors also prove that there exists a Banach space such that is a nearly dentable Banach space and not a nearly dentable Banach space. First, we give the definition of Orlicz function space.

*Definition 13. * is called an function if it has the following properties: (1) is even, convex, and ;(2) for all ;(3) and .

Let denote the right derivative of at and let be the generalized inverse function of defined on by . Then we call the complementary function of . It is well known that there holds the Young inequality and or . Moreover, it is well known that and are complementary to each other. A continuous function is called strictly convex if for all .

Let be a finite nonatomic and complete measure space. Denote by and the right derivative of and , respectively. We define It is well known that the Orlicz function space is a Banach space when it is equipped with the Luxemburg norm or equipped with the Amemiya-Orlicz norm, denote Orlicz function spaces equipped with the Luxemburg norm. , denote Orlicz function spaces equipped with the Orlicz norm. It is well known that , .

*Definition 14 (see [13]). *We say that an -function if there exist and such that whenever .

Lemma 15 (see [13, 14]). *Let be Orlicz function space. Then the following statements are equivalent:*(1)*Every hyperplane of is approximatively compact;*(2)* is smooth space and .*

Lemma 16 (see [15]). *() has the Radon-Nikodym property if and only if .*

Lemma 17 (see [15]). * is a strictly convex space if and only if is strictly convex.*

Lemma 18 (see [15]). * is reflexive if and only if .*

Theorem 19. *If every hyperplane of is approximatively compact, then .*

*Proof. *Since every hyperplane of is approximatively compact, we obtain that has the Radon-Nikodym property (see [14]). Therefore, by Lemma 16, we have . This completes the proof.

*Example 20. *If is strictly convex and , then is not nearly dentable. In fact, suppose that is nearly dentable. Since is strictly convex, we obtain that is a strictly convex space by Lemma 17. Since , we obtain that is a smooth space. This implies that if is a separable closed subspace of , then is a singleton. Therefore, by Theorem 10, we obtain that has the Radon-Nikodym property. Then, by Lemma 16, we obtain that , a contradiction.

*Example 21. *If is strictly convex, and , then is nearly dentable not nearly dentable. In fact, by Theorem 19, we obtain that every hyperplane of is approximatively compact. Therefore, by Theorem 11, we obtain that is nearly dentable. Since , we obtain that is not reflexive. This implies that is not nearly dentable.

#### 5. Applications to Theory of Generalized Inverses of Banach Space

Let be a linear bounded operator from into . Let and denote the domain, range, and null space of , respectively. If or , the operator equation is generally ill-posed. In applications, one usually looks for the best approximative solution (b.a.s.) to the equation (see [16]).

A point is called the best approximative solution to the operator equation , if where (see [16]). Nashed and Votruba [16] introduced the concept of the (set-valued) metric generalized inverse .

*Definition 22. *Let be Banach spaces and let be a linear operator from to . The set-valued mapping defined by