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Existence and approximation for a solution of a generalized equilibrium problem on the dual space of a Banach space
Fixed Point Theory and Applications volume 2013, Article number: 264 (2013)
Abstract
In this paper, we first prove the existence of a solution for a generalized equilibrium problem with a bifunction defined on the dual space in a Banach space setting. Second, by the virtue of this result, we construct the hybrid projection method for solving a solution of a generalized equilibrium problem. Consequently, we establish the strong convergence theorem by using sunny generalized nonexpansive retraction in the dual of Banach spaces.
MSC:47H09, 47H10, 47J25.
Dedication
Dedicated to Prof. W Takahashi on the occasion of his 70th birthday
1 Introduction
Let ℝ be the set of real numbers. Let E be a real Banach space with the norm , and is the dual pair between E and , where is the dual space of E. Let C be a closed and convex subset of a real Banach space E with the dual space , and let be a closed and convex subset of . We recall the following definitions:
-
(1)
A mapping is said to be monotone if for each such that
-
(2)
A mapping is said to be δ-strongly monotone if there exists a constant such that
-
(3)
A mapping is said to be δ-inverse strongly monotone if there exists a constant such that
-
(4)
A mapping is said to be skew monotone if for each such that
-
(5)
A mapping is said to be α-inverse strongly skew monotone if there exists a constant such that
-
(6)
A mapping is said to be hemicontinuous if for all , the mapping f of into E defined by is continuous.
Let C be a nonempty, closed and convex subset of E, and let J be the duality mapping from E into such that is closed and convex of , let us assume that a bifunction satisfies suitable conditions, and is a skew monotone operator from into E.
The generalized equilibrium problem is to find such that
The set of solutions of (1.1) is denoted by , that is,
If , then problem (1.1) reduces to the equilibrium problem, which is to find such that
The set of solutions of problem (1.3) is denoted by , that is,
The above formulation (1.3) was considered in Takahashi and Zembayashi [1], and they proved a strong convergence theorem for finding a solution of the equilibrium problem (1.3) in Banach spaces.
If , then problem (1.1) reduces to variational inequality, which is to find such that
The set of solutions of problem (1.5) is denoted by , that is,
In the sequel, let be a mapping, we denote by the set of fixed points of T, that is,
We denote the strong convergence, weak convergence by , , respectively.
Let C be a nonempty, closed subset of a smooth, strictly convex and reflexive Banach space E such that is closed and convex. For solving the equilibrium problem, let us assume that a bifunction satisfies the following conditions:
-
(DA1)
for all ;
-
(DA2)
F is monotone, i.e., for all ;
-
(DA3)
for all ,
-
(DA4)
for all , is convex and lower semicontinuous.
The following result is in Blum and Oettli [2], and see the proof in [3].
Definition 1.1 Let E be a Banach space. Then,
-
(1)
E is said to be strictly convex if for all with .
-
(2)
E is said to be uniformly convex if for each , there exists such that for all with .
-
(3)
E is said to be smooth if the limit (1.8)
(1.8)
exists for each .
-
(4)
E is said to be uniformly smooth if the limit (1.8) is attained uniformly for all .
-
(5)
E is said to have uniformly Gâteaux differentiable norm if for all , the limit (1.8) converges uniformly for .
Definition 1.2 Let E be a Banach space. Then a function is said to be the modulus of smoothness of E if
-
(1)
E is said to be smooth if , .
-
(2)
E is said to be uniformly smooth if and only if .
Definition 1.3 Let E be a Banach space. Then the modulus of convexity of E is the function defined by
-
(1)
E is said to be uniformly convex if and only if for all .
-
(2)
Let p be a fixed real number . Then E is said to be p-uniformly convex if there exists a constant such that for all .
Observe that every p-uniformly convex is uniformly convex. One should note that no Banach space is p-uniformly convex for . It is well known that a Hilbert space is 2-uniformly convex and uniformly smooth.
For any , the generalized duality mapping is defined by
In particular, is called the normalized duality mapping. If E is a Hilbert space, then , where I is the identity mapping. That is,
Remark 1.4 The basic properties below hold, see [4–6].
-
(1)
If E is a uniformly smooth real Banach space, then J is uniformly continuous on each bounded subset of E.
-
(2)
If E is a uniformly smooth real Banach space, then is a normalized duality mapping on , then , and , where on and are the identity mappings on E and , respectively.
-
(3)
Let E be a smooth, strictly convex reflexive Banach space and J be the duality mapping from E into . Then is also single-valued, one-to-one, onto, and it is also the duality mapping from into E.
-
(4)
If E is a reflexive, strictly convex Banach space, then is hemicontinuous, that is, is norm-to-weak∗-continuous.
-
(5)
If E is a reflexive, smooth and strictly convex Banach space, then J is single-valued, one-to-one, and onto.
-
(6)
A Banach space E is uniformly smooth if and only if is uniformly convex.
-
(7)
Each uniformly convex Banach space E has the Kadec-Klee property, that is, for any sequence , if , and , then .
-
(8)
A Banach space E is strictly convex if and only if J is strictly monotone, that is,
-
(9)
Both, uniformly smooth Banach space and uniformly convex Banach space, are reflexive.
-
(10)
If is uniformly convex, then J is uniformly norm-to-norm continuous on each bounded subset of E.
-
(11)
If is a strictly convex Banach space, then J is one-to-one, that is, implies that .
Let J be the normalized duality mapping, then J is said to be weakly sequentially continuous if the strong convergence of a sequence to implies the weak∗ convergence of a sequence to Jx in .
Let E be a smooth and strictly convex reflexive Banach space, and let C be a nonempty, closed and convex subset of E. We assume that the Lyapunov functional is defined by [6, 7]
Let C be a nonempty, closed and convex subset of a Banach space E. The generalized projection [6] is defined by, for each ,
Remark 1.5 From the definition of ϕ. It is easy to see that
-
(1)
for all .
-
(2)
for all .
-
(3)
for all .
-
(4)
If E is a real Hilbert space H, then , and (the metric projection of H onto C).
If C is a nonempty, closed and convex subset of a smooth and strictly convex reflexive real Banach space E, then
-
(1)
for , and , one has
-
(2)
, , .
-
(3)
if and only if , .
In 2007, Takahashi and Zembayashi [1] introduced an iterative algorithm for finding a solution of an equilibrium problem with a bifunction defined on the dual space of a Banach space by using the shrinking projection method, and they established the strong convergence as follows.
Theorem TZ Let E be a uniformly convex Banach space whose norm is uniformly Gâteaux differentiable, and let C be a nonempty, closed and convex subset of E such that is closed and convex of . Assume that a mapping satisfies the condition (DA1)-(DA4) such that . Let be a sequence generated by the following algorithm:
where J is the duality mapping on E, the sequence such that , for some , and is the sunny generalized nonexpansive retraction from E onto .
Then the sequence converges strongly to some point , where is the sunny generalized nonexpansive retraction from E to .
In 2010, Plubtieng and Sriprad [8] proved the existence theorem of the variational inequality problem for skew monotone operator defined on the dual space of a smooth Banach space, and they established a weak convergence theorem for finding a solution of the variational inequality problem using the projection algorithm method with a new projection, which was introduced by Ibaraki and Takahashi [9] and Iiduka and Takahashi [10] in Banach spaces.
Theorem PS Let E be a uniformly convex and 2-uniformly smooth Banach space whose duality mapping J is weakly sequentially continuous. Let C be a nonempty, closed and convex subset of E such that is closed and convex, and let A be an α-inverse-strongly-skew-monotone operator of into E such that and , for all and . Let be a sequence defined by and
for every , where is the sunny generalized nonexpansive retraction of E into C, , for some a, b with , where is a constant satisfying for all . Then the sequence converges weakly to some element . Further .
In this paper, motivated and inspired by the previously mentioned results, we study the existence theorem for a generalized equilibrium problem with a bifunction defined on the dual space of a Banach space, and we also construct an iterative procedure generated by a hybrid method for solving the solution of a generalized equilibrium problem by using the sunny generalized nonexpansive retraction. Under some suitable assumptions, the strong convergence theorems are established in Banach spaces. The results obtained in this paper extend and improve several recent results in this area.
2 Preliminaries
Definition 2.1 Let C be a nonempty and closed subset of a smooth Banach space.
-
(1)
A mapping is said to be closed if for each , and imply that .
-
(2)
A mapping is said to be nonexpansive if
-
(3)
A mapping is said to be ϕ-nonexpansive if , and
-
(4)
A mapping is said to be generalized nonexpansive [11] if , and
Definition 2.2 [11]
Let C be a nonempty and closed subset of a smooth Banach space E. A mapping is called
-
(1)
a retraction if ;
-
(2)
a sunny if for all and .
We also know that if E is a smooth, strictly convex and reflexive Banach space, and C is nonempty, closed and convex subset of E, then there exists a sunny generalized nonexpansive retraction of E onto C if and only if is closed and convex. In this case, is given by
Definition 2.3 [11]
Let C be a nonempty and closed subset of a smooth Banach space E. The set C is called a sunny generalized nonexpansive retraction of E if there exists a sunny generalized nonexpansive R from E onto C.
Lemma 2.4 [9]
Let C be a nonempty and closed subset of a smooth and strictly convex Banach space E, and let R be a retraction from E onto C. Then the following are equivalent:
-
(1)
R is sunny generalized nonexpansive;
-
(2)
for all and .
Lemma 2.5 [9]
Let C be a nonempty, closed and sunny generalized nonexpansive retraction of a smooth and strictly convex Banach space E. Then the sunny generalized nonexpansive retraction from E onto C is uniquely determined.
Lemma 2.6 [9]
Let C be a nonempty and closed subset of a smooth and strictly convex Banach space E such that there exists a sunny generalized nonexpansive retraction R from E onto C. Let and . Then the following hold:
-
(1)
if and only if for all ;
-
(2)
.
Lemma 2.7 [12]
Let C be a nonempty and closed subset of a smooth, strictly convex and reflexive Banach space E. Then the following are equivalent:
-
(1)
C is a sunny generalized nonexpansive retraction of E;
-
(2)
is closed and convex.
Remark 2.8 From Lemmas 2.5 and 2.7. If E is a Hilbert space, then a sunny generalized nonexpansive retraction from E onto C reduces to a metric projection operator P from E onto C.
Lemma 2.9 [12]
Let E be a smooth, strictly convex and reflexive Banach space, let C be a nonempty, closed and sunny generalized nonexpansive retraction of E, and let R be the sunny generalized nonexpansive retraction from E onto C. Let and . Then the following are equivalent:
-
(1)
;
-
(2)
.
Lemma 2.10 [7]
Let E be a uniformly smooth and strictly convex real Banach space, and let and be two sequences of E. If and either or is bounded, then .
Lemma 2.11 [13]
Let E be a uniformly smooth and strictly convex real Banach space with the Kadec-Klee property, and let C be a nonempty, closed and convex subset of E. Let and be two sequences in C and . If and , then .
Lemma 2.12 [14]
Let and be two sequences of nonnegative real numbers satisfying the inequality
If , then exists.
Now, let us recall the following well-known concept and the result.
Definition 2.13 [15]
Let B be a subset of a topological vector space X. A mapping is called a KKM mapping if , for and , where convA denotes the convex hull of the set A.
In [16], Ky Fan gave the following famous infinite-dimensional generalization of Knaster, Kuratowski and Mazurkiewicz’s classical finite-dimensional result.
Lemma 2.14 [16]
Let B be a subset of a Hausdorff topological vector space X, and let be a KKM mapping. If is closed for all and is compact for at least one , then .
3 Existence theorem
In this section, we prove the existence theorem of a solution for a generalized equilibrium problem with a bifunction defined on the dual space of a Banach space. Now, we use the concept of KKM mapping to prove the lemma for our main result.
Lemma 3.1 Let C be a nonempty, compact and convex subset of a uniformly smooth, strictly convex and reflexive Banach space E, and let J be the duality mapping from E into such that is closed and convex, let us assume that a bifunction satisfies the following conditions (DA1)-(DA4), let be a nonempty, closed and convex subset of , and let be an α-inverse strongly skew monotone. Let any be a given real number, and let be any point. Then there exists such that
Proof Let be any point in E. For each , we define the mapping as follows:
It is easy to see that , and hence .
(a) First, we show that H is a KKM mapping.
Suppose that H is not a KKM mapping. Then there exists a finite subset of C and with such that for all .
It follows from the definition of a mapping H that
By the assumptions of (DA1) and (DA4), we get
which is a contradiction. Thus, H is a KKM mapping on C.
(b) Next, we show that is closed for all .
Let be a sequence in such that , as .
It then follows from that
By assumption (DA3), the continuity of J and the lower semicontinuity of , we obtain from (3.1) that
Now, we get
Therefore, , and so is closed for all .
(c) We show that is weakly compact.
Now, we know that is closed and subset of C.
Since C is compact. Therefore, is compact, and then is weakly compact.
By using (a), (b), (c) and Lemma 2.14, we can conclude that .
Therefore, there exists such that
□
Theorem 3.2 Let C be a nonempty, closed and convex subset of a uniformly smooth, strictly convex and reflexive Banach space E, and let J be the duality mapping from E into such that is closed and convex, let us assume that a bifunction satisfies the following conditions (DA1)-(DA4), let be a nonempty, closed and convex subset of , and let be an α-inverse strongly skew monotone and hemicontinuous. Let any be a given real number, and let be any point. We define a mapping as follows:
Then the following conclusions hold:
-
(1)
is single-valued;
-
(2)
, ;
-
(3)
;
-
(4)
is closed and convex.
Proof We complete this proof by four items below.
(1) We show that is single-valued.
From the definition of , it is easy to see that
Therefore, . Hence, .
Indeed, for any and , let . Then
and
Adding the two inequalities, we have
Therefore, we obtain
From condition (DA2) and the fact that A is an α-inverse strongly skew monotone, we have
Since , J is monotone, and E is strictly convex, we obtain
This implies that is single-valued.
(2) We show that for all .
Indeed, for any and , we have
and
Adding the two inequalities, we have
It follows that
From condition (DA2) and the fact that A is an α-inverse strongly skew monotone, we get
Since , we have
Therefore, we also have
This implies that
(3) We show that .
It is easy to see that
This implies that .
(4) We show that is closed and convex.
For each , we define the mapping as follows:
It is easy to see that , so that .
Next, we show that G is a KKM mapping.
Suppose that G is not a KKM mapping. Then there exists a finite subset of C, and with such that for all . Then we have
It follows from (DA1) and (DA4) that
which is the contradiction. Hence, G is a KKM mapping on C.
(4.1) Next, we show that is closed for each .
For any , let be any sequence in such that , as .
Hence, , as .
Next, we show that . Then for each , we have
It follows from assumption (DA3) that
This implies that , and hence is closed for each .
Since J is continuous. Therefore, is closed.
(4.2) Next, we show that is convex.
Let , then we have and , where .
For , let and for any , we set .
It follows from (DA1) and (DA4) that
and
Adding two inequalities (3.7) and (3.8) and dividing by , we get
Letting t to 0 by (DA3) and the hemicontinuous of A, we obtain
Hence, , and thus, is convex.
This completes the proof. □
4 Convergence theorem
In this section, we use the hybrid projection method for finding a solution of a generalized equilibrium problem in the dual space of Banach spaces.
Theorem 4.1 Let E be a uniformly smooth, strictly convex and reflexive real Banach space, which has a Kadec-Klee property, let C be a nonempty, closed and convex subset of E, and let J be the duality mapping from E into such that is closed and convex of , let us assume that a bifunction satisfies the following conditions (DA1)-(DA4), let be a nonempty, closed and convex subset of , and let be an α-inverse strongly skew monotone. For , we define a mapping as follows:
Suppose that . Let be a sequence generated by
where J is the duality mapping on E, is the sequence in such that , for some and is the sunny generalized nonexpansive retraction from E onto . Then the sequence converges strongly to , where is the sunny generalized nonexpansive retraction from E onto .
Proof We complete this proof by seven steps below.
Step 1. We show that are closed and convex subsets of E for each .
It is obvious that is closed and convex. Suppose that is closed and convex for some .
For each , we see that
Hence, is closed and convex. Therefore, are closed and convex subsets of E for each .
Step 2. We show that for all .
Note that and is generalized nonexpansive.
From , we have . Suppose that , for some .
For any , from algorithm (4.2) and the fact that is generalized nonexpansive, we compute
Therefore, . Hence, we get .
This implies that , , and also the sequence is well defined.
Step 3. We show that is bounded.
From the definition of , we know that and . For all , we have
Then is bounded. Therefore, is bounded, and also is bounded.
Step 4. We show that there exists such that , as .
Since and , we have
Therefore, the sequence is nondecreasing. Hence, exists.
By the definition of , one has that and for any positive integer . It follows that
It follows from Lemma 2.10 that , as .
Thus, the sequence is a Cauchy sequence.
Without loss of generality, we can assume that . Since is bounded, and E is reflexive.
We know that , and is closed and convex, for , we have
It follows that
This implies that
So . Since . By the Kadec-Klee property of E, we obtain that
From J is uniformly norm-to-norm continuous on bounded subset of E, we also have
Step 5. We show that and , as .
Since and exists. We get
From , we have
By Lemma 2.10, we obtain
Therefore,
Since
By the assumption, we have , we obtain
Step 6. We show that , that is, .
From (4.3), (4.5) and (4.6), it follows that
From J is uniformly norm-to-norm continuous on bounded subset of E, we also have
It follows from (4.5) and (4.6) and the property of J that
Since is bounded. Therefore, and are bounded.
Hence, , and are also bounded.
So, there exists a subsequence of such that , and there exists a subsequence of such that .
From (4.6) and , we have
Since , we have
It follows from (DA2) that
From and , we get
Therefore,
For any , and setting . Then we get , and so
It follows from (DA1) and (DA4) that
Letting , we obtain from assumption (DA3) that
This implies that
Step 7. We show that the sequence converges strongly to .
We know that .
Let . It follows from Lemma 2.9 that
Since the norm is weakly lower semicontinuous, and from (DA4), we have
From the definition of , we have .
Finally, we show that , where .
Now, we have
and from Remark 1.5(2), we get
Therefore,
This implies that
By Lemma 2.10, we get
Hence,
Therefore, the sequence converges strongly to . This completes the proof. □
If we substitute and in equation (4.1), then we obtain the following result which extends the following results by Takahashi and Zembayashi [1] from an equilibrium problem to a generalized equilibrium problem.
Corollary 4.2 Let E be a uniformly smooth, strictly convex and reflexive real Banach space which has a Kadec-Klee property, let C be a nonempty, closed and convex subset of E, and let J be the duality mapping from E into such that is closed and convex of , let us assume that a bifunction satisfies the following conditions (DA1)-(DA4), let be a nonempty, closed and convex subset of , and let be an α-inverse strongly skew monotone. Suppose that . Let be a sequence generated by
where J is the duality mapping on E, is the sequence in such that , for some and is the sunny generalized nonexpansive retraction from E onto . Then the sequence converges strongly to , where is the sunny generalized nonexpansive retraction from E onto .
If we set in Corollary 4.2, then we obtain the following result which extends the following results by Takahashi and Zembayashi [1].
Corollary 4.3 Let E be a uniformly smooth, strictly convex and reflexive real Banach space which has Kadec-Klee property, let C be a nonempty, closed and convex subset of E, and let J be the duality mapping from E into such that is closed and convex of , let us assume that a bifunction satisfies the following conditions (DA1)-(DA4). Suppose that . Let be a sequence generated by
where J is the duality mapping on E, is the sequence in such that , for some and is the sunny generalized nonexpansive retraction from E onto . Then the sequence converges strongly to , where is the sunny generalized nonexpansive retraction from E onto .
If we set in Corollary 4.2, then Corollary 4.2 is reduced to the following corollary.
Corollary 4.4 Let E be a uniformly smooth, strictly convex and reflexive real Banach space which has a Kadec-Klee property, let C be a nonempty, closed and convex subset of E such that is closed and convex of , let be a nonempty, closed and convex subset of and be an α-inverse strongly skew monotone. Suppose that . Let be a sequence generated by
where J is the duality mapping on E, is the sequence in such that , for some , and is the sunny generalized nonexpansive retraction from E onto . Then the sequence converges strongly to , where is the sunny generalized nonexpansive retraction from E onto .
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Acknowledgements
The first author would like to thank the Bansomdejchaopraya Rajabhat University for financial support. The authors would like to thank the Higher Education Research Promotion and National Research University Project of Thailand’s Office of the Higher Education Commission for financial support (Under NRU-CSEC Project No. NRU56000508).
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Phuangphoo, P., Kumam, P. Existence and approximation for a solution of a generalized equilibrium problem on the dual space of a Banach space. Fixed Point Theory Appl 2013, 264 (2013). https://doi.org/10.1186/1687-1812-2013-264
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DOI: https://doi.org/10.1186/1687-1812-2013-264