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A new general iterative algorithm with Meir-Keeler contractions for variational inequality problems in q-uniformly smooth Banach spaces
Fixed Point Theory and Applications volume 2013, Article number: 292 (2013)
Abstract
In this paper, we generalize the iterative scheme and extend the space studied in (Fixed Point Theory and Applications 2012:46, 2012). Further, we prove some strong convergence theorems of the new iterative scheme for variational inequality problems in q-uniformly smooth Banach spaces under very mild conditions. Our results improve and extend corresponding ones announced by many others.
MSC:47H09, 47H10.
1 Introduction
Throughout this paper, we denote by X and a real Banach space and the dual space of X, respectively. Let C be a nonempty closed convex subset of X.
The duality mapping is defined by , . It is well known that if X is smooth, then J is single-valued, which is denoted by j. Let be a real number. The generalized duality mapping is defined by
for all , where denotes the generalized duality pairing between X and . In particular, is called the normalized duality mapping and for . It is well known that if X is smooth, then is single-valued, which is denoted by .
Recall that a mapping is a contraction on C if there exists a constant such that
A mapping is said to be nonexpansive if
A mapping is said to be L-Lipschitzian if there exists a positive constant L such that
A mapping is said to be η-strongly accretive if there exist and such that
Without loss of generality, we can assume that and .
Recall that if C and D are nonempty subsets of a Banach space X such that C is nonempty closed convex and , then a mapping is sunny [1] provided for all and , whenever . A mapping is called a retraction if for all . Furthermore, P is a sunny nonexpansive retraction from C onto D if P is a retraction from C onto D which is also sunny and nonexpansive. A subset D of C is called a sunny nonexpansive retraction of C if there exists a sunny nonexpansive retraction from C onto D.
Let be a family of mappings from a subset C of a Banach space X into itself with . We say that satisfies the AKTT-condition if for each bounded subset D of C,
Proposition 1.1 (Banach [2])
Let be a complete metric space, and let f be a contraction on X, then f has a unique fixed point.
Proposition 1.2 (Meir and Keeler [3])
Let be a complete metric space, and let ϕ be a Meir-Keeler contraction (MKC, for short) on X, that is, for every , there exists such that implies for all . Then ϕ has a unique fixed point.
This proposition is one of generalizations of Proposition 1.1, because the contractions are Meir-Keeler contractions.
Proposition 1.3 [4]
Let C be a closed convex subset of a smooth Banach space X. Let be a nonempty subset of C. Let be a retraction, and let J be the normalized duality mapping on X. Then the following are equivalent:
-
(i)
is sunny and nonexpansive.
-
(ii)
, .
-
(iii)
, .
Variational inequality theory has emerged as a great important tool in studying a wide class of unilateral, free, obstacle, moving and equilibrium problems arising in several branches of pure and applied sciences in a unified and general framework. This field is dynamics and it is experiencing an explosive growth in both theory and applications. Several numerical methods have been developed for solving variational inequalities and related optimization problems; see [4–7] and the references therein.
Let C be a nonempty closed convex subset of a real Hilbert space H. Recall that the classical variational inequality is to find an such that
where is a nonlinear mapping. The set of solutions of (1.5) is denoted by .
In 2008, Yao et al. [8] modified Mann’s iterative scheme by using the viscosity approximation method which was introduced by Moudafi [1]. More precisely, they introduced and studied the following iterative algorithm:
where T is a nonexpansive mapping of K into itself and f is a contraction on K. They obtained a strong convergence theorem under some mild restrictions on the parameters.
Zhou [9] and Qin et al. [10] modified normal Mann’s iterative process (1.6) for k-strictly pseudo-contractions to have strong convergence in Hilbert spaces. Qin et al. [10] introduced the following iterative algorithm scheme:
where T is a k-strictly pseudo-contraction, f is a contraction and A is a strong positive linear bounded operator, is the metric projection. They proved, under certain appropriate assumptions on the sequences and , that defined by (1.7) converges strongly to a fixed point of the k-strictly pseudo-contraction, which solves some variational inequality.
Very recently, Song et al. [11] introduced the following iteration process:
where is a -strictly pseudo-contraction, ϕ is an MKC contraction and is an L-Lipschitzian and η-strongly monotone mapping in a Hilbert space, is the metric projection. Under certain appropriate assumptions on the sequences , , and , the sequence defined by (1.8) converges strongly to a common fixed point of an infinite family of -strictly pseudo-contractions, which solves some variational inequality.
Question 1 Can the space in Song [11] be extended from a Hilbert space to a q-uniformly smooth Banach space?
Question 2 Can the projection in Song [11] be changed to the sunny nonexpansive retraction and be put to other place of the iteration process?
Question 3 Can we extend the iterative scheme of algorithm (1.8) to a more general iterative scheme?
Question 4 Can we remove the very strict condition which is necessary in Lemma 3.1 and Theorem 3.2 of Song [11]?
The purpose of this paper is to give affirmative answers to these questions mentioned above. In this paper we study a new general iterative scheme as follows:
where is a -strictly pseudo-contraction, ϕ is an MKC contraction, is the sunny nonexpansive retraction and is an L-Lipschitzian and η-strongly accretive mapping in a q-uniformly smooth Banach space. Under some suitable assumptions on the sequences , , and , the sequence defined by (1.9) converges strongly to a common fixed point of an infinite family of -strictly pseudo-contractions, which solves some variational inequality.
2 Preliminaries
In this section, we first recall some notations. T is said to be a λ-strict pseudo-contraction in the terminology of Browder and Petryshyn [12] if there exists a constant such that
for every and for some . It is clear that (2.1) is equivalent to the following:
A Banach space X is said to be strictly convex if whenever x and y are not collinear, then .
Then the modulus of convexity of X is defined by
for all . X is said to be uniformly convex if and for all , and if with , then X is said to be p-uniformly convex. A Hilbert space H is 2-uniformly convex, while is -uniformly convex for every . Let be the modulus of smoothness of X defined by
A Banach space X is said to be uniformly smooth if as . A Banach space X is said to be q-uniformly smooth if there exists a fixed constant such that with . A typical example of uniformly smooth Banach spaces is , where . More precisely, is -uniformly smooth for every .
The norm of a Banach space X is said to be Gâteaux differentiable if the limit
exists for all x, y on the unit sphere . If, for each , the limit (2.3) is uniformly attained for , then the norm of X is said to be uniformly Gâteaux differentiable. The norm of X is said to be Fréchet differentiable if, for each , the limit (2.3) is attained uniformly for .
In order to prove our main results, we need the following lemmas.
Lemma 2.1 [13]
Let ϕ be an MKC on a convex subset C of a Banach space X. Then, for each , there exists such that
Lemma 2.2 [14]
Let X be a real q-uniformly smooth Banach space, then there exists a constant such that
Lemma 2.3 [15]
Let be a sequence of nonnegative numbers satisfying the property
where , , satisfy the restrictions:
-
(i)
, ;
-
(ii)
, ;
-
(iii)
. Then .
Lemma 2.4 [16]
Let C be a nonempty convex subset of a real q-uniformly smooth Banach space X, and let be a λ-strict pseudo-contraction. For , we define . Then, as , , is nonexpansive such that .
Lemma 2.5 [17]
Let , then the following inequality holds:
for arbitrary positive real numbers a, b.
Lemma 2.6 Let F be an L-Lipschitzian and η-strongly accretive operator on a nonempty closed convex subset C of a real q-uniformly smooth Banach space X with and . Then is a contraction with contraction coefficient .
Proof From the definition of η-strongly accretive and L-Lipschitzian operator, we have
Therefore, we have
for all . From and , we have and
where . Hence, G is a contraction with contraction coefficient . This completes the proof. □
Lemma 2.7 ([18], Demiclosedness principle)
Let C be a nonempty closed convex subset of a reflexive Banach space X which satisfies Opial’s condition, and suppose that is nonexpansive. Then the mapping is demiclosed at zero, that is, , implies .
Lemma 2.8 Let C be a closed convex subset of a smooth Banach space X. Let be a nonempty subset of C. Let be a retraction, and let j, be the normalized duality mapping and generalized duality mapping on X, respectively. Then the following are equivalent:
-
(i)
is sunny and nonexpansive.
-
(ii)
, .
-
(iii)
, .
-
(iv)
, .
Proof From Proposition 1.3, we have (i) ⇔ (ii) ⇔ (iii). We need only to prove (iii) ⇔ (iv).
Indeed, if , it follows from the fact that , .
If , then , . This completes the proof. □
Lemma 2.9 [19]
Suppose that satisfies the AKTT-condition, then for each bounded subset D of C:
-
(i)
converges strongly to some point in C for each ;
-
(ii)
Furthermore, if the mapping is defined by for all , then .
Lemma 2.10 Let C be a closed convex subset of a reflexive Banach space X which admits a weakly sequentially continuous duality mapping from X to . Let be a nonexpansive mapping with and ϕ be an MKC on C. Suppose that is an η-strongly accretive and L-Lipschitzian mapping with coefficient and . Then the sequence defined by converges strongly as to a fixed point of S, which solves the variational inequality
Proof The definition of is a good definition. Indeed, from the definition of MKC, we can see that an MKC is also a nonexpansive mapping. Consider a mapping on C defined by
It is easy to see that is a contraction when . Indeed, by Lemmas 2.1 and 2.5, we have
where . Hence has a unique fixed point, denoted by , which uniquely solves the fixed point equation
Next we show the uniqueness of a solution of the variational inequality (2.4). Suppose that and are solutions to (2.4), then, without loss of generality, we may assume that there is a number ε such that . Then, by Lemma 2.1, there is a number such that . From (2.4) we have
Adding up (2.6) and (2.7), we obtain
Meanwhile, we notice that
Thus and the uniqueness is proved. Below, we use to denote the unique solution of (2.3).
First, we prove that is bounded.
Assume that for , fixed for each t.
Case 1. . In this case, we can see easily that is bounded.
Case 2. . In this case, by Lemma 2.1, there is a number such that , then we have
which implies . Thus is bounded.
Then, we prove that () as .
Since X is reflexive and is bounded, there exists a subsequence of such that . Setting , we obtain .
We claim .
It follows from Lemma 2.8 that
then we have
which implies that
Since as , by (2.10) we obtain that . Hence, we have .
Now, we prove that solves the variational inequality (2.4).
Since
we get that
Notice that
Then, for ,
where . Notice
Thus, we have
Now replacing t in (2.13) with and letting , notice that for , we obtain , i.e., is a solution of (2.4). Hence by uniqueness. Thus, we have shown that every cluster point of (at ) equals , therefore as . □
Lemma 2.11 Let X be a q-uniformly smooth Banach space, and let C be a nonempty convex subset of X. Assume that is a countable family of -strict pseudo-contractions for some and such that . Assume that is a positive sequence such that . Then is a λ-strict pseudo-contraction with and .
Proof Let , where . Then is a λ-strict pseudo-contraction with .
Step 1. We firstly prove the case of .
where , which shows that is a λ-strict pseudo-contraction.
Using the same means, our proof method can easily carry over to the general finite case.
Step 2. We prove the infinite case. From the definition of λ-strict pseudo-contraction, we have
then we obtain
Taking , it follows from (2.15) that
Thus, for , if with and , then strongly converges.
Let
then we obtain
Therefore
Thus, H is a λ-strict pseudo-contraction.
Step 3. We prove .
Let , then, for , we obtain
where . Thus, we obtain , it follows that . □
3 Main results
Lemma 3.1 Let C be a nonempty closed convex subset of a q-uniformly smooth Banach space X. Let be the sunny nonexpansive retraction from X onto C, and let ϕ be an MKC on C. Let be η-strongly accretive and L-Lipschitzian with , and let be a -strictly pseudo-contractive non-self-mapping such that . Assume . Let be a sequence of C generated by (1.9). We assume that the following parameters are satisfied:
-
(i)
, , , ;
-
(ii)
, ;
-
(iii)
, ;
-
(iv)
, , ;
-
(v)
, .
Then .
Proof Let , by Lemma 2.11 we obtain that for each , is a λ-strict pseudo-contraction on C and . Further, we can get that
thus satisfies the AKTT-condition. Let , where , . From Lemma 2.4 and Lemma 2.11 we have that is a nonexpansive mapping and , then the iterative algorithm (1.9) can be rewritten as follows:
We divide the rest of the proof into two parts.
Step 1. We will prove that the sequence is bounded.
Letting
from Lemma 2.4 and condition (ii), we get that is nonexpansive. Taking a point , we have and . Therefore, we obtain
From the definition of MKC and Lemma 2.1, for any , there is a number , if , then ; if , then . It follows from (3.1) and Lemma 2.5 that
By induction, we obtain
Hence, we obtain
where M is a constant such that
which implies that is bounded, so are and .
Step 2. We claim that as .
From (3.1) we have
Next, we estimate and . Notice that
Substituting (3.3) and (3.4) into (3.2) and using condition (iv), we have
where is an appropriate constant such that
Since satisfies the AKTT-condition, we get that
Noticing conditions (i), (iii) and (iv) and applying Lemma 2.3 to (3.5), we obtain
This completes the proof. □
Lemma 3.2 Let C be a nonempty closed convex subset of a q-uniformly smooth Banach space X. Let be the sunny nonexpansive retraction from X onto C, and let ϕ be an MKC on C. Let be η-strongly accretive and L-Lipschitzian with , and let be a -strictly pseudo-contractive non-self-mapping such that . Assume . Let be a sequence of C generated by (1.9). We assume that the parameters , , and satisfy the conditions (i), (ii), (iii), (iv), (v) in Lemma 3.1 and (vi) . Then converges strongly to , which solves the following variational inequality:
Proof The proof of the lemma will be split into three parts.
Step 1. We will prove that , where is defined by and . From (3.1) we have
Using conditions (i) and (vi), we obtain
Since satisfies the AKTT-condition, from Lemma 2.9 we can obtain . Furthermore, notice that
therefore, we deduce that is a λ-strict pseudo-contraction. Applying Lemma 2.4, we obtain that T is nonexpansive with . Notice that
Using (3.6), (3.7) and (vi), we obtain
Step 2. We will show that
where with being the fixed point of the contraction
From the above, we know that , then we take a subsequence of and assume that , where . Since the Banach space X has a weakly sequentially continuous generalized duality mapping , by using Lemma 2.7, 2.10 and (3.8), we have
Step 3. We will prove that .
By contradiction, there is a number such that
First, let
Now, we will obtain the contradiction from two cases.
Case 1. Fix (), if for some such that , and for the other such that .
Let
From (3.9) we know , thus there are two numbers h and N. When , we have , where . We extract a number satisfying , then from Lemma 2.5 and (3.10), we have
which implies that
Hence, we have
In the same way, we obtain
It contradicts .
Case 2. Fix (), if for all . In this case, from Lemma 2.1 there exists a number such that
From (3.10) we have
which implies that
Put and . Applying Lemma 2.3 to (3.11), we obtain as , which contradicts . Thus . This completes the proof. □
Theorem 3.1 Let C be a nonempty closed subset of a q-uniformly smooth Banach space X. Let be the sunny nonexpansive retraction from X onto C, and let ϕ be an MKC on C. Let be an η-strongly accretive L-Lipschitzian and linear mapping with , and let be a -strictly pseudo-contractive non-self-mapping such that . Assume . Let be a sequence of C generated by (1.9). We assume that the following parameters are satisfied:
-
(i)
, , , ;
-
(ii)
, ;
-
(iii)
, ;
-
(iv)
, , ;
-
(v)
, ;
-
(vi)
.
Then generated by (3.1) converges strongly to , which solves the following variational inequality:
Proof Combining the proof of Lemma 3.1 with Lemma 3.2, we can obtain the conclusion. □
Remark 3.1 Compared with Theorem 3.2 of Song [11], our results are different from those in the following aspects:
-
(i)
Theorem 3.1 improves and extends Theorem 3.2 of Song [11]. Especially, our results extend the above results from a Hilbert space to a more general q-uniformly smooth and uniformly convex Banach space.
-
(ii)
We change the metric projection in Song [11] into the sunny nonexpansive retraction and put it to the other place of the iteration process so that our iteration process is better defined.
-
(iii)
We generalize the iteration process so that our iteration process is more general.
-
(iv)
We remove the very strict condition in Lemma 3.1 and Theorem 3.2 of Song [11], and it is worth stressing that the strict condition is also very necessary in Qin et al. [10] and Cai et al. [20].
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Guan, J., Hu, C. & Wang, X. A new general iterative algorithm with Meir-Keeler contractions for variational inequality problems in q-uniformly smooth Banach spaces. Fixed Point Theory Appl 2013, 292 (2013). https://doi.org/10.1186/1687-1812-2013-292
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DOI: https://doi.org/10.1186/1687-1812-2013-292