# Lower semicontinuity of the solution sets of parametric generalized quasiequilibrium problems

Tạp chí KHOA HỌC ĐHSP TPHCM

Nguyen Van Hung

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LOWER SEMICONTINUITY OF THE SOLUTION SETS
OF PARAMETRIC GENERALIZED QUASIEQUILIBRIUM PROBLEMS
NGUYEN VAN HUNG*

ABSTRACT
In this paper we establish sufficient conditions for the solution sets of parametric
generalized quasiequilibrium problems with the stability properties such as lower
semicontinuity and Hausdorff lower semicontinuity.
Keyword: parametric generalized quasiequilibrium problems, lower semicontinuity,
Hausdorff lower semicontinuity.
TÓM TẮT
Tính chất nửa liên tục dưới của các tập nghiệm
của các bài toán tựa cân bằng tổng quát phụ thuộc tham số
Trong bài báo này, chúng tôi thiết lập điều kiện đủ cho các tập nghiệm của các bài
toán tựa cân bằng tổng quát phụ thuộc tham số có các tính chất ổn định như: tính nửa liên

tục dưới và tính nửa liên tục dưới Hausdorff.
Từ khóa: các bài toán tựa cân bằng tổng quát phụ thuộc tham số, tính nửa liên tục
dưới, tính nửa liên tục dưới Hausdorff.

1.

Introduction and Preliminaries
Let X , Y , Λ, Γ, M be a Hausdorff topological spaces, let Z be a Hausdorff
topological vector space, A ⊆ X and B ⊆ Y be a nonempty sets. Let K1 : A× Λ → 2 A ,
K 2 : A× Λ → 2 A , T : A × A × Γ → 2 B , C : A× Λ → 2 B and F : A × B × A × M → 2 Z
multifunctions with C is a proper solid convex cone values and closed.

be

For the sake of simplicity, we adopt the following notations. Letters w, m and s
are used for a weak, middle and strong, respectively, kinds of considered problems. For
ubsets U and V under consideration we adopt the notations.
(u, v) w U × V means ∀u ∈ U , ∃v ∈ V ,
(u, v) m U × V

*

means

∃v ∈ V , ∀u ∈ U ,

(u, v) s U × V

means

∀u ∈ U , ∀v ∈ V ,

ρ1 (U , V )

means

U ∩V ≠ ∅ ,

ρ 2 (U , V )

means

U ⊆V ,

(u, v) wU × V

means

∃u ∈ U , ∀v ∈ V and similarly for m, s ,

MSc., Dong Thap University

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ρ1 (U , V )

means

U ∩ V = ∅ and similarly for ρ 2 .

Let α ∈ {w, m, s} , α ∈ {w, m, s } , ρ ∈ {ρ1 , ρ 2 } and ρ ∈ {ρ1 , ρ 2 } . We consider the
following parametric generalized quasiequilibrium problems.
(QEP αρ ): Find x ∈ K1 ( x , λ ) such that ( y, t )α K 2 ( x , λ ) × T ( x , y, γ ) satisfying

ρ ( F ( x , t , y, µ ); C ( x , λ )).
*
We consider also the following problem (QEP αρ
) as an auxiliary problem to

(QEP αρ ):
*
): Find x ∈ K1 ( x , λ ) such that ( y, t )α K 2 ( x , λ ) × T ( x , y, γ ) satisfying
(QEP αρ

ρ ( F ( x , t , y, µ );int C ( x , λ )).
For each λ ∈ Λ, γ ∈ Γ, µ ∈ M , we let E (λ ) := {x ∈ A | x ∈ K1 ( x, λ )} and let
%αρ : Λ × Γ × M → 2 A be a set-valued mapping such that Σ (λ , γ , µ ) and
Σαρ , Σ
αρ
%αρ (λ , γ , µ ) are the solution sets of (QEP ) and (QEP * ), respectively, i.e.,
Σ
αρ

αρ

Σαρ (λ , γ , µ ) = {x ∈ E (λ ) | ( y, t )α K 2 ( x , λ ) × T ( x , y, γ ) : ρ ( F ( x , t , y, µ ); C ( x , λ ))},
%αρ (λ , γ , µ ) = {x ∈ E (λ ) | ( y, t )α K ( x , λ ) × T ( x , y, γ ) : ρ ( F ( x , t , y, µ );int C ( x , λ ))}.
Σ
2

Clearly Σ%αρ (λ , γ , µ ) ⊆ Σαρ (λ , γ , µ ) . Throughout the paper we assume that
%αρ (λ , γ , µ ) ≠ ∅ for each (λ , γ , µ ) in the neighborhood of
Σαρ (λ , γ , µ ) ≠ ∅ and Σ
(λ0 , γ 0 , µ0 ) ∈ Λ × Γ × M .

By the definition, the following relations are clear:
% sρ ⊆ Σ
% mρ ⊆ Σ
% wρ .
Σ ⊆Σ ⊆Σ
and Σ

The parametric generalized quasiequilibrium problems is more general than many
following problems.
(a) If T ( x, y, γ ) = {x}, Λ = Γ = M , A = B, X = Y , K1 = K 2 = K , ρ = ρ 2 , ρ = ρ1 and
replace C ( x, λ ) by − int C ( x, λ ) . Then, (QEP α ρ2 ) and (QEP α ρ1 ) becomes to (PGQVEP)
and (PEQVEP), respectively, in Kimura-Yao .
(PGQVEP): Find x ∈ K ( x , λ ) such that
F ( x , y, λ ) ⊂/ − intC ( x , λ )), for all y ∈ K ( x, λ ).

and
(PEQVEP): Find x ∈ K ( x , λ ) such that
F ( x , y, λ ) ∩ (− int C ( x , λ )) = ∅, for all y ∈ K ( x, λ ).

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(b) If T ( x, y, γ ) = {x}, Λ = Γ, A = B, X = Y , K1 = clK , K 2 = K , ρ = ρ1 , ρ = ρ 2 and
replace C ( x, λ ) by Z \ − int C with C ⊆ Z be closed and int C ≠ ∅ . Then, (QEP αρ1 ) and
(QEP αρ2 ) becomes to (QEP) and (SQEP), respectively, in Anh - Khanh .
(QEP): Find x ∈ clK ( x , λ ) such that
F ( x , y, λ ) ∩ ( Z \ − int C ) ≠ ∅, for all y ∈ K ( x, λ ).

and
(SQEP): Find x ∈ K ( x , λ ) such that
F ( x , y, λ ) ⊆ Z \ − int C , for all y ∈ K ( x, λ ).

(c) If T ( x, y, γ ) = {x}, Λ = Γ = M , A = B, X = Y , K1 = K 2 = K , ρ = ρ 2 and replace
C ( x, λ ) by − int C ( x, λ ) , replace F by f be a vector function. Then, (QEP α ρ )
2
becomes to (PVQEP) in Kimura-Yao .
(PQVEP): Find x ∈ K ( x , λ ) such that
f ( x , y, λ ) ∈
/ − int C ( x , λ )), for all y ∈ K ( x, λ ).

Note that generalized quasiequilibrium problems encompass many optimizationrelated models like vector minimization, variation inequalities, Nash equilibrium, fixed
point and coincidence-point problems, complementary problems, minimum
inequalities, etc. Stability properties of solutions have been investigated even in models
for vector quasiequilibrium problems [1, 2, 3, 6, 7, 8], variation problems [4, 5, 9, 10]
and the references therein.
In this paper we establish sufficient conditions for the solution sets Σαρ to have
the stability properties such as the lower semicontinuity and the Hausdorff lower
semicontinuity with respect to parameter λ , γ , µ under relaxed assumptions about
generalized convexity of the map F .
The structure of our paper is as follows. In the remaining part of this section, we
recall definitions for later uses. Section 2 is devoted to the lower semicontinuity and the
Hausdorff lower semicontinuity of solution sets of problems (QEP αρ ).
Now we recall some notions. Let X and Z be as above and G : X → 2Z be a
multifunction. G is said to be lower semicontinuous (lsc) at x0 if G ( x0 ) ∩ U ≠ ∅ for
some open set U ⊆ Z implies the existence of a neighborhood N of x0 such that, for all
x ∈ N , G ( x) ∩ U ≠ ∅ . An equivalent formulation is that: G is lsc at x0 if ∀xα → x0 ,
∀z0 ∈ G ( x0 ), ∃zα ∈ G ( xα ), zα → z0 . G is called upper semicontinuous (usc) at x0 if for
each open set U ⊇ G ( x0 ) , there is a neighborhood N of x0 such that U ⊇ G ( N ) . Q is
said to be Hausdorff upper semicontinuous (H-usc in short; Hausdorff lower
semicontinuous, H-lsc, respectively) at x0 if for each neighborhood B of the origin in
Z , there exists a neighborhood N of x0 such that, Q( x) ⊆ Q( x0 ) + B, ∀x ∈ N
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( Q( x0 ) ⊆ Q( x) + B, ∀x ∈ N ). G is said to be continuous at x0 if it is both lsc and usc at
x0 and to be H-continuous at x0 if it is both H-lsc and H-usc at x0 . G is called closed
at x0 if for each net {( xα , zα )} ⊆ graphG := {( x, z )∣ z ∈ G ( x)}, ( xα , zα ) → ( x0 , z0 ) , z0 must
belong to G ( x0 ) . The closeness is closely related to the upper (and Hausdorff upper)
semicontinuity. We say that G satisfies a certain property in a subset A ⊆ X if G
satisfies it at every points of A . If A = X we omit ``in X " in the statement.
Let A and Z be as above and G : A → 2Z be a multifunction.
(i) If G is usc at x0 then G is H -usc at x0 . Conversely if G is H -usc at x0 and
if G ( x0 ) compact, then G usc at x0 ;
(ii) If G is H-lsc at x0 then G is lsc. The converse is true if G ( x0 ) is compact;
(iii) If G has compact values, then G is usc at x0 if and only if, for each net
{xα } ⊆ A which converges to x0 and for each net { yα } ⊆ G ( xα ) , there are y ∈ G ( x) and
a subnet { yβ } of { yα } such that yβ → y.
Definition. (See , ) Let X and Z be as above. Suppose that A is a nonempty
convex set of X and that G : X → 2Z be a multifunction.
(i) G is said to be convex in A if for each x1 , x2 ∈ A and t ∈ [0,1]
G (tx1 + (1 − t ) x2 ) ⊃ tG ( x1 ) + (1 − t )G ( x2 )

(ii) G is said to be concave A if for each x1 , x2 ∈ A and t ∈ [0,1]
G (tx1 + (1 − t ) x2 ) ⊂ tG ( x1 ) + (1 − t )G ( x2 )

2.

Main results
In this section, we discuss the lower semicontinuity and the Hausdorff lower
semicontinuity of solution sets for parametric generalized quasiequilibrium problems
(QEP αρ ).
Definition 2.1
Let A and Z be as above and C : A → 2 Z with a proper solid convex cone values.
Suppose G : A → 2Z . We say that G is generalized C -concave in A if for each
x1 , x2 ∈ A , ρ (G ( x1 ), C ( x1 )) and ρ (G ( x2 ),int C ( x2 )) imply

ρ (G (tx1 + (1 − t ) x2 ),int C (tx1 + (1 − t ) x2 )), for all t ∈ (0,1).
Theorem 2.2
Assume for problem (QEP αρ ) that
(i) E is lsc at λ0 , K 2 is usc and compact-valued in K1 ( A, Λ ) × {λ0 } and E (λ0 ) is
convex;

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(ii) in K1 ( A, Λ ) × K 2 ( K1 ( A, Λ ), Λ) × {γ 0 } , T is usc and compact-valued if α = s ,
and lsc if α = w (or α = m );
(iii) ∀t ∈ T ( K1 ( A, Λ ) × K 2 ( K1 ( A, Λ ), Λ ), Γ), ∀µ0 ∈ M , ∀λ0 ∈ Λ , K 2 (., λ0 ) is concave
in
K1 ( A, Λ )
and
F (., t ,., µ0 )
is
generalized
C (., λ0 ) -concave
in
K1 ( A, Λ ) × K 2 ( K1 ( A, Λ), Λ ) ;
(iv) the set {(x, t, y, µ, λ) ∈ K1( A, Λ) ×T (K1( A, Λ), K2 (K1( A, Λ), Λ), Γ) × K 2 ( K1 ( A, Λ), Λ ) ×
{µ0 } × {λ0 }: ρ ( F ( x, t , y, µ ); C ( x, λ ))} is closed.

Then Σαρ is lower semicontinuous at (λ0 , γ 0 , µ0 ) .
Proof.
Since α = {w, m, s} and ρ = {ρ1 , ρ 2 } , we have in fact six cases. However, the
proof techniques are similar. We consider only the cases α = s, ρ = ρ 2 . We prove that
% s ρ is lower semicontinuous at (λ , γ , µ ) . Suppose to the contrary that Σ
% s ρ is not lsc
Σ
2
2
0

0

0

at (λ0 , γ 0 , µ0 ) , i.e., ∃x0 ∈ Σ% sρ2 (λ0 , γ 0 , µ0 ) , ∃(λn , γ n , µn ) → (λ0 , γ 0 , µ0 ) , ∀xn ∈ Σ% sρ2 (λn , γ n , µn ),
xn →
/ x0 . Since E is lsc at λ0 , there is a net xn′ ∈ E (λn ) , xn′ → x0 . By the above
contradiction assumption, there must be a subnet xm′ of xn′ such that, ∀m ,
% sρ (λ , γ , µ ) , i.e., ∃y ∈ K ( x′ , λ ) , ∃t ∈ T ( x′ , y , γ ) such that
x′ ∈
/ Σ
m

2

m

m

m

m

2

m

m

m

m

m

m

F ( xm′ , tm , ym , µm ) ⊆/ int C ( xm′ , λm ).

(2.1)

As K 2 is usc at ( x0 , λ0 ) and K 2 ( x0 , λ0 ) is compact, one has y0 ∈ K 2 ( x0 , λ0 ) such
that ym → y0 (taking a subnet if necessary). By the lower semicontinuity of T at
( x0 , y0 , γ 0 ) ,
one has tm ∈ T ( xm , ym , γ m ) such that tm → t0 .
Since ( xm′ , tm , ym , λm , γ m , µm ) → ( x0 , t0 , y0 , λ0 , γ 0 , µ0 ) and by condition (iv) and (2.1)
yields that
F ( x0 , t0 , y0 , µ0 ) ⊆/ int C ( x0 , λ0 ) ,

which is impossible since x0 ∈ Σ% sρ (λ0 , γ 0 , µ0 ) . Therefore, Σ% s ρ is lsc at (λ0 , γ 0 , µ0 ) .
2

2

Now we check that
% sρ (λ , γ , µ )).
Σ s ρ2 (λ0 , γ 0 , µ0 ) ⊆ cl(Σ
2
0
0
0

Indeed, let x1 ∈ Σ s ρ (λ0 , γ 0 , µ0 ) , x2 ∈ Σ% sρ (λ0 , γ 0 , µ0 ) and xα = (1− t ) x1 + tx2 , t ∈ (0,1) .
2

2

By the convexity of E , we have xα ∈ E (λ0 ) . By the generalized C (., λ0 ) -concavity of
F (., t , y, µ0 ) , we have
F ( xα , t , y, µ0 ) ⊆ int C ( xα , λ0 ),
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and since K 2 (., λ0 ) is concave, one implies that for each yα ∈ K 2 ( xα , λ0 ) , there exist
y1 ∈ K 2 ( x1 , λ0 ) and y2 ∈ K 2 ( x2 , λ0 ) such that yα = ty1 + (1 − t ) y2 . By the generalized
C (., λ0 ) -concavity of F (., t ,., µ0 ) , we have
F ( xα , t , yα , µ0 ) ⊆ int C ( xα , λ0 ),

i.e., xα ∈ Σ% s ρ (λ0 , γ 0 , µ0 ) . Hence Σ s ρ (λ0 , γ 0 , µ0 ) ⊆ cl(Σ% s ρ (λ0 , γ 0 , µ0 )) . By the lower
2

2

2

semicontinuity of Σ% s ρ at (λ0 , γ 0 , µ0 ) , we have
2

% sρ (λ , γ , µ )) ⊆ lim inf Σ
% sρ (λ , γ , µ ) ⊆ lim inf Σ (λ , γ , µ ),
Σ sρ2 (λ0 , γ 0 , µ0 ) ⊆ cl (Σ
2
2
0
0
0
n
n
n
sρ2
n
n
n

i.e., Σ s ρ is lower semicontinuous at (λ0 , γ 0 , µ0 ) .
2

The following example shows that the lower semicontinuity of E is essential.
Example 2.3
Let A = B = X = Y = Z = , Λ = Γ = M = [0,1], λ0 = 0, C ( x, λ ) = [0, +∞ ) and let
F ( x, t , y, λ ) = 2λ , T ( x, y, λ ) = {x}, K 2 ( x, λ ) = [0,1]

and
⎧[-1,1]
K1 ( x, λ ) = ⎨
⎩[-1-λ , 0]

if λ = 0,
otherwise.

We have E (0) = [−1,1] , E (λ ) = [−λ − 1, 0], ∀λ ∈ (0,1] . Hence K 2 is usc and the
condition (ii), (iii) and (iv) of Theorem 2.2 is easily seen to be fulfilled. But Σαρ is not
upper semicontinuous at λ0 = 0 . The reason is that E is not lower semicontinuous. In
fact Σαρ (0, 0, 0) = [−1,1] and Σαρ (λ , γ , µ ) = [−λ − 1, 0], ∀λ ∈ (0,1] .
The following example shows that in this the special case, assumption (iv) of
Theorem 2.2 may be satisfied even in cases, but both assumption (ii 1 ) and (iii 1 ) of
Theorem 2.1 in Anh-Khanh  are not fulfilled.
Example 2.4
Let A, B, X , Y , Z , T , Λ, Γ, M , λ0 , C as in Example 2.3, and let K1 ( x, λ ) =
K 2 ( x, λ ) = [0,1] and
⎧[-4,0]
K1 ( x, λ ) = ⎨
⎩[-1-λ , 0]

if λ = 0,
otherwise.

We shows that the assumptions (i), (ii) and (iii) of Theorem 2.2 satisfied and
Σαρ (λ , γ , µ ) = [0,1], ∀λ ∈ [0,1] . But both assumption (ii 1 ) and (iii 1 ) of Theorem 2.1

in Anh-Khanh  are not fulfilled.

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The following example shows that in this the special case, assumption of
Theorem2.2 may be satisfied even in cases, but Theorem 2.1 and Theorem 2.3 in AnhKhanh  are not fulfilled.
Example 2.5
Let A, B, X , Y , T , Λ, Γ, M , λ0 , C as in Example 2.4, and let K1 ( x, λ ) = K 2 ( x, λ ) =
λ
[0, ] and
2
⎧[0,1]
K1 ( x, λ ) = ⎨
⎩[2, 4]

if λ = 0,
otherwise.

We shows that the assumptions (i), (ii) and (iii) of Theorem 2.2 satisfied and
λ
Σαρ (λ , γ , µ )) = [0, ], ∀λ ∈ [0,1] . Theorem 2.1 and Theorem 2.3 in Anh-Khanh  are
2
not fulfilled. The reason is that F is neither usc nor lsc at ( x, y, 0) .
Remark 2.6
In special cases, as in Section 1 (a) and (c). Then, Theorem 2.2 reduces to
Theorem 5.1 in Kimura-Yao [7, 6]. However, the proof of the theorem 5.1 is in a
different way. Its assumption (i) - (v) of Theorem 5.1 coincides with (i) of Theorem 2.2
and assumption (vi), (vii) coincides with (iii), (iv) of Theorem 2.2 Theorem 2.2 slightly
improves Theorem 5.1 in Kimura-Yao [7, 6], since no convexity of the values of E is
imposed.
The following example shows that the convexity and lower semicontinuity of K
is essential.
Example 2.7
Let A, X , Y , Z , C , Λ, M , Γ, λ0 as in Example 2.5 and let
⎧⎪{−1, 0,1}
K1 ( x, λ ) = ⎨
⎪⎩{0,1}

if λ = 0,
otherwise.

Then, we shows that K 2 is usc and has compact-valued K1 ( X , A) × {λ0 } and assumption
(ii), (iii) and (iv) of Theorem 2.2 are fulfilled. But Σαρ (λ , γ , µ )) is not lsc at (0, 0, 0) .
The reason is that E is not lsc at λ0 = 0 and E (0) is also not convex. Indeed, let
1
x1 = −1, x2 = 0 ∈ E (0) and t = ∈ (0,1) but tx1 + (1 − t ) x2 ∈
/ E (0) .
2
In fact, Σαρ (0, 0, 0) = {−1, 0,1} and Σαρ (λ , γ , µ ) = {0,1}, ∀λ ∈ (0,1] .
The following example shows that the concavity of F (., t., µ0 ) is essential.

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Example 2.8
Let A, X , Y , Z , C , Λ, M , Γ, λ0 as in Example 2.6 and let K1 ( x, λ ) = K 2 ( x, λ )
= [λ , λ + 3] and F ( x, t , y, µ ) = F ( x, y, λ ) = x 2 − (1 + λ ) x . We show that K 2 (., λ0 ) is
concave and the assumptions (i), (ii), (iv) of Theorem 2.2. are satisfied. But Σαρ is not

lsc at (0, 0, 0) . The reason is that the concavity of F is violated. Indeed, taking
3
x1 = 0, x2 =
∈ E (0) = [0,3] ,
then
for
all
y ∈K2 ( A,0) = [0,3] ,
we
2
1
1
3
have F ( x1 , y, 0) = 0, F ( x2 , y, 0) = 3 / 4 , but F ( x1 + x2 , y, 0) = − ∈/ (0, +∞) .
2
2
16
Theorem 2.9
Impose the assumption of Theorem 2.2 and the following additional conditions:
(v) K 2 is lsc in K1 ( A, Λ ) × {λ0 } and E (λ0 ) is compact;
(vi) the set {( x, t , y ) ∈ K1 ( A, Λ) × T ( K1 ( A, Λ ), K 2 ( K1 ( A, Λ ), Λ ), Γ) × K 2 ( K1 ( A, Λ ), Λ ) :

ρ ( F ( x, t , y, µ0 ); C ( x, λ0 ))} is closed.
Then Σαρ is Hausdorff lower semicontinuous at (λ0 , γ 0 , µ0 ) .
Proof.
We consider only for the cases: α = s, ρ = ρ 2 . We first prove that Σ s ρ (λ0 , γ 0 , µ0 )
2

is closed. Indeed, we let xn ∈ Σ s ρ (λ0 , γ 0 , µ0 ) such that xn → x0 . If x0 ∈/ Σ sρ (λ0 , γ 0 , µ0 ) ,
2

2

∃y0 ∈ K 2 ( x0 , λ0 ), ∃t0 ∈ T ( x0 , y0 , γ 0 ) such that
F ( x0 , t0 , y0 , µ0 ) ⊆/ C ( x0 , λ0 ) .

(2.2)

By the lower semicontinuity of K 2 (., λ0 ) at x0 , one has yn ∈ K 2 ( xn , λ0 ) such that
yn → y0 . Since xn ∈ Σ s ρ (λ0 , γ 0 , µ0 ) , ∀tn ∈ T ( xn , yn , γ 0 ) such that
2

F ( xn , tn , yn , µ0 ) ⊆ C ( xn , λ0 ) .

(2.3)

By the condition (vi), we see a contradiction between ( 2.2) and (2.3). Therefore,
Σ s ρ (λ0 , γ 0 , µ0 ) is closed.
2

On the other hand, since Σsρ (λ0 , γ 0 , µ0 ) ⊆ E(λ0 ) is compact by E (λ0 ) compact.
2

Since Σ s ρ is lower semicontinuous at (λ0 , γ 0 , µ0 ) and Σ s ρ (λ0 , γ 0 , µ0 ) compact. Hence
2

2

Σ s ρ2 is Hausdorff lower semicontinuous at (λ0 , γ 0 , µ0 ) . So we complete the proof.

The following example shows that the assumed compactness in (v) is essential.
Example 2.10
Let X = Y = A = B =
x = ( x − 1, x2 ) ∈

26

2

2

, Z = , Λ = M = Γ = [0,1], C ( x, λ ) =

+

, λ0 = 0 ,

and

for

, K1 ( x, λ ) = K1 ( x, λ ) = {( x1 , λ x1 )} and F ( x, t , y, µ ) = 1 + λ . We shows

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that the assumptions of Theorem 2.8 are satisfied, but the compactness of E (λ0 ) is not
satisfied. Direct computations give Σαρ (λ , γ , µ ) = {( x1 , x2 ) ∈

2

| x2 = λ x1} and then Σαρ

is not Hausdorff lower semicontinuous at (0, 0, 0) (although Σαρ is lsc at (0,0,0)).

1.

2.
3.
4.

5.

6.
7.

8.

9.
10.
11.

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(Ngày Tòa soạn nhận được bài: 08-11-2011; ngày chấp nhận đăng: 23-12-2011)

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