O Fixed Point Theory and Applications

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Feasibility-solvability theorems for generalized vector equilibrium problem in reflexive Banach spaces

Gang Wang1*, Hai-tao Che2 and Hai-bin Chen1

* Correspondence: wgglj1977@163. com

1Schoolof Management Science, Qufu NormalUniversity, Shandong Rizhao 276826, China Fulllist of author information is available at the end of the article

Abstract

In this article, the solvability of generalized vector equilibrium problem (GVEP) with set-valued mapping in reflexive Banach spaces is considered. Under suitable conditions, we establish a link between SK and SDloc for the GVEP. Furthermore, new sufficient conditions are provided for the nonemptiness and boundedness of the solution set of the GVEP if it is strictly feasible in the strong sense. The new results extend and improve some existence theorems for vector equilibrium problem in some sense.

Mathematical subject classification: 49K30; 90C29.

Keywords: generalized vector equilibrium problem, C-pseudomonotone, strict feasibility, asymptotic cone

1 Introduction

Let X be a real reflexive Banach space and U be a metric space, and K £ X, D £ U be two nonempty and closed sets. Let T: K ®2D be a nonempty-compact-valued mapping, i.e., T(x) is a nonempty compact subset for any x e K, and upper semicontinuous on K. Let F:D x K x K x Y bea vector-valued map, where Y is a real normed space with an ordered cone C, that is, a proper, closed and convex cone such that int C = 0. The generalized vector equilibrium problem [1], abbreviated by GVEP, is to find x e K and u e T(x) such that

(GVEP) F(u, x, y) e -int C, Vy e K.

The GVEP includes vector optimization problem, vector variational inequality problem, vector complementarity problem, vector Nash equilibrium problem, and fixed point problem, which has made notable influence in several branches of pure and applied sciences.

For the GVEP, its dual problem is to find x e K such that

(DGVEP) F(u,y,x) e int C, Vy e K, u e T(y).

Throughout this article, we denote the solution set of the GVEP and the solution set of the DGVEP by SK and S^D, respectively.

Springer

© 2012 Wang et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

According to the definition of local solution of the dual problem for the equilibrium problem introduced in [2], we use

SK,ioc= {x e K : 3r > 0, F(u, y, x)/int C, Vy e K, u e T(y), \\y - x\\ < r}

to denote the local solutions of the DGVEP. Obviously, SD <Z SD

SK - SK,loc'

It is well known that the solvability of the (vector) equilibrium problem is an important issue. Based on the coercivity assumption, the existence of solution for (vector) equilibrium problem received much attention of researchers, see e.g., [1-8]. For the vector equilibrium problem, Bianchi discussed the existence of solution under the condition that F is pseu-domonotone or quasimonotone [3]. Later, Gong established the existence of the solution for strong vector equilibrium problem via the separation theorem for convex sets [6]. Recently, Long considered the existence, connectedness, and compactness of the solutions for vector equilibrium problem [8-10]. In this article, motivated by the work of Bianchi for equilibrium problem [2], we establish the relation between SK and SD ioc for the GVEP.

It should be noted that for the vector equilibrium problem, the existence of solution can also be established on the strict feasibility condition which was originally used in scalar variational inequality and vector variational inequality [11-14], and this can be extended to the scalar equilibrium problem by establishing solvability of a scalar monotone equilibrium problem when it is strictly feasible [15]. On other way, Hu and Fang [16] generalized the concept of strict feasibility to the vector equilibrium problem and established the nonemptiness and boundedness of the solution set for C-pseudo-monotone vector equilibrium problem under suitable conditions. In this article, we establish the nonemptiness and boundedness of the solution set for the GVEP under a relatively weaker condition.

In summary, in this article, we first establish the relation SK and SKD,loc for the GVEP under suitable conditions, and then establish the equivalence between solution set of the GVEP and solution set of the DGVEP. In addition, some new sufficient conditions are presented for the nonemptiness and boundedness of the solution set for the GVEP are proposed under the condition that it is strictly feasible in the strong sense.

2 Notations and preliminaries

In this section, we mainly give some notations and some preliminary results needed in the following.

Definition 2.1 Let K be a nonempty convex subset of X and C be a closed convex cone in real normed space Y.

(i) The mapping F: K ® Y is said to be C-convex if

aF(x) + (1 - a)F(y) e F(ax +(1 - a)y) + C, Vx,y e K, a e [0,1].

(ii) The mapping F: K ® Y is said to be C-quasiconvex if the set {x e K | F(x) e aC} is convex for any a e Y.

The mapping F: K ® Y is said to be C-explicitly quasiconvex if F is quasiconvex and F(y) - F(x) e int C ^ F(y) - F(tx + (1 - t)y) e int C, Vx,y e K, t e (0,1).

(iii) [3] The mapping F:K ® Y is said to be C-lower semicontinuous if the set {x e K | F(x) -a € int C} is closed on Kfor any a e Y. F is said to be weakly C-lower semi-continuous if F is C-lower semicontinuous with respect to the weak topology of X. The map F is said to be weakly lower semicontinuous on K if it is weakly lower semicontinuous onK.

(iv) The mapping F: D x K x K®Y is said to be generalized hemicontinuous if the map t ® F(u0 x + t(y-x),y) is continuous at 0+ for any x,ye K and ut _ T(x + t(y-x)).

(v) The mapping F:D x K x K®Y is said to be C-pseudomonotone iffor all x,ye K, ue T(x),ve T(y),

3u e T(x) such that F(u, x, y) e -int C ^Vv e T(y) such that F(v, y, x) e int C, or equivalently,

3v e T(y) such that F(v, y, x) e int C ^Vu e T(x) such that F(u, x, y) e -int C.

(vi) The asymptotic cone K^ and barrier cone barr(K) of K are, respectively defined by

d e X|3tfc +oo, e Kwith--^ d\

barr(K) = {x* e X*|supx e K{x*, x) < ,

where X* denotes the dual space of X and ^ stands for the weak convergence. Remark 2.1 The explicit quasiconvexity of the function F(.) implies that [3]

(a) for all c € C, the set {y e K | F(y) <C c} is convex;

(b) if F(z) - F(y) e int C and F(z) € int C, then F(z) - F(zt) e int C, for z = ty + (1 -t)z,te (0,1).

The asymptotic cone K^ has the following useful properties [3,17]. Lemma 2.1 Let K c X be nonempty and closed. Then the followings hold:

(i) K^ is a closed cone;

(ii) ifK is convex, then K^ = {d e X | K + d c K} = {d e X | x + td e K, V t >0}, where x e K is arbitrary point;

(iii) if K is convex cone, then K^ = K.

Lemma 2.2 Let a,be Y be such that a e -int C and b € C. Then, the set of upper bounds of a and b is nonempty and intersects with Y\C.

Definition 2.2 The GVEP is said to be strictly feasible in the strong sense if + =A where

Ф5 + = {x e K\F(u,x,x + y) e int C, Vy e KTO\{0}, u e T(x)}.

Definition 2.3 [18]A set-valued map F: E ® 2X is said to be KKM mapping if coA с un=1F(xi)(xi) for each finite set Л = Xi,..., Xn £ E, where co (.) stands for the convex hull.

The main tools for proving our results are the following well-known KKM theorems.

Lemma 2.3 [19]For topological vector space X, let E £ X be a nonempty convex and F: E ® 2X be a KKM mapping with closed values. If there is a subset X0 contained in a compact convex subset of E such that nxe XF(x) is compact, then nxe£F(x) = 0.

Definition 2.4 [20,21]Let K be a nonempty, closed and convex subset of a real reflexive Banach space X with its dual X*. K is said to be well-positioned if there exist x0 e X and ge X* such that

g, x — x0) > ||x — x0 У, Vx e K.

Lemma 2.4 [20,21]Let K be a nonempty, closed and convex subset of a real reflexive Banach space X with its dual X*. Then K is well-positioned if and only if the barrier cone barr(K) of K has a nonempty interior. Furthermore, if K is well-positioned then there is no sequence {xn} £ K with ||xn||® such that origin is a weak limit of

Il.tJ J'

Lemma 2.5 [22]Let X and Y be two metric spaces and T: X ® 2Y be a nonempty-compact-valued mapping and upper semicontinuous at x*. Then, for any sequences xn ® x* and un e T(xn), there exist a subsequence {unil} of {un} and some u* ® T (x*) such that Unk ^ u*.

3 Feasibility-solvability theorems for the GVEP

In this section, we mainly establish a link between SK and SD loc of the GVEP. First, we make the following assumptions.

Assumption 3.1 The mapping F: D x K x K®Y is such that

(i) for all xe K, F(u, x,x) = 0, Vue T (x);

(ii) F is generalized hemicontinuous.

Lemma 3.1 Suppose that Assumption 3.1 holds andF(u,x,.) is C-convex for all x e K, u e T(x) and SD,loc =,0. Then, SD,loc с Sk .

Proof. Take x e SD,joc and y e K. From the definition of SD,ioc, there exists r >0 such that

F(u, w,x) e int C, Vw e K, и e T(w), ||x — w|| < r.

Take any y e (x, y] with ||x — y|| < r, and set yt = (1 — t)x + ty for t e (0,1). Obviously,

F (и, yt, X) e int C, Vu e T(yt ).

Since F(u,x,.) is C-convex for all x e K,u e T(x), it holds that

(1 — t)F(u, yt, X) + tF(u, yt, y) e F(u, yt, yt) + C.

Furthermore, it follows from F(u,x,x) = 0 that

tF(u,yt,y) e —(1 — t)F(u,yt,X)+F(u,yt,yt) + C ç —(1 — t)F(u,yt,X) + C ç Y\ — int C.

Hence,

F(u, yt, y) e —int C, Vu e T(yt).

Letting t ® 0+, we obtain by generalized hemicontinuity of F(.,.,y) and Lemma 2.5 that there exists u e T(X) such that

F(u, Xc, y) e —int C.

Now, we show that F(u, X, y) e —int C by contradiction. On the contrary, suppose that F(u, X, y) e —int C, then by the C-convexity of F(u,x,.),

tF(u,X,X) + (1 — t)F(u,X,y) e F(u,X, tX +(1 — t)y) + C, Vt e (0,1).

It follows from F(u, X, X) = 0 and int C + C £ int C that

F(U, X, tX+( 1 — t)y) e tF(u,X, X) + (1 — t)F(U,xx, y) — C ç (1 — t)F(U,xx, y) — C ç —int C,

that is,

F(u, X, tX +(1 — t)y) e —int C, Vt e (0,1).

Since ye (X, y], there exists t0 such that y = (1 — to)X + to y with F(u, X, y) e —int C which contradicts F(u, X, y) e —int C . So, F(u, y) e —int C . By the arbitrariness of y e K, we have X e SK and SD loc ç Sk .

Lemma 3.2 Suppose that Assumption 3.1 holds and F(u,x,.) is C-explicitly quasiconvex for allxe K,u e T(x) and SD,loc =0. Then, SD,loc ç Sk .

Proof. Take X e SD,joc and y e K. From the definition of SD,ioc, there exists r >0 such that

F(u, w,X) e int C, Vw e K, u e T(w), ||X — w\\ < r.

Take any y e (X, y] with ||X — y| < r r, and set yt = (1 — t)X + ty for t e (0,1). Obviously,

F(u, yt, X) e int C, Vu e T(yt).

Now we show that F(u, yt, y) e —int C, Vu e T(yt). by contradiction. On the contrary, suppose the conclusion does not hold, then there exists t e (0,1), u e T(yt) such that

F(u, y-t, y) e -int C.

We will break the arguments into two cases.

Case 1. If F(u, y, x) e d C c C, then

F(u, yt, x) — F(u, yt, y) e C + int C c int C,

where 9C is the boundary of C. Since F(u,x,.) is C-explicitly quasiconvex, we have

F(u, yt, x) — F(u, yt, yt) e int C.

It follow from F(u, yt, yt) = 0 that

F(u, yt, x) e int C,

which contradicts the assumption that F(u, yt, x) e dC .

Case 2. If F(u, yt, x) e C, then by Lemma 2.2, there exists p € C such that

F(u, yt, x) < cp, F(u, yt, y) < cp.

By the quasiconvexity of F(u,x,.), one has

F(u, yt, yt) < cp.

Noticing F(u, yt, yt) = 0, we obtain that p e C, which contradicts p € C.

From Cases (1) and (2), it holds that F(u, yt, y) e —int C, Vu e T(yt). Letting t e 0+, we obtain by generalized hemicontinuity of F(.,.,y) and Lemma 2.5 that there exists u e T(x) such that

F(u, 3c, y) e —int C.

Now we show that F(u, x, y) e —int C. Suppose on the contrary, F(u, x, y) e —int C , then from the facts that F(u, x, x) = 0, F(u, x, y) e —int C., it follows that

F(u, x, x) — F(u, x, y) e int C.

By the C-explicitly quasiconvexity of F(u,x,.), one has

0 — F(u, x, tx +(1 — t)y) e int C, Vt e (0,1).

That is,

F(u, x, tx +(1 — t)y) e —int C, Vt e (0,1).

Since ye y]], there exists t0 such that y = (1 — fo)x +10y with F(u, x, y) e —int C which contradicts F(u, x, y) e —int C . So, F(ii, xx, y) e —int C . By the arbitrariness of y e K, we have x e SK and S^ loc c Sk .

By virtue of C-pseudomonotonity of F, the equivalence between solution set of the GVEP and that of the DGVEP can be established.

Theorem 3.1 Let K c X be a nonempty and convex closed bounded set and suppose Assumption 3.1 holds. If F:D x K x K®Y satisfies the followings

(i) F is C-pseudomonotone;

(ii) F(u,x,.) is C-convex and weakly lower semicontinuous for x L K,u L T(x), then Sk = SD =0.

Proof. For any y L K, set

Г(у) = {x e K\F(v, y, x) e int C, Vu e T(y)}.

We claim that Г is a KKM mapping with closed values. Suppose on the contrary, it does not hold, then there exists a finite set {x^..., xn} £ K and z L co{x1t..., xn} such that z e un=1 r(xi), where co{x1,..., xn} denotes the convex hull generated by x1,... , xn. Thus, there exists vi L T(xi), such that F(vi, xy z) L int C. Since F is C-pseudomonotone, it follows that

F(w,z,xi) e —int C, Vw e T(z), i = 1,2,...,n.

So, Хл tiF(w, z, xi) e —int C, where ^Zj ^ = 1, ^ > 0, i =1,2,..., n For z = X1 tixi, due to that the function F(u,x,.) is C-convex, we have

tiF(w, z, xi) e F(w, z, z) + C с C с Y\ — int C,

which contradicts X1 tiF(w,z, xi) e —int C. So, {r(y) |y L K} satisfies the finite-intersection property. In combination with the assumption in (ii), we know that Г is a KKM mapping with closed values. Since K c X is a nonempty and convex closed bounded set, we deduce that K is weakly compact. From Lemma 2.3, there exists x* L K such that x* e ny eK r(y) = S^ .It follows from Lemma 3.1 that с S^,loc С Sk . Furthermore, SK с SD due to the C-pseudomonotonity of the F. Thus, SK = SD = 0 and the proof is completed.

Theorem 3.2 Let K с X be a nonempty and convex closed bounded set and assume Assumption 3.1 holds. IfF:D x K x K®Y satisfies that

(i) F is C-pseudomonotone;

(ii) F(u, x,.) is C-explicitly quasiconvex and weakly lower semicontinuous for x e K, u e T( x),

then Sk = SD = $.

Proof. For any y e K, set

r(y) = {x e K|F(v, y, x) e int C, Vv e T(y)}.

We claim that Г is a KKM mapping. Suppose on the contrary, it does not hold. Then there exists a finite set {x1,..., xn} £ K and z e co{x1,..., xn} such that z g Ц^Г^). Thus, there exists vi e T(xi), such that F(vi, xi, z) e int C. Since F is C-pseudomono-tone, it follows that

F(w,z,xi) e —int C, Vw e T(z), i = 1,2,...,n.

By the quasiconvexity of F(w,z,.), we deduce the set {y e K | F(w,z,y) e -int C} is convex. For z = ^ 1 Uxi,Yl 1 ti = 1, ti — 0, i = 1,2,..., n i = 1, 2,...,n, one has

F(w, z, z) e -int C,

which contradicts F(w, z, z) = 0. So, {r(y) | y e K} satisfies the finite-intersection property. By the assumption (ii), we conclude that the r is closed value. Hence r is a KKM mapping with closed values. Following the similar arguments in the proof of Theorem 3.1, we can obtain the desired result.

In the sequel, we shall present some sufficient conditions for the nonemptiness and bound-edness of the solution set of the GVEP provided that it is strictly feasible in the strong sense.

Theorem 3.3 Let K be a nonempty, closed, convex and well-positioned subset of a real reflexive Banach space X and Assumption 3.1 hold. If F: D x K x K®Y satisfies the followings

(i) F is C-pseudomonotone;

(ii) F(u,x,.) is C-convex and weakly lower semicontinuous for x e K,u e T(x), then, the GVEP has a nonempty bounded solution set whenever it is strictly feasible in the strong sense.

Proof. Suppose that the GVEP is strictly feasible in the strong sense. Then there exists x0 e K such that x0e Ts+, i.e.,

F(u, x0, x0 + z) e int C, Vu e T(x0), z e KTO.

M = {x e K|F(u,x0,x) e int C}, Vu e T(x0).

By Assumption 3.1 and (ii), x0 e M and M is weakly closed. We assert that M is bounded. Suppose on the contrary it does not holds, then there exists a sequence {xn} £ M with ||xn|| ® + ^ as n ® Since X is a real reflexive Banach space, without loss of generality, we may take a subsequence xnu of {xn} such that

1 e (0,1), lim ^ z e Kiyo

\\xnu x0 \

xnu - x0 ,. xnu jr

lim -¡j—--r- = lim -¡j—i-jr z e

fe^+TO \\xni - x0\\ \\xni \\

Indeed, since +<x. m m - there holds

|| xnu x0 ||

xnu x0 j. xnu !• x0 limfe^+(XJ-Tj-r- = lim^^-jj-¡j- - limfe^+(XJ |j-r

|| xnu x01| ||xnu x0 || ||xnu x01

1- \\xni \\ xnu = Imi ; ... || _ || . 11-IT .

II xnu x0 || || xnu ||

Noting that

\\xnk Il - llxoll \\ xnk x0 xnJ + llxoM

< ^—i--—!L < 11 1

one has limfc_+(XJ-7i— "''' —r- = 1,, which yields

|| xnk x0 ||

,. xnk x0 ,. xnk jr

lim -Tj—--r- = lim -Tj—i-jr z e

fe^+œ \\xnk - Xo\\ k^+œ \\xnk \\

Since K is well-positioned, by Lemma 2.4, we have z * 0. It follows from x0 e that

F(u, x0, x0 + z) e int C. Noting that F is C-convex, we have

tj( Xnk - xo \ J i, 1 \ Xnk \

F U, Xo, Xo + "¡1-rr = F \ U, Xo, 1 — "ii-rr Xo + 1

xnk x0 W J \ \ Wxnk x0 W / Wxnk x01 J e ( 1 - -ij---¡j- J F(u,x0,x0) + |j---¡j-F(u,x0,x„fe) - C.

xnk - x0 xnk - x0

It follows from F(u,x0,x0) = 0 that

xnk - x0 1 F I u,xo,xo + -¡j-¡j- I e -¡j-¡¡-F(u,xo,xnJ — C.

xnk - x0 xnk - x0

By virtue of F(u, x0, xnk) e int C, one has F(u, x0, xnk) e Y\int C. Consequently,

F ( u, Xq,Xq + -¡p-^—^-¡j- J e -¡j-¡j- F(m, Xo,xnk) — C c Y\int C-Cc Y\int C,

\ ||Xnk x0 || / ||xn„ x01|

that is,

F(u,xo,xo + ———£ int C. l|Xn - X0II

Taking into account that F(u,x,.) is weakly lower semicontinuous, we obtain F(u, x0, x0 + z) e int C,

which contradicts F(u, x0,x0 + z)e int C. Thus, M is bounded and so it is weakly compact. For each p e K, set

Mp = {x e M\F(v,p,x) e int C}, u e T(p).

We assert Mp , V p e K,v e T(p). Indeed, given p e K,v e T(p), set K0 = conv (MUp) £ K, where conv means the convex hull of a set. Then K0 is nonempty, convex and weakly compact. By Theorem 3.1, there exists X e K0 such that

F(u, y, X) e int C, Vy e K0, u e T(p).

Since F(u, x0, x) e int C implies x e M and F(v, p, x) e int C implies x e Mp, we obtain Mp =,0. Obviously, Mp is nonempty and weakly compact.

Next we prove that {Mp | p e K} has the finite intersection property. For any finite set {p! = 1,2,...,n}£K, let Ki = conv{Mu{p1,p2,..., pn}}. Then Ki is nonempty, convex and weakly compact. By Theorem 3.1, there exists x e Ki such that

F(v, y, x) e int C, Vy e K1, v e T(y).

In particular, we have

F(u, x0 x) e int C, F(v, pi7 x) e int C, i =1,2,..., n.

This means that x e nn=1Mpi. Thus {Mp | e K} has the finite intersection property. Since M is weakly compact and Mp £ M is weakly closed for all p e K, v e T(p), it follows that

n Mp = 0.

Let x* e nueK Mp, then

F(v,y,x*) e int C, Vy e K, v e T(y).

By Theorem 3.1, x* is a solution of the GVEP. As for the boundedness of the solution set of the GVEP, it follows from Theorem 3.1 that the solution set of the GVEP is a subset of M.

Remark 3.1 The authors of [16]discuss a special case of the GVEP when T(x) is singleton. In general the GVEP, the condition that F is positively homogeneous with degree a> 0 in [16], is not easily satisfied. Compared with [16], we remove the condition that F is positively homogeneous with degree a > 0, when F(u,x,.) is C-convex rather than C-explicitly quasiconvex. As an application of Theorem 3.3, we can obtain the solvability of generalized vector variational inequality under strict feasibility in the strong sense.

In the sequel, we present the new solvability condition for the GVEP, when F(u,x,.) is C-explicitly quasiconvex. First, we present a technical lemma.

Lemma 3.3 Suppose that a,be Y, with a = 0 and b € int C. Then, there exists c € int C such that a<cc and b <c c.

Proof. Since int C =j0, there exists d e int C such that d - b e C, see [23]. For t e [0,1], set dt = td+(1- t)b. Since C is closed and convex, there exists t0 e (0,1) such that

dt e C, Vt e [fo, 1].

Furthermore, there exists t* e [t0,1] such that dt. e dC. That is, dt* e C and dt* € int C. Set c = dt*. We can verify c - b = t*(d - b) e C and c - 0 = dt* e C.

Theorem 3.4 Let K be a nonempty, closed, convex and well-positioned subset of a real reflexive Banach space X and Assumption 3.1 hold. If F: D x K x K_Y satisfies the followings

(i) F is C-pseudomonotone;

(ii) F(u, x,.) is C-explicitly quasiconvex and weakly lower semicontinuous for xeK,ue

(iii) there exists b € int C such that F(u,x0,m) <C b for m e M, where M is defined in Theorem 3.3, then, the GVEP has a nonempty bounded solution set whenever it is strictly feasible in the strong sense.

Proof. Suppose that the GVEP is strictly feasible in the strong sense. Then there exists x0 e K such that x0e Ys+, ]i.e.,

F(u, x0, xo + z) e int C, Vu e T(x0), z e Km.

D = {x e K|F(u, x0, x) e int C}, Vu e T(x0).

By Assumption 3.1 and assumption (ii), x0 e D and D is weakly closed. We assert that D is bounded. Indeed, if it is not the case, there exists a sequence {xn} £ D with l|xn|| ® as n ® Without loss of generality, we may take a subsequence {xnkj of {xn} such that

1 6 (0/1), lim H*"""*0,, = lim Ar^ze

\\Xni - Xo|| fe^+TO \\xnk - xo || fe^+TO ||Xn,

By Lemma 2.3, z 0 since K is well-positioned. It follows from x0 L that F(u, x0, x0 + z) e int C.

Since F(u,x0,x0) = 0, Vm e T(x) and condition (iii) holds, i.e., F(u, Xo, xnt)<cb, b e intC, by Lemma 3.3, there exists c € int C such that

0 = F(u, xo, xo)<cc, F(u, xo, xnk )<cc.

Taking into account that F(u,x,.) is C-explicitly quasiconvex, we obtain

F ( u, Xq, Xo + -¡p^—~-"-jr ) = F I u, Xo I 1 — -¡j---ii- ) Xo + -¡i———¡r ) <cC-

V — v , / I I v — / v —

•*nk 0 / \ \ ||Ank ^0 || / ||Ank ^0

Thus, F ( u, Xo, Xo + —-—I & int C . Since F(u,x,.) is weakly lower semicontinu-V llxn - x0y/

ous, one has

F(u, x0, x0 + z) e int C

This is a contradiction to F(u,x0,x0 + z) e int C. Thus, D is bounded and it is weakly compact. Following the similar arguments in the proof of Theorem 3.3, we can obtain the desired result.

Remark 3.2 Compared with [16], we substitute the condition that F is positively homogeneous by the condition (iii), when F(u,x,.) is C-explicitly quasiconvex.

Similar to [16], we can establish the existence of the solution of the GVEP, when F is positively homogeneous with a >0.

Theorem 3.5 Let K be a nonempty, closed, convex and well-positioned subset of a real reflexive Banach space X and Assumption 3.1 hold. IfF: D x K x K ® Y satisfies the followings

(i) F is C-pseudomonotone;

(ii)F(u,x,.) is C-explicitly quasiconvex and weakly lower semicontinuous for x e K,u e T(x);

(iii) F is positively homogeneous with degree a > 0, i.e., there exists a>0 such that F(u, x, x + t(y — x)) = taF(u, x, y), Vx, y e K, u e T(x), t e (0,1),

then, the GVEP has a nonempty bounded solution set whenever it is strictly feasible in the strong sense.

The following example shows that the converse of Theorem 3.3 is not true in general.

Example 3.1 Let X = R, K = R, D = [0,1],Y = R, C = R+ and

f 1, if x > 0 T(x) = i

[ {0,1} if x = 0.

Let F:D x K x K ® 2Y be defined by

|(u, y2 — x2), Vx, y e K, u e T(x), [u,y — x, Vx,y e K, u e T(x).

It is easy to see that K is well-positioned and F satisfies assumptions of Theorem 3.3. It can be verified that the GVEP has a nonempty bounded solution set SK = SD = {0}. On the other hand, it can also be verified that =,0. The following example illustrates the conclusion of Theorem 3.4. Example 3.2 Let X = R, K = -1], D = [0,1],Y = R, C = R+ and

11, if x < —1 {0,1}, if x = —1.

Let F:D x K x K® 2Y be defined by

F(u, x, y) =

u,y2 — x2}, Vx,y e K, u e T(x),

u, — — — ), Wx,y e K, u e Tfx). yx

For this problem, it can be verified that K is well-positioned and F satisfies assumptions (i) and (ii) of Theorem 3.4 and F(u,x,.) is not C-convex. However, Theorem 3.3 is not applicable. Furthermore, we can verify that -1 e Ys+ and M = {-1}. So, there exists 0 € int C such that F(u,x0,x0) <c 0. This means that assumptions (iii) in Theorem 3.4 holds. In summary, all the assumptions of Theorem 3.4 are satisfied for this example. Thus, the GVEP is solvable. In fact, x* = -1 is its a solution. However, F is not positively homogeneous with a 0. Thus, Theorem 3.5 is not applicable.

Acknowledgements

This research is supported by the NaturalScience Foundation of China (Grant Nos. 11126233, 11171180, 71101081), and Specialized Research Fund for the doctoralProgram of Chinese Higher Education (Grant Nos. 20113705110002, 20113705120004). The authors are in debt to the anonymous referees for their numerous insightfulcomments and constructive suggestions which help improve the presentation of the article. The authors thank Prof. Yiju Wang for his carefulreading of the manuscript.

Author details

1Schoolof Management Science, Qufu NormalUniversity, Shandong Rizhao 276826, China 2Schoolof Mathematics

and Information Science, Weifang University, Shandong Weifang 261000, China

Authors' contributions

Allauthors carried out the proof. Allauthors conceived of the study, and participated in its design and coordination.

Allauthors read and approved the finalmanuscript.

Competing interests

The authors declare that they have no competing interests.

Received: 17 November 2011 Accepted: 7 March 2012 Published: 7 March 2012

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doi:10.1186/1687-1812-2012-38

Cite this article as: Wang et al.: Feasibility-solvability theorems for generalized vector equilibrium problem in reflexive Banach spaces. Fixed Point Theory and Applications 2012 2012:38.