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# Sensitivity analysis for generalized quasi-variational relation problems in locally *G*-convex spaces

- Nguyen Van Hung
^{1}Email author

**2012**:158

https://doi.org/10.1186/1687-1812-2012-158

© Hung; licensee Springer 2012

**Received:**22 July 2012**Accepted:**4 September 2012**Published:**19 September 2012

## Abstract

In this paper, we study generalized quasi-variational relation problems in locally *G*-convex spaces. Using the Kakutani-Fan-Glicksberg fixed-point theorem for upper semicontinuous set-valued mapping with nonempty closed acyclic values, we establish an existence theorem of a solution set for these problems. Moreover, the stability and closedness of the solution set for these problems are also obtained. The results presented in the paper improve and extend the main results in the literature.

**MSC:**47J20, 49J40.

## Keywords

- generalized quasi-variational relation problems
*G*-convex spaces- Kakutani-Fan-Glicksberg fixed-point theorem
- quasiconvexity
- existence
- closedness
- upper semicontinuity
- compactness

## 1 Introduction and preliminaries

The generalized quasi-variational relation problems include, as special cases, the generalized variational inclusion problems, the generalized vector equilibrium problems, the generalized vector variational inequality problems *etc.* In recent years, a lot of results for the existence and stability of solutions for variational relation problems, vector equilibrium problems and vector variational inequality problems have been established by many authors in different ways. For example, variational relation problems [1–4], vector equilibrium problems [5–18], vector variational inequality problems [19, 20] and the references therein.

*X*, we shall denote by ${2}^{X}$ and $\u3008X\u3009$ the families of all subsets of

*X*and the family of all nonempty finite subsets of

*X*, respectively. For each $A\in \u3008X\u3009$, $|A|$ denotes the cardinality of

*A*. Let ${\mathrm{\Delta}}_{n}$ denote the standard

*n*-dimensional simplex in ${R}^{n+1}$ with vertices $\{{e}_{1},{e}_{2},\dots ,{e}_{n+1}\}$, that is,

where ${e}_{i}$ is the *i* th unit vector in ${\mathbb{R}}^{n+1}$.

For any nonempty subset *J* of $\{0,1,2,\dots ,n\}$, we denote ${\mathrm{\Delta}}_{J}$ by the convex hull of the vertices $\{{e}_{j}:j\in J\}$.

A convex set *A* in a vector space is called a convex space if it is equipped with a topology which includes the Euclidean topology on convex hulls of any nonempty finite subsets of *A*.

*G*-convex space was introduced by Park and Kim in [21]. Let

*X*be a topological space, $A\subseteq X$ be a nonempty subset and a function $\mathrm{\Gamma}:\u3008A\u3009\to {2}^{X}\setminus \{\mathrm{\varnothing}\}$ be such that the following conditions hold:

- (a)
for each $M,N\in \u3008A\u3009$, $\mathrm{\Gamma}(M)\subset \mathrm{\Gamma}(N)$ if $M\subset N$,

- (b)
for each $M\in \u3008A\u3009$ with $|M|=n+1$, there exists a continuous mapping ${\varphi}_{M}:{\mathrm{\Delta}}_{n}\to \mathrm{\Gamma}(M)$ such that, for each $J\in \u3008M\u3009$, ${\varphi}_{M}({\mathrm{\Delta}}_{J})\subset \mathrm{\Gamma}(J)$, where ${\mathrm{\Delta}}_{J}$ denotes the face of ${\mathrm{\Delta}}_{n}$ corresponding to $J\in \u3008M\u3009$.

Then $(X,A,\mathrm{\Gamma})$ is called a generalized convex space (or a *G*-convex space). If $A=X$, we omit *A* simply write $(X,\mathrm{\Gamma})$.

For a *G*-convex space $(X,A,\mathrm{\Gamma})$, a subset *B* of *X* is said to be *G*-convex if, for each $M\in \u3008A\u3009$, $M\subseteq B$ implies $\mathrm{\Gamma}(M)\subseteq B$. A space *X* is said to have a *G*-convex structure if and only if *X* is a *G*-convex space. A *G*-convex *X* is said to be a locally *G*-convex space if *X* is a uniform topological space with uniformity *U*, which has an open base $B=\{{V}_{i}:i\in I\}$ of symmetric entourages such that for each $v\in B$, the set $V(x):=\{y\in X:(y,x)\in V\}$ is a *G*-convex set for each $x\in X$.

*X*,

*Y*,

*Z*be real locally

*G*-convex Hausdorff topological vector spaces, $A\subseteq X$, $B\subseteq Y$ and $D\subseteq Z$ be nonempty compact convex subsets. Let ${K}_{1}:A\to {2}^{A}$, ${K}_{2}:A\to {2}^{A}$, $T:A\to {2}^{B}$ be multifunctions and $R(x,z,y)$ be a relation linking $x\in A$, $z\in D$ and $y\in B$. We adopt the following notations (see [2]). Letters w, m and s are used for weak, middle and strong kinds of considered problems respectively. For subsets

*U*and

*V*under consideration, we adopt the following notations:

Let $\alpha \in \{\mathrm{w},\mathrm{m},\mathrm{s}\}$ and $\rho \in \{{\rho}_{1},{\rho}_{2},{\rho}_{3},{\rho}_{4}\}$. We consider the following for a generalized quasi-variational relation problem (in short, (QVR_{
α
})):

_{ α }): Find $\overline{x}\in A$ such that $\overline{x}\in {K}_{1}(\overline{x})$ and $(y,z)\alpha {K}_{2}(\overline{x})\times T(\overline{x})$ satisfying

Let ${\mathrm{\Sigma}}_{\alpha}(R)$ be the solution set of (QVR_{
α
}).

_{ α }) are as follows:

- (I)If we let
*A*,*D*,*B*,*X*,*Y*,*Z*, ${K}_{1}$, ${K}_{2}$,*T*be as in (QVR_{ α }) and $F:A\times D\times B\to {2}^{Z}$ be a multifunction, the relation*R*is defined as follows:$R(x,z,y)\text{holds}\phantom{\rule{1em}{0ex}}\text{iff}\phantom{\rule{1em}{0ex}}0\in F(x,z,y).$

Then (QVR_{
α
}) becomes the generalized quasi-variational inclusion problem:

- (II)If we let
*A*,*D*,*B*,*X*,*Y*,*Z*, ${K}_{1}$, ${K}_{2}$,*T*,*R*be as in (QVR_{ α }) and $F:A\times D\times B\to {2}^{Z}$ and $G:A\times D\to {2}^{Z}$ be multifunctions, the relation*R*is defined as follows:$R(x,z,y)\text{holds}\phantom{\rule{1em}{0ex}}\text{iff}\phantom{\rule{1em}{0ex}}\rho (F(x,z,y),G(x,z)).$

Then (QVR_{
α
}) becomes the generalized quasi-variational inclusion problem:

- (III)If we let
*A*,*D*,*B*,*X*,*Y*,*Z*, ${K}_{1}$, ${K}_{2}$,*T*be as in (QVR_{ α }) and $F:A\times D\times B\to {2}^{Z}$, $C:A\to {2}^{Z}$ be multifunctions such that $C(x)$ is a closed convex cone with $intC(x)\ne \mathrm{\varnothing}$, the relation*R*is defined as follows:$R(x,z,y)\text{holds}\phantom{\rule{1em}{0ex}}\text{iff}\phantom{\rule{1em}{0ex}}\rho (F(x,z,y),C(x)).$

Then (QVR_{
α
}) becomes the generalized vector quasi-equilibrium problem:

- (IV)If we let
*A*,*D*,*B*,*X*,*Y*,*Z*, ${K}_{1}$, ${K}_{2}$ be as in (QVR_{ α }), $f:A\times D\times B\to Z$ be a vector function, and $C:A\to {2}^{Z}$ be a multifunction such that $C(x)$ is a closed convex cone with $intC(x)\ne \mathrm{\varnothing}$, the relation*R*is defined as follows:$R(x,z,y)\text{holds}\phantom{\rule{1em}{0ex}}\text{iff}\phantom{\rule{1em}{0ex}}f(x,z,y)\in C(x).$

Then (QVR_{
α
}) becomes the vector quasi-equilibrium problem:

- (V)If we let
*D*, $A=B$, $X=Y$,*Z*, ${K}_{1}$, ${K}_{2}$,*T*be as in (QVR_{ α }), $L(X,Z)$ be the space of all linear continuous operators from*X*to*Z*and $H:L(X,Z)\to L(X,Z)$, $Q:A\times A\to X$, $F:A\times A\to Z$ be continuous single-valued mappings, $C:A\to {2}^{Z}$ be a multifunction such that $C(x)$ is a closed convex cone with $intC(x)\ne \mathrm{\varnothing}$, the relation*R*is defined as follows:$R(x,z,y)\text{holds}\phantom{\rule{1em}{0ex}}\text{iff}\phantom{\rule{1em}{0ex}}\u3008H(z),Q(y,x)\u3009+F(y,x)\in C(x).$

Then (QVR_{
α
}) becomes the generalized mixed vector quasi-variational inequality problem:

*X*,

*Y*be two topological vector spaces,

*A*be a nonempty subset of

*X*and $F:A\to {2}^{Z}$ be a multifunction.

- (i)
*F*is said to be lower semicontinuous (lsc) at ${x}_{0}\in A$ if $F({x}_{0})\cap U\ne \mathrm{\varnothing}$ for some open set $U\subseteq Y$ implies the existence of a neighborhood*N*of ${x}_{0}$ such that $F(x)\cap U\ne \mathrm{\varnothing}$, $\mathrm{\forall}x\in N$.*F*is said to be lower semicontinuous in*A*if it is lower semicontinuous at all ${x}_{0}\in A$. - (ii)
*F*is said to be upper semicontinuous (usc) at ${x}_{0}\in A$ if for each open set $U\supseteq F({x}_{0})$, there is a neighborhood*N*of ${x}_{0}$ such that $U\supseteq F(x)$, $\mathrm{\forall}x\in N$.*F*is said to be upper semicontinuous in*A*if it is upper semicontinuous at all ${x}_{0}\in A$. - (iii)
*F*is said to be continuous in*A*if it is both lsc and usc in*A*. - (iv)
*F*is said to be closed if $Graph(F)=\{(x,y):x\in A,y\in F(x)\}$ is a closed subset in $A\times Y$.

**Definition 2** ([22])

*X*,

*Y*be two topological vector spaces,

*A*be a nonempty subset of

*X*, $F:A\to {2}^{Z}$ be a multifunction and $C\subset Y$ be a nonempty closed convex cone.

- (i)
*F*is called upper*C*-continuous at ${x}_{0}\in A$ if for any neighborhood*U*of the origin in*Y*, there is a neighborhood*V*of ${x}_{0}$ such that$F(x)\subset F({x}_{0})+U+C,\phantom{\rule{1em}{0ex}}\mathrm{\forall}x\in V.$ - (ii)
*F*is called lower*C*-continuous at ${x}_{0}\in A$ if for any neighborhood*U*of the origin in*Y*, there is a neighborhood*V*of ${x}_{0}$ such that$F({x}_{0})\subset F(x)+U-C,\phantom{\rule{1em}{0ex}}\mathrm{\forall}x\in V.$

**Definition 3** ([23])

*X*and

*Y*be two topological vector spaces and

*A*be a nonempty convex subset of

*X*. A set-valued mapping $F:A\to {2}^{Y}$ is said to be properly

*C*-quasiconvex if for any $x,y\in A$ and $t\in [0,1]$, we have

**Lemma 4** ([23])

*Let*

*X*,

*Y*

*be two topological vector spaces*,

*A*

*be a nonempty convex subset of*

*X*

*and*$F:A\to {2}^{Y}$

*be a multifunction*.

- (i)
*If**F**is upper semicontinuous at*${x}_{0}\in A$*with closed values*,*then**F**is closed at*${x}_{0}\in A$. - (ii)
*If**F**is closed at*${x}_{0}\in A$*and**Y**is compact*,*then**F**is upper semicontinuous at*${x}_{0}\in A$. - (iii)
*If**F**has compact values*,*then**F**is usc at*${x}_{0}\in A$*if and only if*,*for each net*$\{{x}_{\alpha}\}\subseteq A$*which converges to*${x}_{0}\in A$*and for each net*$\{{y}_{\alpha}\}$*with*${y}_{\alpha}\subseteq F({x}_{\alpha})$,*there are*${y}_{0}\in F({x}_{0})$*and a subnet*$\{{y}_{\beta}\}$*of*$\{{y}_{\alpha}\}$*such that*${y}_{\beta}\to {y}_{0}$.

**Definition 5** ([24])

Let *X* be a topological space. A subset *A* of *X* is called contractible at ${x}_{0}\in A$, if there is a continuous $F:A\times [0,1]\to A$ such that $F(z,0)=z$ for all $z\in A$ and $F(z,1)={x}_{0}$ for all $z\in A$.

A topological space *X* is said to be acyclic if all of its reduced Čech homology groups over the rationals vanish. In particular, each contractible space is acyclic, and thus any nonempty convex or star-shaped set is acyclic. Moreover, by the definition of a contractible set, we see that each convex space is contractible.

We now have the following fixed-point theorem in locally *G*-convex spaces given by Yuan [25] which is a generalization of the Fan-Glickberg-type fixed-point theorem for an upper semicontinuous set-valued mapping with nonempty closed acyclic values.

**Theorem 6** ([25], Theorem 2.1)

*Let* *X* *be a compact locally* *G*-*convex space and* $F:X\to {2}^{X}$ *be an upper semicontinuous set*-*valued mapping with nonempty closed acyclic values*. *Then* *F* *has a fixed*-*point*; *that is*, *there exists an* ${x}^{\ast}\in X$ *such that* ${x}^{\ast}\in F({x}^{\ast})$.

## 2 Existence of solutions

In this section, we apply the Kakutani-Fan-Glicksberg fixed-point theorem for upper semicontinuous set-valued mapping with nonempty closed acyclic values to establish sufficient conditions for the existence of a solution set of generalized quasi-variational relation problems. Moreover, the closedness of the solution set for these problems is obtained.

**Definition 7**Let

*X*be a topological vector space,

*A*be a nonempty convex subset of

*X*and $R(x)$ be a relation linking $x\in A$. We say that

*R*is quasiconvex at ${x}_{0}\in A$ if $\mathrm{\forall}{x}_{1},{x}_{2}\in A$, $\mathrm{\forall}\lambda \in [0,1]$ such that $R({x}_{1})$ holds and $R({x}_{2})$ holds, we have

*R* is said to be quasiconvex in *A* if it is quasiconvex at all ${x}_{0}\in A$.

**Remark 8**In the Definition 7, if we let $X=A=\mathbb{R}$, and let mapping $F:\mathbb{R}\to \mathbb{R}$, then the relation

*R*defined by $R(x)$ holds iff $F(x)\subseteq {\mathbb{R}}_{-}$. We have $\mathrm{\forall}{x}_{1},{x}_{2}\in A$, $\mathrm{\forall}\lambda \in [0,1]$, if $F({x}_{1})\le 0$, $F({x}_{2})\le 0$, then $F((1-\lambda ){x}_{1}+\lambda {x}_{2})\le 0$. This means that

*R*is modified 0-level quasiconvex, since the classical quasiconvexity says that $\mathrm{\forall}{x}_{1},{x}_{2}\in A$, $\mathrm{\forall}\lambda \in [0,1]$,

**Theorem 9**

*Assume for the problem*(

*QVR*

_{ α })

*that*

- (i)
${K}_{1}$

*is upper semicontinuous in**A**with nonempty closed contractible values*,*and*${K}_{2}$*is lower semicontinuous**A**with nonempty closed values*; - (ii)
*T**is upper semicontinuous in**A**with nonempty closed acyclic values if*$\alpha =\mathrm{w}$ (*or*$\alpha =\mathrm{m}$)*and lower semicontinuous in**A**with nonempty acyclic values if*$\alpha =\mathrm{s}$; - (iii)
*for all*$(x,z)\in A\times D$, $R(x,z,{K}_{2}(x))$*holds*; - (iv)
*for all*$(z,y)\in D\times B$, $R(\cdot ,z,y)$*is quasiconvex in**A*; - (v)
*the set*$\{(x,z,y)\in A\times D\times B:R(x,z,y)\mathit{\text{holds}}\}$*is closed*.

*Then*,

*the*(

*QVR*

_{ α })

*has a solution*,

*i*.

*e*.,

*there exist*$\overline{x}\in A$

*such that*$\overline{x}\in {K}_{1}(\overline{x})$

*and*$(y,z)\alpha {K}_{2}(\overline{x})\times T(\overline{x})$

*satisfying*

*Moreover*, *the solution set of the* (*QVR*_{
α
}) *is closed*.

*Proof* Since $\alpha =\{\mathrm{w},\mathrm{m},\mathrm{s}\}$, we have in fact three cases. However, the proof techniques are similar. We present only the proof for the case where $\alpha =\mathrm{m}$.

- (I)
We show that ${\mathrm{\Psi}}_{m}(x,z)$ is acyclic.

*F*is a continuous mapping, and we see that $F(b,0)=b$ for all $b\in {\mathrm{\Psi}}_{m}(x,z)$ and $F(b,1)=a$ for all $b\in {\mathrm{\Psi}}_{m}(x,z)$. Let $(b,\lambda )\in {\mathrm{\Psi}}_{m}(x,z)\times [0,1]$, we need to prove that $F(b,\lambda )\in {\mathrm{\Psi}}_{m}(x,z)$. Since $a,f(b,\lambda )\in {K}_{1}(x)$, and ${K}_{1}(x)$ is contractible, thus, for $a,f(b,\lambda )\in {\mathrm{\Psi}}_{m}(x,z)$, it follows that

*A*, we have

*i.e.*, $F(b,\lambda )\in {\mathrm{\Psi}}_{m}(x,z)$. Therefore, ${\mathrm{\Psi}}_{m}(x,z)$ is contractible.

- (II)
We will prove ${\mathrm{\Psi}}_{m}$ is upper semicontinuous in $A\times D$ with nonempty closed values.

*A*is a compact set and ${\mathrm{\Psi}}_{m}(x,z)\subset A$. Hence $\mathrm{\Psi}(x,z)$ is compact. We need to show that ${\mathrm{\Psi}}_{m}$ is a closed mapping. Indeed, let a net $\{({x}_{n},{z}_{n})\}\subseteq A\times D$ such that $({x}_{n},{z}_{n})\to (x,z)\in A\times D$, and let ${a}_{n}\in {\mathrm{\Psi}}_{m}({x}_{n},{z}_{n})$ such that ${a}_{n}\to {a}_{0}$. Now, we need only prove that ${a}_{0}\in {\mathrm{\Psi}}_{m}(x,z)$. Since ${a}_{n}\in {K}_{1}({x}_{n})$ and ${K}_{1}$ is upper semicontinuous at $x\in A$ with nonempty closed values, by Lemma 4(i), we have ${K}_{1}$ is closed at $x\in A$, thus ${a}_{0}\in {K}_{1}(x)$. Suppose, to the contrary, ${a}_{0}\notin {\mathrm{\Psi}}_{m}(x,z)$. Then $\mathrm{\exists}{y}_{0}\in {K}_{2}(x)$ such that

- (III)
Now, we shall show that the solution set ${\mathrm{\Sigma}}_{m}(R)\ne \mathrm{\varnothing}$.

*i.e.*, $\overline{x}\in {\mathrm{\Sigma}}_{m}(R)$.

- (IV)
Next, we prove that ${\mathrm{\Sigma}}_{m}(R)$ is closed.

*T*is upper semicontinuous in

*A*and $T({x}_{0})$ is compact, there exists ${z}_{0}\in T({x}_{0})$ such that ${z}_{\alpha}\to {z}_{0}$. By the condition (v), we have

This means that ${x}_{0}\in {\mathrm{\Sigma}}_{m}(R)$. Thus ${\mathrm{\Sigma}}_{m}(R)$ is a closed set. □

**Remark 10**If we let $A=B$,

*D*, $X=Y$,

*Z*, ${K}_{1}={K}_{2}=K$,

*T*,

*R*, $\alpha =\mathrm{m}$ as in (QVR

_{ α }), $F:A\times D\times A\to {2}^{Z}$ be a multifunction and $C\subset Z$ be a nonempty closed convex cone, the relation

*R*is defined as follows:

Then, (QVR_{
α
}) becomes the generalized strong vector quasi-equilibrium problem of type (I) and (II) (in short, (GSVQEP I) and (GSVQEP II)) studied in [17].

and

The following example shows that all the assumptions of Theorem 9 are satisfied, but Theorem 3.1 in [17] does not work. The reason is that *F* is not lower (−*C*)-continuous.

**Example 11**Let $X=Y=Z=\mathbb{R}$, $A=B=D=[0,1]$, $C={\mathbb{R}}_{+}$, ${K}_{1}(x)={K}_{2}(x)=[0,1]$ and

We let the relation *R* be defined by $R(x,z,y)$ holds iff $F(x,z,y)\subseteq {\mathbb{R}}_{+}$. We can show that all the assumptions of Theorem 9 are satisfied. However, *F* is not lower (−*C*)-continuous at ${x}_{0}=\frac{1}{2}$. Also, Theorem 3.1 in [17] does not work.

The following example shows that all the assumptions of Theorem 9 are satisfied, but Theorem 3.1 in [17] is not fulfilled. The reason is that *F* is not upper *C*-continuous.

**Example 12**Let

*A*,

*B*,

*D*,

*X*,

*Y*,

*Z*,

*K*,

*C*be as in Example 11 and $T(x)=\{z\}$ and

We let the relation *R* be defined by $R(x,z,y)$ holds iff $F(x,z,y)\subseteq {\mathbb{R}}_{+}$. It is easy to check that all the assumptions of Theorem 9 are satisfied. So, (QVR_{
α
}) has a solution. However, *F* is not upper *C*-continuous at ${x}_{0}=\frac{1}{2}$. Also, Theorem 3.1 in [17] does not work.

The following example shows that all assumptions of Theorem 9 are satisfied, but Theorem 3.1 in [17] is not fulfilled. The reason is that *F* is not *C*-quasiconvex.

**Example 13**Let

*A*,

*B*,

*D*,

*X*,

*Y*,

*Z*,

*K*,

*C*,

*T*be as in Example 12 and

We let the relation *R* be defined by $R(x,z,y)$ holds iff $F(x,z,y)\subseteq {\mathbb{R}}_{+}$. It is easy to check that all the assumptions of Theorem 9 are satisfied. However, *F* is not *C*-quasiconvex at ${x}_{0}=\frac{1}{2}$. Thus, it gives also cases where Theorem 9 can be applied but Theorem 3.1 in [17] does not work.

If we let *X*, *Y*, *Z* be real locally convex Hausdorff topological vector spaces, then we have the following corollary.

**Corollary 14**

*Assume for problem*(

*QVR*

_{ α })

*that*

- (i)
${K}_{1}$

*is upper semicontinuous in**A**with nonempty closed convex values*,*and*${K}_{2}$*is lower semicontinuous**A**with nonempty closed values*; - (ii)
*T**is upper semicontinuous in**A**with nonempty closed convex values if*$\alpha =\mathrm{w}$ (*or*$\alpha =\mathrm{m}$)*and lower semicontinuous in**A**with nonempty convex values if*$\alpha =\mathrm{s}$; - (iii)
*for all*$(x,z)\in A\times D$, $R(x,z,{K}_{2}(x))$*holds*; - (iv)
*for all*$(z,y)\in D\times B$, $R(\cdot ,z,y)$*is quasiconvex in**A*; - (v)
*the set*$\{(x,z,y)\in A\times D\times B:R(x,z,y)\mathit{\text{holds}}\}$*is closed*.

*Then the*(

*QVR*

_{ α })

*has a solution*,

*i*.

*e*.,

*there exist*$\overline{x}\in A$

*such that*$\overline{x}\in {K}_{1}(\overline{x})$

*and*$(y,z)\alpha {K}_{2}(\overline{x})\times T(\overline{x})$

*satisfying*

*Moreover*, *the solution set of the* (*QVR*_{
α
}) *is closed*.

**Remark 15**

- (i)
If we let

*X*,*Y*,*Z*be real locally convex Hausdorff topological vector spaces, then (GSVQEP I) becomes the problem (GSVQEP) studied in [15]. - (ii)If $A=B$, $X=Y$,
*Z*, ${K}_{1}={K}_{2}=K$,*R*as in (QVR_{ α }) and $T(x)=\{z\}$, $F:A\times A\to {2}^{Z}$ is a multifunction, $C\subset Z$ is a nonempty closed convex cone, the relation*R*is defined as follows:$R(x,z,y)\text{holds}\phantom{\rule{1em}{0ex}}\text{iff}\phantom{\rule{1em}{0ex}}F(x,y)\subset C.$

Then (QVR_{
α
}) becomes strong vector quasi-equilibrium problem (in short, (SVQEP)) studied in [18].

## 3 Stability

_{ α }). Throughout this section, let

*X*,

*Y*,

*Z*be Banach spaces,

*N*be a real locally

*G*-convex Hausdorff topological vector space. Let $A\subset X$, $B\subset Y$ and $D\subset Z$ be nonempty compact convex subsets, ${K}_{1}\equiv {K}_{2}\equiv K:A\to {2}^{A}$, $T:A\to {2}^{B}$ be multifunctions, and $R(x,z,y)$ be a relation linking $x\in A$, $z\in D$ and $y\in B$. Now, we let

where ${H}^{\ast}({E}_{1},{E}_{2}):={sup}_{{e}_{1}\in {E}_{1}}d({e}_{1},{E}_{2})$ and $d({e}_{1},{E}_{2}):={inf}_{{e}_{2}\in {E}_{2}}\parallel {e}_{1}-{e}_{2}\parallel $.

where ${H}_{1}$, ${H}_{2}$ are the appropriate Hausdorff metrics. Obviously, $(\mathrm{\Xi},\xi )$ is a metric space.

*R*satisfies the conditions of Theorem 9. Then for each $(T,K)\in \mathrm{\Xi}$, (QVR

_{ α }) has a solution $\overline{x}$,

*i.e.*, there exists $\overline{x}\in A$ such that $\overline{x}\in K(\overline{x})$ and $(y,\overline{z})\alpha K(\overline{x})\times T(\overline{x})$ satisfying

Then ${\mathrm{\Theta}}_{\alpha}(T,K)\ne \mathrm{\varnothing}$, and so ${\mathrm{\Theta}}_{\alpha}(T,K)$ defines a set-valued mapping from Ξ into *A*.

**Lemma 17** ([26])

*Let* *Z* *be a metric space and let* *M*, ${M}_{n}$ ($n=1,2,\dots $) *be compact sets in* *Z*. *Suppose that for any open set* $O\supset M$, *there exists* ${n}_{0}$ *such that* ${M}_{n}\subset O$, $\mathrm{\forall}n\ge {n}_{0}$. *Then any sequence* $\{{x}_{n}\}$ *satisfying* ${x}_{n}\in {M}_{n}$ *has a convergent subsequence with limit in* *M*.

**Theorem 18** ${\mathrm{\Theta}}_{\alpha}:\mathrm{\Xi}\to {2}^{A}$ *is upper semicontinuous with compact values*.

*Proof* Similar arguments can be applied to three cases. We present only the proof for the cases where $\alpha =\mathrm{m}$. Indeed, since *A* is compact, we need only show that ${\mathrm{\Theta}}_{m}$ is a closed mapping. Let a sequence $\{({T}_{n},{K}_{n},{x}_{n})\}\subset Graph({\mathrm{\Theta}}_{m})$ be given such that $({T}_{n},{K}_{n},{x}_{n})\to (T,K,{x}_{0})$. We now show that $\{(T,K,{x}_{0})\}\subset Graph({\mathrm{\Theta}}_{m})$.

*n*, since ${x}_{n}\in {\mathrm{\Theta}}_{m}({T}_{n},{K}_{n})$, we have that ${x}_{n}\in {K}_{n}({x}_{n})$ and $\mathrm{\exists}{z}_{n}\in {T}_{n}({x}_{n})$, $\mathrm{\forall}{y}_{n}\in {K}_{n}({x}_{n})$ such that

where $d(z,T({x}_{0}))={inf}_{{z}^{\prime}\in T({x}_{0})}\parallel z-{z}^{\prime}\parallel $.

*T*is upper semicontinuous at ${x}_{0}$, $\mathrm{\exists}{n}_{0}$ such that

Since $T({x}_{0})\subset O$ and ${z}_{n}\in {T}_{n}({x}_{n})$, we can apply Lemma 17. There exists a subsequence $\{{z}_{{n}_{k}}\}$ of $\{{z}_{n}\}$ such that $\{{z}_{{n}_{k}}\}$ convergent to ${z}_{0}$, it follows that ${z}_{0}\in T({x}_{0})$. By using the same argument as above, we can show that ${x}_{0}\in K({x}_{0})$.

Next, we need only show that $R({x}_{0},{z}_{0},{y}_{0})\text{holds}$. Since ${x}_{n}\to {x}_{0}$ and *K* is upper semicontinuous at ${x}_{0}$, $K({x}_{0})$ is closed, there exists ${y}_{0}\in K({x}_{0})$ such that ${y}_{n}\to {y}_{0}$ (taking a subsequence if necessary).

Since ${x}_{0}\in K({x}_{0})$ and ${z}_{0}\in T({x}_{0})$ and (10) yields that $(T,K,{x}_{0})\in Graph({\mathrm{\Theta}}_{m})$ and so $Graph({\mathrm{\Theta}}_{m})$ is closed. Therefore, ${\mathrm{\Theta}}_{m}$ is closed. Since *A* is a compact set and ${\mathrm{\Theta}}_{m}(T,K)\subset A$. Hence ${\mathrm{\Theta}}_{m}$ has a compact valued mapping. □

**Remark 19** Theorem 18 improves and extends Theorems 3.1 and 3.3 in [17], Theorem 3.1 in [15].

## Declarations

### Acknowledgements

The author thanks the two anonymous referees for their valuable remarks and suggestions, which helped him to improve the article considerably.

## Authors’ Affiliations

## References

- Balaj M, Lin LJ: Generalized variational relation problems with applications.
*J. Optim. Theory Appl.*2011, 148: 1–13. 10.1007/s10957-010-9741-yMathSciNetView ArticleMATHGoogle Scholar - Hung NV: Continuity of solutions for parametric generalized quasivariational relation problems.
*Fixed Point Theory Appl.*2012., 2012: Article ID 102Google Scholar - Khanh PQ, Luc DT: Stability of solutions in parametric variational relation problems.
*Set-Valued Anal.*2008, 16: 1015–1035. 10.1007/s11228-008-0101-0MathSciNetView ArticleMATHGoogle Scholar - Luc DT, Sarabi E, Soubeyran A: Existence of solutions in variational relation problems without convexity.
*J. Math. Anal. Appl.*2010, 364: 544–555. 10.1016/j.jmaa.2009.10.040MathSciNetView ArticleMATHGoogle Scholar - Fu JY: Symmetric vector quasiequilibrium problems.
*J. Math. Anal. Appl.*2003, 285: 708–713. 10.1016/S0022-247X(03)00479-7MathSciNetView ArticleMATHGoogle Scholar - Kien BT, Huy NQ, Wong NC: On the solution existence of generalized vector quasi-equilibrium problems with discontinuous multifunctions.
*Taiwan. J. Math.*2009, 13: 757–775.MathSciNetMATHGoogle Scholar - Lin Z, Yang H, Yu J: On existence and essential components of the solution set for the system of vector quasi-equilibrium problems.
*Nonlinear Anal.*2005, 63: 2445–2452. 10.1016/j.na.2005.03.049View ArticleGoogle Scholar - Lin Z, Yu J: The existence of solutions for the system of generalized vector quasi-equilibrium problems.
*Appl. Math. Lett.*2005, 18: 415–422. 10.1016/j.aml.2004.07.023MathSciNetView ArticleMATHGoogle Scholar - Lin LJ, Liu YH: Existence theorems for systems of generalized vector quasiequilibrium problems and optimization problems.
*J. Optim. Theory Appl.*2006, 130: 461–475.MathSciNetView ArticleMATHGoogle Scholar - Hai NX, Khanh PQ: Existence of solution to general quasiequilibrium problem and applications.
*J. Optim. Theory Appl.*2007, 133: 317–327. 10.1007/s10957-007-9170-8MathSciNetView ArticleMATHGoogle Scholar - Hai NX, Khanh PQ: Systems of set-valued quasivariational inclusion problems.
*J. Optim. Theory Appl.*2007, 135: 55–67. 10.1007/s10957-007-9222-0MathSciNetView ArticleMATHGoogle Scholar - Liu QY, Long XJ, Huang NJ: Connectedness of the sets of weak efficient solutions for generalized vector equilibrium problems.
*Math. Slovaca*2012, 62: 123–136. 10.2478/s12175-011-0077-3MathSciNetMATHGoogle Scholar - Long XJ, Peng JW: Connectedness and compactness of weak efficient solutions for vector equilibrium problems.
*Bull. Korean Math. Soc.*2011, 48: 1225–1233.MathSciNetView ArticleMATHGoogle Scholar - Long XJ, Huang YQ, Peng ZY: Optimality conditions for the Henig efficient solution of vector equilibrium problems with constraints.
*Optim. Lett.*2011, 5: 717–728. 10.1007/s11590-010-0241-7MathSciNetView ArticleMATHGoogle Scholar - Long XJ, Huang NJ, Teo KL: Existence and stability of solutions for generalized strong vector quasi-equilibrium problems.
*Math. Comput. Model.*2008, 47: 445–451. 10.1016/j.mcm.2007.04.013MathSciNetView ArticleMATHGoogle Scholar - Song QQ, Wang LS: The existence of solutions for the system of vector quasi-equilibrium problems in topological order spaces.
*Comput. Math. Appl.*2011, 62: 1979–1983. 10.1016/j.camwa.2011.06.041MathSciNetView ArticleMATHGoogle Scholar - Plubtieng S, Sitthithakerngkiet K: Existence result of generalized vector quasiequilibrium problems in locally
*G*-convex spaces.*Fixed Point Theory Appl.*2011., 2011: Article ID 967515Google Scholar - Yang Y, Pu YJ: On the existence and essential components for solution set for system of strong vector quasiequilibrium problems.
*J. Glob. Optim.*2011. doi:10.1007/s10898–011–9830-yGoogle Scholar - Hou SH, Gong XH, Yang XM: Existence and stability of solutions for generalized Ky Fan inequality problems with trifunctions.
*J. Optim. Theory Appl.*2010, 146: 387–398. 10.1007/s10957-010-9656-7MathSciNetView ArticleMATHGoogle Scholar - Lin Z: Existence of solutions to the system of generalized implicit vector quasivariational inequality problems.
*Fixed Point Theory Appl.*2009., 2009: Article ID 654370Google Scholar - Park S, Kim H: Admissible classes of multifunctions on generalized convex spaces.
*Proc. Coll. Nat. Sci.*1993, 18: 1–21.Google Scholar - Berge C:
*Topological Spaces*. Oliver & Boyd, London; 1963.MATHGoogle Scholar - Luc DT Lecture Notes in Economics and Mathematical Systems. In
*Theory of Vector Optimization*. Springer, Berlin; 1989.View ArticleGoogle Scholar - Bardaro C, Ceppitelli R: Some further generalizations of Knaster-Kuratowski-Mazurkiewicz theorem and minimax inequalities.
*J. Math. Anal. Appl.*1988, 132: 484–490. 10.1016/0022-247X(88)90076-5MathSciNetView ArticleMATHGoogle Scholar - Yuan GXZ: Fixed points of upper semicontinuous mappings in locally
*G*-convex spaces.*Bull. Aust. Math. Soc.*1998, 58: 469–478. 10.1017/S0004972700032457View ArticleMathSciNetMATHGoogle Scholar - Yu J: Essential weak efficient solution in multiobjective optimization problems.
*J. Math. Anal. Appl.*1992, 166: 230–235. 10.1016/0022-247X(92)90338-EMathSciNetView ArticleMATHGoogle Scholar

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