Abstract and Applied Analysis

Volume 2014, Article ID 478486, 5 pages

http://dx.doi.org/10.1155/2014/478486

## Existence Theorems of -Cone Saddle Points for Vector-Valued Mappings

College of Culture and Tourism, Yunnan Open University, Kunming 650223, China

Received 19 November 2013; Accepted 22 January 2014; Published 25 February 2014

Academic Editor: Qamrul Hasan Ansari

Copyright © 2014 Tao Chen. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A new existence result of -vector equilibrium problem is first obtained. Then, by using the existence theorem of -vector equilibrium problem, a weakly -cone saddle point theorem is also obtained for vector-valued mappings.

#### 1. Introduction

Saddle point problems are important in the areas of optimization theory and game theory. As for optimization theory, the main motivation of studying saddle point has been their connection with characterized solutions to minimax dual problems. Also, as for game theory, the main motivation has been the determination of two-person zero-sum games based on the minimax principle.

In recent years, based on the development of vector optimization, a great deal of papers have been devoted to the study of cone saddle points problems for vector-valued mappings and set-valued mappings, such as [1–8]. Nieuwenhuis [5] introduced the notion of cone saddle points for vector-valued functions in finite-dimensional spaces and obtained a cone saddle point theorem for general vector-valued mappings. Gong [2] established a strong cone saddle point theorem of vector-valued functions. Li et al. [4] obtained an existence theorem of lexicographic saddle point for vector-valued mappings. Bigi et al. [1] obtained a cone saddle point theorem by using an existence theorem of a vector equilibrium problem. Zhang et al. [9] established a general cone loose saddle point for set-valued mappings. Zhang et al. [8] obtained a minimax theorem and an existence theorem of cone saddle points for set-valued mappings by using Fan-Browder fixed point theorem. Some other types of existence results can be found in [3, 10–18].

On the other hand, in some situations, it may not be possible to find an exact solution for an optimization problem, or such an exact solution simply does not exist, for example, if the feasible set is not compact. Thus, it is meaningful to look for an approximate solution instead. There are also many papers to investigate the approximate solution problem, such as [19–21]. Kimura et al. [20] obtained several existence results for -vector equilibrium problem and the lower semicontinuity of the solution mapping of -vector equilibrium problem. Anh and Khanh [19] have considered two kinds of solution sets to parametric generalized -vector quasiequilibrium problems and established the sufficient conditions for the Hausdorff semicontinuity (or Berge semicontinuity) of these solution mappings. X. B. Li and S. J. Li [21] established some semicontinuity results on -vector equilibrium problem.

The aim of this paper is to characterize the -cone saddle point of vector-valued mappings. For this purpose, we first establish an existence theorem for -vector equilibrium problem. Then, by this existence result, we obtain an existence theorem for -cone saddle point of vector-valued mappings.

#### 2. Preliminaries

Let be a real Hausdorff topological vector space and let be a real local convex Hausdorff topological vector space. Assume that is a pointed closed convex cone in with nonempty interior . Let be the topological dual space of . Denote the dual cone of by : Note that from Lemma 3.21 in [22] we have

*Definition 1 (see [7, 23]). *Let be a vector-valued mapping. is said to be -upper semicontinuous on if and only if, for each and any , there exists an open neighborhood of such that
is said to be -lower semicontinuous on if and only if is -upper semicontinuous on .

Lemma 2 (see [17]). *Let be a vector-valued mapping and . If is -lower semicontinuous, then is lower semicontinuous.*

*Definition 3 (see [24]). *Let and be nonempty subsets of and be a vector-valued mapping.(i)is said to be -concavelike in its first variable on if and only if, for all and , there exists such that
(ii) is said to be -convexlike in its second variable on if and only if, for all and , there exists such that
(iii) is said to be -concavelike-convexlike on if and only if is -concavelike in its first variable and -convexlike in its second variable.

*Definition 4. *Let be a nonempty subset and .(i)A point is said to be a weak -minimal point of if and only if and denotes the set of all weak -minimal points of .(ii)A point is said to be a weak -maximal point of if and only if and denotes the set of all weak -maximal points of .

*Definition 5. *Let be a vector-valued mapping and . A point is said to be a weak --saddle point of on if

#### 3. Existence of -Vector Equilibrium Problem

In this section, we deal with the following -vector equilibrium problem (for short VAEP). Find such that where is a vector-valued mapping, is a nonempty subset of and .

If , , and if is a solution of VAEP, then is a solution of -vector optimization of , where is a vector-valued mapping.

Denote the -solution set of (VAEP) by

Lemma 6 (see [20]). *Let be a nonempty subset of . Suppose that is a vector-valued mapping and the following conditions are satisfied:*(i)* is a compact set;*(ii)*;*(iii)* is -lower semicontinuous on .**Then, for each , .*

Next, we give a sufficient condition for the condition (ii) in Lemma 6.

Lemma 7. *Let be a nonempty subset of . Suppose that is a vector-valued mapping with for all and the following conditions are satisfied:*(i)* is a compact set;*(ii)* is -concavelike-convexlike on ;*(iii)*for each , is -lower semicontinuous on .**Then, there exists such that
*

*Proof. *
For any and , we define a multifunction by

First, by assumptions, we must have
In fact, if there exists such that , for all , then
Particularly, taking , we have , which contradicts the assumption about .

Then, by Lemma 2, is a closed set, for each . By (11), for any , we have
Since is compact, there exists a finite point set in such that
Namely, for each , there exists such that
Now, we consider the set
Obviously, by the condition (ii), is a convex set. By (15), we have the fact that .

By the separation theorem of convex sets, there exists such that
Since , we can get and , for all . By the definition of , for each ,
By (18), . Then, by (17) and (19),
By (20), . Thus, by the condition (ii), for each , there exists such that
By the assumption about and , there exists such that
This completes the proof.

*By Lemmas 6 and 7, we can get the following result.*

*Theorem 8. Let be a nonempty subset of . Suppose that is a vector-valued mapping with for all and the following conditions are satisfied:(i) is a compact set;(ii) is -concavelike-convexlike on ;(iii) is -lower semicontinuous on .Then, for each , .*

*Remark 9. *Note that the condition (i) does not require the fact that is a convex set. So Theorem 8 is different from Theorem 3.2 in [20]. The following example explains this case.

*Example 10. *Let , , and ,
Obviously, is a compact set. However, is not a convex set. So, Theorem 3.2 in [20] is not applicable. By the definition of , is -concavelike-convexlike on and -lower semicontinuous on . Thus, all conditions of Theorem 8 hold. Indeed, for each ,
Namely, .

*4. Existence of -Cone Saddle Points*

*Lemma 11. Let be a nonempty subset of and . Let and let be a vector-valued mapping with , where , , , and . If there exists such that
then is a weak --saddle point of on .*

*Proof. *By assumptions, we have
Then,
By (27), taking ,
which implies . Then, by (27), taking ,
which implies . Thus, is a weak --saddle point of on . This completes the proof.

*Theorem 12. Let and be nonempty sets and . Suppose that is a vector-valued mapping and the following conditions are satisfied:(i) and are compact sets;(ii) is -concavelike-convexlike on ;(iii) is -upper semicontinuous on ;(iv) is -lower semicontinuous on .Then, has a weak --saddle point on .*

*Proof. *Let and be a vector-valued mappings by
Next, we show that all assumptions of Theorem 8 are satisfied by .

Clearly, by the condition (i), is compact. Then, by the condition (ii), we have the fact that, for each and , there exists such that
and, for each and , there exists
By (31) and (32), for each and , there exists such that
Namely, is -concavelike-convexlike on .

Now, we show that is -lower semicontinuous on . By the condition (iii), for each and , there exists an open neighborhood of and of such that
and, for each and , there exists an open neighborhood of and of such that
By (34) and (35), we have the fact that, for any ,
Namely, is -lower semicontinuous on . Therefore, by Lemma 11, has a weak --saddle point on . This completes the proof.

*Remark 13. *The conditions (iii) and (iv) of Theorem 12 do not imply that is continuous (see [23]).

*Conflict of Interests*

*The author declares that there is no conflict of interests regarding the publication of this paper.*

*Acknowledgments*

*The author would like to thank the anonymous referees for their valuable comments and suggestions, which helped to improve the paper. This paper is dedicated to Professor Miodrag Mateljevi’c on the occasion of his 65th birthday.*

*References*

- G. Bigi, A. Capătă, and G. Kassay, “Existence results for strong vector equilibrium problems and their applications,”
*Optimization*, vol. 61, no. 5, pp. 567–583, 2012. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - X.-H. Gong, “The strong minimax theorem and strong saddle points of vector-valued functions,”
*Nonlinear Analysis. Theory, Methods & Applications*, vol. 68, no. 8, pp. 2228–2241, 2008. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - X. Gong, “Strong vector equilibrium problems,”
*Journal of Global Optimization*, vol. 36, no. 3, pp. 339–349, 2006. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - X. B. Li, S. J. Li, and Z. M. Fang, “A minimax theorem for vector-valued functions in lexicographic order,”
*Nonlinear Analysis. Theory, Methods & Applications*, vol. 73, no. 4, pp. 1101–1108, 2010. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - J. W. Nieuwenhuis, “Some minimax theorems in vector-valued functions,”
*Journal of Optimization Theory and Applications*, vol. 40, no. 3, pp. 463–475, 1983. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - T. Tanaka, “A characterization of generalized saddle points for vector-valued functions via scalarization,”
*Nihonkai Mathematical Journal*, vol. 1, no. 2, pp. 209–227, 1990. View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - T. Tanaka, “Generalized semicontinuity and existence theorems for cone saddle points,”
*Applied Mathematics and Optimization*, vol. 36, no. 3, pp. 313–322, 1997. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Y. Zhang, S. J. Li, and S. K. Zhu, “Minimax problems for set-valued mappings,”
*Numerical Functional Analysis and Optimization*, vol. 33, no. 2, pp. 239–253, 2012. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Q.-b. Zhang, M.-j. Liu, and C.-z. Cheng, “Generalized saddle points theorems for set-valued mappings in locally generalized convex spaces,”
*Nonlinear Analysis. Theory, Methods & Applications*, vol. 71, no. 1-2, pp. 212–218, 2009. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Q. H. Ansari, I. V. Konnov, and J. C. Yao, “Existence of a solution and variational principles for vector equilibrium problems,”
*Journal of Optimization Theory and Applications*, vol. 110, no. 3, pp. 481–492, 2001. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Q. H. Ansari, I. V. Konnov, and J. C. Yao, “Characterizations of solutions for vector equilibrium problems,”
*Journal of Optimization Theory and Applications*, vol. 113, no. 3, pp. 435–447, 2002. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Q. H. Ansari, X. Q. Yang, and J.-C. Yao, “Existence and duality of implicit vector variational problems,”
*Numerical Functional Analysis and Optimization*, vol. 22, no. 7-8, pp. 815–829, 2001. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - J. W. Chen, Z. Wan, and Y. J. Cho, “The existence of solutions and well-posedness for bilevel mixed equilibrium problems in Banach spaces,”
*Taiwanese Journal of Mathematics*, vol. 17, no. 2, pp. 725–748, 2013. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Y. J. Cho, S. S. Chang, J. S. Jung, S. M. Kang, and X. Wu, “Minimax theorems in probabilistic metric spaces,”
*Bulletin of the Australian Mathematical Society*, vol. 51, no. 1, pp. 103–119, 1995. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - Y. J. Cho, M. R. Delavar, S. A. Mohammadzadeh, and M. Roohi, “Coincidence theorems and minimax inequalities in abstract convex spaces,”
*Journal of Inequalities and Applications*, vol. 2011, article 126, 2011. View at Publisher · View at Google Scholar - Y.-P. Fang and N.-J. Huang, “Vector equilibrium type problems with ${(S)}_{+}$-conditions,”
*Optimization*, vol. 53, no. 3, pp. 269–279, 2004. View at Publisher · View at Google Scholar · View at MathSciNet - B. T. Kien, N. C. Wong, and J.-C. Yao, “Generalized vector variational inequalities with star-pseudomonotone and discontinuous operators,”
*Nonlinear Analysis. Theory, Methods & Applications*, vol. 68, no. 9, pp. 2859–2871, 2008. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - X. B. Li and S. J. Li, “Existence of solutions for generalized vector quasi-equilibrium problems,”
*Optimization Letters*, vol. 4, no. 1, pp. 17–28, 2010. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - L. Q. Anh and P. Q. Khanh, “Semicontinuity of the approximate solution sets of multivalued quasiequilibrium problems,”
*Numerical Functional Analysis and Optimization*, vol. 29, no. 1-2, pp. 24–42, 2008. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - K. Kimura, Y. C. Liou, Yao, and J. C. :, “Semicontinuty of the solution mapping of $\epsilon $-vector equilibrium problem,” in
*Globalization Challenge and Management Transformation*, J. L. Zhang, W. Zhang, X. P. Xia, and J. Q. Liao, Eds., pp. 103–113, Science Press, Beijing, China, 2007. View at Google Scholar - X. B. Li and S. J. Li, “Continuity of approximate solution mappings for parametric equilibrium problems,”
*Journal of Global Optimization*, vol. 51, no. 3, pp. 541–548, 2011. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet - J. Jahn,
*Vector Optimization: Theory, Applications, and Extensions*, Springer, Berlin, Germany, 2004. View at MathSciNet - D. H. Luc,
*Theory of Vector Optimization*, vol. 319 of*Lecture Notes in Economics and Mathematical Systems*, Springer, Berlin, Germany, 1989. View at MathSciNet - Y. Zhang and S. J. Li, “Minimax theorems for scalar set-valued mappings with nonconvex domains and applications,”
*Journal of Global Optimization*, vol. 57, no. 4, pp. 1359–1373, 2013. View at Publisher · View at Google Scholar · View at Zentralblatt MATH · View at MathSciNet

*
*