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Journal of Operators
Volume 2013 (2013), Article ID 124962, 10 pages
Evolution of the Minimax Inequality of Ky Fan
1The National Academy of Sciences, ROK, Seoul 137-044, Republic of Korea
2Department of Mathematical Sciences, Seoul National University, Seoul 151-747, Republic of Korea
Received 14 November 2012; Accepted 27 February 2013
Academic Editor: Ram U. Verma
Copyright © 2013 Sehie Park. 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.
There are quite a few generalizations or applications of the 1984 minimax inequality of Ky Fan compared with his original 1972 minimax inequality. In a certain sense, the relationship between the 1984 inequality and several hundreds of known generalizations of the original 1972 inequality has not been recognized for a long period. Hence, it would be necessary to seek such relationship. In this paper, we give several generalizations of the 1984 inequality and some known applications in order to clarify the close relationship among them. Some new types of minimax inequalities are added.
The KKM theory is originated from the Knaster-Kuratowski-Mazurkiewicz (KKM for short) theorem of 1929 . Since then, it has been found a large number of results which are equivalent to the KKM theorem; see [2, 3]. Typical examples of the most remarkable and useful equivalent formulations are Ky Fan's KKM lemma of 1961  and his minimax inequality of 1972 . The inequality and its various generalizations are very useful tools in various fields of mathematical sciences.
Since 1961, Ky Fan showed that the KKM theorem provides the foundation for many of the modern essential results in diverse areas of mathematical sciences. Actually, a milestone in the history of the KKM theory was erected by Fan in 1961 . His 1961 KKM Lemma (or the Fan-KKM theorem) extended the KKM theorem to arbitrary topological vector spaces and had been applied to various problems in his subsequent papers [5–10].
Recall that, at the beginning, the basic theorems in the KKM theory and their applications were established for convex subsets of topological vector spaces mainly by Fan in 1961–1984 [4–10]. A number of intersection theorems and their applications to various equilibrium problems followed. In our previous review , we recalled Fan's contributions to the KKM theory based on his celebrated 1961 KKM lemma, and introduced relatively recent applications of the lemma due to other authors in the twenty-first century.
Then, the KKM theory was extended to convex spaces by Lassonde in 1983  and to -spaces (or H-spaces) by Horvath in 1983–1993 [13–16] and others. Since 1993, the theory has been extended to generalized convex (G-convex) spaces in a sequence of papers of the present author and others; see . Since 2006, the main theme of the theory has become abstract convex spaces in the sense of Park [17–30]. The basic theorems in the theory have numerous applications to various equilibrium problems in nonlinear analysis and other fields. In our previous review , we recalled our versions of general KKM type theorems for abstract convex spaces and introduced relatively recent applications of various generalized KKM type theorems due to other authors in the twenty-first century.
While we were studying on [11, 30], we noticed that there are quite a few generalizations and applications of the 1984 minimax inequality of Fan compared with his 1972 inequality. In a certain sense, the 1984 inequality is not connected with several hundreds of known generalizations of the original minimax inequality. Hence, it would be necessary to seek such relationship. In this paper, we introduce several generalizations of two minimax inequalities of Fan and some known direct applications in order to clarify the close relationship among them.
This paper is organized as follows. In Section 2, we introduce Fan's 1961 KKM lemma, 1972 minimax inequality, and 1984 minimax inequality with some applications of them. Section 3 deals with the evolution of the 1984 inequality. Actually, we give its generalizations to convex spaces of Lassonde, convex spaces with the coercivity due to Chang, H-spaces originated by Horvath, and G-convex spaces and -spaces of Park. In Section 4, we introduce recently obtained basic concepts or results of the KKM theory such as the abstract convex spaces, generalized KKM theorems and general minimax inequalities. We show that one of our general minimax inequalities subsumes previously obtained inequalities on various types of KKM spaces in Section 3. Section 5 deals with other generalizations of the Fan minimax inequality and vector minimax inequality. Finally, we add some related historical remarks in Section 6.
2. The Origin and Fan's Applications
The 1961 KKM Lemma of Fan. Let be an arbitrary set in a Hausdorff topological vector space . To each , let a closed set in be given such that the following two conditions are satisfied: (i)convex hull of any finite subset of is contained in ,(ii) is compact for at least one .Then .
Five decades after the birth of the lemma, the above original form is still adopted by many authors in each year. But, it was found quite a long time ago by Lassonde  that the Hausdorffness is redundant. Moreover, note that can be any convex subset of a topological vector space.
Recall that an extended real-valued function , where is a topological space, is lower semicontinuous (l.s.c.) if is open for each .
For a convex set , a function is said to be quasiconcave if is convex for each .
Similarly, the upper semicontinuity (u.s.c.) and the quasiconvexity can be defined.
One of the most remarkable equivalent formulations of the KKM theorem is the minimax inequality established by Fan from his KKM lemma. The following is the original form given by Fan in 1972 .
The Fan Minimax Inequality. Let be a compact convex set in a Hausdorff topological vector space. Let be a real-valued function defined on such that(a)for each fixed , is a lower semicontinuous function of on ;(b)for each fixed , is a quasiconcave function of on . Then, the minimax inequality holds.
In , Fan applied his inequality to the following:(1)a variational inequality (extending Hartman-Stampacchia (1966) and Browder (1967));(2)a geometric formulation of the inequality (equivalent to the Fan-Browder fixed point theorem (1968));(3)separation properties of u.d.c. multimaps, coincidence and fixed point theorems;(4)properties of sets with convex sections (from which the Sion minimax theorem (1958), the equilibrium theorem of Nash (1951), and a variant of a theorem of Debrunner and Flor (1964) on extension of monotone sets easily follow);(5)a fundamental existence theorem in potential theory.
The inequality became a crucial tool in proving many existence problems in various fields of mathematical sciences, for example, nonlinear analysis, especially in fixed point theory, variational inequality problems, various equilibrium theory, mathematical programming, partial differential equations, game theory, impulsive control, and mathematical economics; see [21, 26, 31] and the references therein.
Theorem 1 (see ). Let be a nonempty convex set in a Hausdorff topological vector space. Let be a real-valued function defined on such that(a)for each fixed , is a lower semicontinuous function of on ;(b)for each fixed y ∈ X, is a quasi-concave function of on ;(c) for all ;(d) has a nonempty compact convex subset such that the set is compact.
Then, there exists a point such that for all .
The following proof was given by Fan .
Theorem 1 The Fan Minimax Inequality. Using condition (a), we see that in the case compact , condition (d) is fulfilled by taking as any nonempty closed convex subset of . Thus, Theorem 1 reduces to the minimax inequality.
In , Theorem 1 was applied to a best approximation theorem, coincidence and fixed point theorems, a matching theorem for two closed covers of a convex set, another proof of the Brouwer fixed point theorem, and a generalization of Shapley's KKM theorem.
Recall that the original Nash equilibrium theorem was proved by the Brouwer or the Kakutani fixed point theorem; see [33, 34]. Later, Fan  proved it by applying his result on sets with convex sections. Nowadays, it is known to be one of the most important applications of the Fan minimax inequality; see [10, 27]. Note that, in a wide sense, the Brouwer theorem, the KKM theorem, the Kakutani theorem, the Nash theorem, Fan's theorem on sets with convex sections, the Fan inequality, the Fan-Browder fixed point theorem, and many others are mutually equivalent; see [2, 3].
3. Evolution of the Minimax Inequality of Ky Fan
The Fan minimax inequality has been followed by a large number of generalizations and applications in the KKM theory on convex subsets of topological vector spaces, Lassonde type convex spaces, Horvath type H-spaces, generalized convex spaces due to Park, and other types of spaces. Furthermore, many authors generalized the lower semicontinuity and quasiconcavity in the inequality or replaced them by other requirements. Therefore, even for convex spaces, it is necessary to establish proper forms of the Fan minimax inequality which unify as many particular cases as possible.
Multimaps are also simply called maps. Let denote the set of all nonempty finite subsets of a set .
In this section, we show several major generalizations of the Fan minimax inequality in the chronological order.
3.1. Lassonde's Theorem
The concept of convex sets in a topological vector space was extended to convex spaces by Lassonde in 1983 , and further to c-spaces by Horvath in 1983–1993 [13–16]. A number of other authors also extended the concept of convexity for various purposes.
Definition 2. Let be a subset of a vector space and a nonempty subset of . One calls a convex space if co and has a topology that induces the Euclidean topology on the convex hulls of any (see Park ). For a convex space , a subset of is said to be -convex if for each , implies co.
If is convex, then becomes a convex space in the sense of Lassonde .
A nonempty subset of a convex space is called a c-compact set  if for each finite subset there is a compact convex set such that .
Lassonde  presented a simple and unified treatment of a large variety of minimax and fixed point problems. He first noticed that the Hausdorffness in the 1961 Fan-KKM lemma is redundant. More specifically, he gave several KKM type theorems for convex spaces and proposed a systematic development of the method based on the KKM theorem; the principal topics treated by him may be listed as follows: Fixed point theory for multimaps; Minimax equalities; Extensions of monotone sets; Variational inequalities; Special best approximation problems.
The following [12, Proposition I.1.4] extends the 1984 inequality of Fan.
Theorem 3 (see ). Let X be a convex space and a convex l.s.c. function satisfying the following:(i)for each fixed , is l.s.c. on (compact subsets of) ;(ii)for each fixed , is concave on ;(iii)for all , . Furthermore, assume that the following “coercivity” condition holds:(iv)there are a compact set and a c-compact set such that for each , there is an with .
Then, there exists such that for all .
Consider that the following extends Theorem 3.
Theorem 3′. In Theorem 3, assume that (ii)′for each fixed , is quasiconcave on instead of (ii).
Theorem 3′ Theorem 1. Note that (a)–(c) of Theorem 1 imply (i), (ii)′, and (iii) of Theorem 3′, respectively. Now, we show that (d) implies (iv). Let for all . Then for all , there exists such that . Since is a compact convex subset of a Hausdorff topological vector space, it is a c-compact set. Then, condition (iv) holds with and . Therefore, by Theorem 3′, there exists a point such that for all .
Remark 4. As shown in the above proof, the Hausdorffness in the 1984 inequality is essential. However, it is redundant in the original 1972 minimax inequality.
The compactly closed (resp., open) sets adopted by Lassonde and many followers can be replaced simply by closed (resp., open) sets; see [22, 36]. Similarly, “l.s.c. on compact subsets of X” in (i) can be simply “l.s.c.”
3.2. Chang Type Coercivity
In 1989, Chang  obtained a KKM theorem with a coercivity condition which eliminated the concept of c-compact sets. From a Fan-Browder type fixed point theorem equivalent to her KKM theorem, we noticed that the following holds .
Theorem 5 (see ). Let be a convex space, and and be functions satisfying the following:(i) for and for all ;(ii)for each , is convex or empty;(iii)for each , is open; (iv)there exist a nonempty compact subset of and, for each finite subset of , a compact convex subset of containing such that implies for some .
Then, there exists a point such that Moreover, the set of all such solutions is a compact subset of .
Actually, Theorem 5 is equivalent to the following.
Theorem 5′. The case of in Theorem 5.
Theorem 5 Theorem 3. We apply Theorem 5 with and . Then we obtain (i)–(iv) in Theorem 5 from Theorem 3 as follows.(i)Clearly holds since for all .(ii)For each , is convex or empty since is concave and is convex. (iii)For each , is open since is l.s.c. and is l.s.c.(iv)There exist a nonempty compact subset of and, for each finite subset of , a compact convex subset of containing (since is -compact) such that implies for some .
3.3. For H-Spaces
In this subsection, we follow .
Definition 6. A triple is called an H-space if is a topological space, is a nonempty subset of , and is a family of contractible (or, more generally, -connected) subsets of indexed by such that whenever .
For an H-space , a subset of is said to be H-convex if for each , implies .
A multimap is said to be H-KKM if for each . A subset of is called an H-subspace of if , and for every , is contractible.
If , we denote instead of , which is called a -space by Horvath [13–16] or an H-space by Bardaro and Ceppitelli . Horvath noted that a torus, the Möbius band, or the Klein bottle can be regarded as c-spaces and are examples of compact c-spaces without having the fixed point property.
In the frame of H-spaces, we obtained several generalized minimax inequalities. The following is one of them in [39, Theorem 7].
Theorem 7 (see ). Let be an H-space, let and be functions, and let such that(1) for all and for all ,(2)for each , is (compactly) closed, (3)for each , is H-convex.Suppose that there exists a nonempty compact subset of such that either(i)there exists an such that (ii)or for each , there exists a compact H-subspace of containing such that for each there exists an satisfying .
Then, there exists a such that
Theorem 7 Theorem 5′. Any convex space is an H-space.
3.4. For G-Convex Spaces
Definition 8. A generalized convex space or a G-convex space consists of a topological space , a nonempty set , and a map such that for each with the cardinality , there exists a continuous function such that implies .
Here, is the standard -simplex, and is the face of corresponding to ; that is, if and , then . We may write for each and .
For a G-convex space , a subset of is said to be G-convex with respect some if for each , we have .
A map is called a KKM map if for each .
Theorem 9. Let be a G-convex space, let and be functions, and let such that(1) for all ,(2)for each , is closed,(3)for each , implies . Suppose that there exists a nonempty compact subset of such that either(i)there exists an such that (ii)or for each , there exists a compact G-convex subset of relative to some such that , and for each there exists a satisfying .
Then, there exists a such that
3.5. For -Spaces
Definition 10. A space having a family or simply a -space
consists of a topological space , a nonempty set , and a family of continuous functions (i.e., singular -simplexes) for with the cardinality .
For a -space , a subset of is said to be -convex relative to some if for each , we have .
We define a KKM map on a -space if, for each and , we have where is the face of corresponding to ; see Park .
The following minimax inequality for -spaces is new.
Theorem 11. Let be a -space, let and be functions, and let such that (1) for all ,(2)for each , is closed,(3)for each and , one has .
Suppose that there exists a nonempty compact subset of such that either(i)there exists an such that (ii)or for each , there exist a compact -convex subset of relative to some such that and for each there exists a satisfying .
Then, there exists a such that
4. Abstract Convex Spaces and General KKM Theorems
Definition 12. An abstract convex space consists of a topological space , a nonempty set , and a multimap with nonempty values for .
For any , the -convex hull of is denoted and defined by
A subset of is called a -convex subset of relative to if for any , we have ; that is, .
In case , let .
Definition 13. Let be an abstract convex space and be a topological space. For a multimap with nonempty values, if a multimap satisfies
then is called a KKM map with respect to . A KKM map is a KKM map with respect to the identity map .
A multimap is called a -map (resp., a -map) if, for any closed-valued (resp., open-valued) KKM map with respect to , the family has the finite intersection property. In this case, we denote (resp., ).
Definition 14. The partial KKM principle for an abstract convex space is the statement ; that is, for any closed-valued KKM map , the family has the finite intersection property. The KKM principle is the statement that the same property also holds for any open-valued KKM map.
An abstract convex space is called a (partial) KKM space if it satisfies the (partial) KKM principle, respectively.
In our recent works [3, 18, 19], we studied the elements or foundations of the KKM theory on abstract convex spaces and noticed that many important results therein are related to the partial KKM principle.
Example 15. We give known examples of partial KKM spaces; see  and the references therein.(1)Every -space is a KKM space. More precisely, for a -space , the corresponding abstract convex space with for , is a KKM space. This KKM space may not be -convex; see .(2)A connected linearly ordered space can be made into a KKM space.(3)The extended long line is a KKM space with the ordinal space . But is not a G-convex space.(4)For a closed convex subset of a complete -tree , and for each , Kirk and Panyanak showed that the triple satisfies the partial KKM principle.(5)For Horvath's convex space with the weak Van de Vel property is a KKM space, where for each .(6)A -space due to Briec and Horvath is a KKM space.(7)Recently, Kulpa and Szymanski  found some partial KKM spaces which are not KKM spaces.
Now we have the following diagram for triples :
The following whole intersection property for the map-values of a KKM map is a standard form of the KKM type theorems [20, 23, 24].
Theorem A. Let be an abstract convex space, the identity map resp., , and be a multimap satisfying the following:(1) has closed (resp., open) values,(2) for any (i.e., is a KKM map).Then has the finite intersection property.
Further, if is compact for some ,then one has
Consider the following related four conditions for a map . (a) implies .(b) ( is intersectionally closed valued). (c) ( is transfer closed-valued). (d) is closed valued.
Theorem B. Let be a partial KKM space (i.e., ) and a map such that(1) is a KKM map (i.e., for all ), (2)there exists a nonempty compact subset of such that either(i) for some ,(ii)or for each , there exists a compact -convex subset of relative to some such that and
Then one has .
Furthermore,(α)if is transfer closed valued, then ; (β)if is intersectionally closed valued, then .
Remark 16. (1) Every -space is a KKM space. Hence, Theorem B works for -spaces.
(2) We may assume that is closed. Then, the closure notations in each coercivity condition can be eliminated.
From the preceding theorem, we obtained several prototypes of minimax inequalities in . Here we give only one of them with its proof for completeness.
Theorem 17 (). Let be a partial KKM space, let and be extended real-valued functions, and let such that (0) for each , ,(1) the map defined by for is intersectionally closed-valued (resp., transfer closed-valued), (2)for each , ,(3)the coercivity condition (2) of Theorem B holds.Then,
(a) there exists a resp., such that for all ; and
(b)if , then one has
Lemma 18. Under the hypothesis of Theorem 17, condition (2) implies that is a KKM map.
Proof. Suppose, on the contrary, that there exists a finite such that . Then, there exist a such that or for all . Hence, and, by (2), we have . Since , we have . This contradicts (1).
Proof of Theorem 17. By Lemma, is a KKM map. Therefore, all the requirements of Theorem B are satisfied. Hence, (a) follows from Theorem B. Note that (b) is a simple consequence of (a).
Remark 19. Comparing Theorem 17 with the original Fan minimax inequality, note that condition (1) generalizes the l.s.c. of , (2) generalizes the quasiconcavity of whenever and , and (3) generalizes the compactness of the underlying space.
In condition (1), transfer closedness can be replaced by mere closedness. For a long period, instead of closedness, some authors adopted the so-called compactly closedness, transfer compactly closedness, or even by the finitely closedness when is a convex subset of a t.v.s. Moreover, (1) holds whenever is l.s.c. This also can be replaced by compactly l.s.c., transfer compactly l.s.c, or finitely l.s.c. This kind of terminology is not essential and generalizes nothing important.
Now we show the following.
Remark 20. The above proof shows that Theorem 17 subsumes all of Theorems 1–11 and numerous variants or particular forms of them appeared in previous works.
There are other theorems also containing Theorem 1–11; see [26, 29].
5. Other Types of Generalizations
This section deals with some of other generalizations of the Fan minimax inequality and vector minimax inequality.
5.1. Generalizations of Quasiconcavity
The compactness, convexity, lower semicontinuity, and quasiconcavity in the inequality are extended or modified by a large number of authors. For example, the quasicocavity is extended to -DQCV by Zhou and Chen in 1988. Further, Lin and Tian in 1993 [31, Theorem 3] defined -DQCV in slightly more general form:
Let be a convex subset of a Hausdorff t.v.s. and let . A functional is said to be -diagonally quasi-concave (-DQCV) in if, for any finite subset and any co, we have .
Theorem 21 (see ). Let be a nonempty convex subset of a Hausdorff t.v.s. , let , and let be a functional such that(i) is l.s.c. in ,(ii) is -DQCV in ,(iii)there exists a nonempty subset of such that is compact and is contained in a compact convex subset of .
Then there exists a point such that for all .
Lin and Tian  proved Theorem 21 by applying the partition of unity argument [this is why Hausdorffness is assumed] and the Brouwer fixed point theorem. Moreover, they showed that Theorem 21 is equivalent to the Fan KKM lemma (where Hausdorffness is redundant). Note that Theorem 21 improves the 1984 inequality of Fan.
Moreover, the -quasiconcavity due to Hou in 2009  unifies the diagonal transfer quasiconcavity (weaker than quasiconcavity) and the -concavity (weaker than concavity).
Here we introduce a new usage of -spaces and a new minimax inequality.
Definition 22 (see ). Let be a topological space, and let . A real function is said to be -quasiconcave on if, for any , there exists a continuous map such that
for all , where .
Note that is a -space.
In , we obtained the following generalization of the Fan minimax inequality:
Theorem 23 (see ). Let be a topological space, a nonempty subset of , and . Assume that:(1) on ,(2)there exist such that is compact where and ;(3) is -quasiconcave on ,(4)for each , is intersectionally closed resp., transfer closed. Then there exists resp., such that holds.
Furthermore, S.-Y. Chang in 2010 extended the -quasiconcavity to 0-pair-concavity and obtained a new Fan type inequality; see Park .
5.2. Vector Ky Fan Minimax Inequality
Definition 24 (see ). Let be a Hausdorff topological space, a Hausdorff topological vector space with nonempty, convex, closed, and pointed cone with Int , and . A function is called --quasiconcave on if and only if, for any finite subset of , there exists a continuous mapping such that, for any , there exists such that
In , the --quasiconcavity is called the generalized -quasiconcave. In the above definition, note that is a -space.
Definition 25 (see ). Let be a Hausdorff topological space, and let be a Hausdorff topological vector space with nonempty, convex, closed, and pointed cone with Int . A vector-valued function is called -continuous at if and only if, for any open neighborhood of the zero element in , there exists an open neighborhood of in such that, for any . In particular, is called -continuous on if and only if is -continuous at every point of .
Theorem 26 (see ). Let be a nonempty and compact subset of a Hausdorff topological vector space , and let be a Hausdorff topological vector space with nonempty, convex, closed, and pointed cone with . A mapping satisfies the following:(i)for any fixed is -continuous;(ii)for any fixed is --quasiconcave on X;(iii)for any . Then there exists such that ∉ for any .
6. Comments and Historical Remarks
Recall that the main applications of recent generalized KKM theorems are as follows; see [11, 30]: Vector variational-type inequalities, Various quasiequilibrium problems, Eigenvector problems, Set-valued minimax inequality, Fixed point theorems, Generalizations of Nash equilibrium theorem, Variational inclusion problem, Simultaneous nonlinear inequalities problem, Iferential inclusion problem, (Vector mixed) quasivariational inequality, (Vector mixed) quasicomplementarity problem, Traffic network problem, Quasi-monotone vector equilibrium problem, Generalized vector equilibrium problem, Generalized (implicit) vector variational-like inequality, Set equilibrium problem, Set-valued mixed (quasi)variational inequalities, Variational-hemivariational inequalities.
In 2006–2009, we proposed new concepts of abstract convex spaces and the (partial) KKM spaces which are proper generalizations of G-convex spaces and adequate to establish the KKM theory; see [3, 18, 19] and the references therein. The partial KKM principle for an abstract convex space is an abstract form of the classical KKM theorem. A partial KKM space is an abstract convex space satisfying the partial KKM principle. A KKM space is an abstract convex space satisfying the partial KKM principle and its “open” version. Now the KKM theory becomes the study of spaces satisfying the partial KKM principle.
In our work , we clearly derived a sequence of a dozen statements which characterize the KKM spaces and equivalent formulations of the partial KKM principle. We add more than a dozen statements as their applications, including generalized formulations of von Neumann minimax theorem, von Neumann intersection lemma, the Nash equilibrium theorem, and the Fan type minimax inequalities for any KKM spaces. Consequently,  unifies and enlarges previously known several proper examples of such statements for particular types of KKM spaces.
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