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Strong convergence for solving a general system of variational inequalities and fixed point problems in Banach spaces
Journal of Inequalities and Applications volume 2013, Article number: 334 (2013)
Abstract
In this paper, we propose and analyze some iterative algorithms by hybrid viscosity approximation methods for solving a general system of variational inequalities and a common fixed point problem of an infinite family of nonexpansive mappings in a uniformly convex Banach space which has a uniformly Gâteaux differentiable norm, and we prove some strong convergence theorems under appropriate conditions. The results presented in this paper improve, extend, supplement and develop the corresponding results recently obtained in the literature.
MSC:49J30, 47H09, 47J20.
1 Introduction
Let X be a real Banach space whose dual space is denoted by . Let denote the unit sphere of X. The Banach space X is said to be uniformly convex if for each there exists such that for all ,
It is known that a uniformly convex Banach space is reflexive and strictly convex. The Banach space X is said to be smooth if the limit
exists for all ; in this case, X is also said to have a Gâteaux differentiable norm. X is said to have a uniformly Gâteaux differentiable norm if for each , the limit is attained uniformly for . Moreover, it is said to be uniformly smooth if this limit is attained uniformly for . The norm of X is said to be the Fréchet differential if for each , this limit is attained uniformly for . In addition, we define a function , called the modulus of smoothness of X, as follows:
It is known that X is uniformly smooth if and only if . Let q be a fixed real number with . Then the Banach space X is said to be q-uniformly smooth if there exists a constant such that for all . As pointed out in [1], no Banach space is q-uniformly smooth for .
Let be the dual of X. The normalized duality mapping is defined by
where denotes the generalized duality pairing. It is an immediate consequence of the Hahn-Banach theorem that is nonempty for each . Moreover, it is known that J is single-valued if and only if X is smooth, whereas if X is uniformly smooth, then the mapping J is norm-to-norm uniformly continuous on bounded subsets of X. If X has a uniformly Gateaux differentiable norm, then the duality mapping J is norm-to-weak∗ uniformly continuous on bounded subsets of X.
Let C be a nonempty closed convex subset of a real Banach space X. A mapping is called nonexpansive if
The set of fixed points of T is denoted by . We use the notation ⇀ to indicate the weak convergence and the one → to indicate the strong convergence.
Definition 1.1 Let be a mapping of C into X. Then A is said to be
-
(i)
accretive if for each there exists such that
where J is the normalized duality mapping;
-
(ii)
α-strongly accretive if for each there exists such that
for some ;
-
(iii)
β-inverse-strongly-accretive if for each there exists such that
for some ;
for some .
In the literature, much work has been done related to strong convergence for solving variational inequalities and fixed point problems, see [1–48]. It is worth emphasizing that the definition of the inverse strongly accretive mapping is based on that of the inverse strongly monotone mapping, which was studied by so many authors; see, e.g., [15, 30, 31].
Very recently, Cai and Bu [35] considered the following general system of variational inequalities (GSVI) in a real smooth Banach space X, which involves finding such that
where C is a nonempty, closed and convex subset of X, are two nonlinear mappings and and are two positive constants. Here the set of solutions of GSVI (1.1) is denoted by . In particular, if , a real Hilbert space, then GSVI (1.1) reduces to the following GSVI of finding such that
where and are two positive constants. The set of solutions of problem (1.2) is still denoted by . In particular, if , then problem (1.2) reduces to the new system of variational inequalities (NSVI), introduced and studied by Verma [5]. Further, if additionally, then the NSVI reduces to the classical variational inequality problem (VIP) of finding such that
The solution set of VIP (1.3) is denoted by . Variational inequality theory has been studied quite extensively and has emerged as an important tool in the study of a wide class of obstacle, unilateral, free, moving, equilibrium problems. It is now well known that the variational inequalities are equivalent to the fixed point problems, the origin of which can be traced back to Lions and Stampacchia [13]. This alternative formulation has been used to suggest and analyze the projection iterative method for solving variational inequalities under the conditions that the involved operator must be strongly monotone and Lipschitz continuous.
Recently, Ceng, Wang and Yao [4] transformed problem (1.2) into a fixed point problem in the following way.
Lemma 1.1 (see [4])
For given, is a solution of problem (1.1) if and only ifis a fixed point of the mappingdefined by
where.
In particular, if the mappingsare-inverse strongly monotone for, then the mappingGis nonexpansive providedfor.
In 1976, Korpelevich [6] proposed an iterative algorithm for solving VIP (1.3) in the Euclidean space :
with a given number, which is known as the extragradient method (see also [7, 8]). The literature on the VIP is vast and Korpelevich’s extragradient method has received great attention given by many authors, who improved it in various ways; see, e.g., [9–27, 35, 40] and references therein, to name but a few.
In particular, whenever X is still a real smooth Banach space, and , then GSVI (1.1) reduces to the variational inequality problem (VIP) of finding such that
which was considered by Aoyama, Iiduka and Takahashi [28]. Note that VIP (1.5) is connected with the fixed point problem for a nonlinear mapping (see, e.g., [41]), the problem of finding a zero point of a nonlinear operator (see, e.g., [36]) and so on. It is clear that VIP (1.5) extends VIP (1.3) from Hilbert spaces to Banach spaces.
In order to find a solution of VIP (1.5), Aoyama, Iiduka and Takahashi [28] introduced the following iterative scheme for an accretive operator A:
where is a sunny nonexpansive retraction from X onto C. Then they proved a weak convergence theorem. For related work, see [29] and the references therein.
Recently, Jung [43] introduced and analyzed a composite iterative algorithm by the viscosity approximation method for solving a fixed point problem of a nonexpansive mapping and VIP (1.3) for an inverse-strongly monotone mapping A in a real Hilbert space H.
Theorem 1.1 (see [[43], Theorem 3.1])
LetCbe a nonempty closed convex subset of a real Hilbert spaceH. Letbe anα-inverse-strongly monotone mapping, letbe a nonexpansive mapping such that, and letbe a contraction with coefficient. Letbe the sequence generated by
where, and. If, andsatisfy the following conditions:
-
(i)
and,
-
(ii)
, for some,
-
(iii)
for some,
-
(iv)
, and,
thenconverges strongly to, which solves the VIP
Beyond doubt, it is an interesting and valuable problem of how to construct some algorithms with strong convergence for solving GSVI (1.1) which contains VIP (1.5) as a special case. Very recently, Cai and Bu [35] constructed an iterative algorithm for solving GSVI (1.1) and a common fixed point problem of an infinite family of nonexpansive mappings in a uniformly convex and 2-uniformly smooth Banach space. They proved strong convergence of the proposed method by virtue of the following inequality in a 2-uniformly smooth Banach space X.
Lemma 1.2 (see [32])
LetXbe a 2-uniformly smooth Banach space. Then
whereκis the 2-uniformly smooth constant ofXandJis the normalized duality mapping fromXinto.
Define the mapping as follows:
The fixed point set of G is denoted by Ω. Then their strong convergence theorem on the proposed method is stated as follows.
Theorem 1.2 (see [[35], Theorem 3.1])
LetCbe a nonempty closed convex subset of a uniformly convex and 2-uniformly smooth Banach spaceX. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-inverse-strongly accretive withfor. Letfbe a contraction ofCinto itself with coefficient. Letbe an infinite family of nonexpansive mappings ofCinto itself such that, whereΩis the fixed point set of the mappingGdefined by (1.7). For arbitrarily given, letbe the sequence generated by
Suppose thatandare two sequences insatisfying the following conditions:
-
(i)
;
-
(ii)
and.
Assume thatfor any bounded subsetDofC, and letSbe a mapping ofCintoXdefined byfor alland suppose that. Thenconverges strongly to, which solves the following VIP:
We remark that in Theorem 1.2, the Banach space X is both uniformly convex and 2-uniformly smooth. According to Lemma 1.2, the 2-uniform smoothness of X guarantees the nonexpansivity of the mapping for an -inverse-strongly accretive mapping with for , and hence the composite mapping is nonexpansive where . In the meantime, for the convenience of implementing the argument techniques in [4], the assumption of real smoothness and uniform convexity on X guarantees that the following inequality holds (see [42]): for any given , there exists a strictly increasing, continuous and convex function , such that
where .
Naturally, we wonder whether the uniform convexity and 2-uniform smoothness of X can be replaced by the weaker geometrical property of X or not. There is no doubt that it is an interesting problem worth investigating.
In this paper, motivated by the above facts, we introduce and study two implicit iterative algorithms and two explicit iterative algorithms by hybrid viscosity approximation methods for finding a common element of the set of solutions of GSVI (1.1) and the set of common fixed points of an infinite family of nonexpansive mappings in a real uniformly convex Banach space which has a uniformly Gâteaux differentiable norm. We prove some strong convergence theorems under appropriate conditions. Our results improve, extend, supplement and develop recent corresponding results in the literature, especially [[35], Theorem 3.1] in the following aspects. First, the assumption of the uniformly convex and 2-uniformly smooth Banach space X in [[35], Theorem 3.1] is weakened to the one of the uniformly convex Banach space X having a uniformly Gâteaux differentiable norm in our results. Second, the proof in [[35], Theorem 3.1] depends on the argument techniques in [4], inequality (1.6) in 2-uniformly smooth Banach spaces and inequality (1.9) in smooth and uniform convex Banach spaces. It is worth emphasizing that the proof in our results depends on no argument techniques in [4] but use the inequality in uniform convex Banach spaces; see Lemma 2.5 in Section 2 of this paper. Third, the four iterative algorithms proposed in this paper are very different from the iterative algorithm in [[35], Theorem 3.1] because two iterative algorithms are implicit ones and the iterative step of computing in other two explicit iterative algorithms involves the sum of three terms.
2 Preliminaries
We list some lemmas that will be used in the sequel. Lemma 2.1 can be found in [34]. Lemma 2.2 is an immediate consequence of the subdifferential inequality of the function .
Lemma 2.1 Let be a sequence of nonnegative real numbers satisfying
where, andsatisfy the conditions:
-
(i)
and;
-
(ii)
;
-
(iii)
, , and.
Then.
Lemma 2.2In a smooth Banach spaceX, the following inequality holds:
Lemma 2.3 (see [39])
Letandbe bounded sequences in a Banach spaceX, and letbe a sequence inwhich satisfies the following condition
Suppose, and. Then.
Let D be a subset of C and let Π be a mapping of C into D. Then Π is said to be sunny if
whenever for and . A mapping Π of C into itself is called a retraction if . If a mapping Π of C into itself is a retraction, then for every , where is the range of Π. A subset D of C is called a sunny nonexpansive retract of C if there exists a sunny nonexpansive retraction from C onto D. The following lemma concerns the sunny nonexpansive retraction.
Lemma 2.4 [33]
LetCbe a nonempty closed convex subset of a real smooth Banach space X. LetDbe a nonempty subset ofC. LetΠbe a retraction ofContoD. Then the following are equivalent:
-
(i)
Πis sunny and nonexpansive;
-
(ii)
, ;
-
(iii)
, , .
It is well known that if , a Hilbert space, then a sunny nonexpansive retraction is coincident with the metric projection from X onto C; that is, . If C is a nonempty closed convex subset of a strictly convex and uniformly smooth Banach space X and if is a nonexpansive mapping with the fixed point set , then the set is a sunny nonexpansive retract of C.
Lemma 2.5 (see [32])
Given a number, a real Banach spaceXis uniformly convex if and only if there exists a continuous strictly increasing function, , such that
for allandsuch thatand.
Lemma 2.6 (see [37])
LetCbe a nonempty closed convex subset of a Banach spaceX. Letbe a sequence of mappings ofCinto itself. Suppose that. Then, for each, converges strongly to some point ofC. Moreover, letSbe a mapping ofCinto itself defined byfor all. Then.
Let C be a nonempty closed convex subset of a Banach space X, and let be a nonexpansive mapping with . As previously, let be the set of all contractions on C. For and , let be the unique fixed point of the contraction on C; that is,
Lemma 2.7 (see [41])
LetXbe a reflexive and strictly convex Banach space with a uniformly Gâteaux differentiable norm. LetCbe a nonempty closed convex subset ofX, letbe a nonexpansive mapping with, and. Then the netdefined byconverges strongly to a point in. If we define a mappingby, , thensolves the VIP
Lemma 2.8 (see [38])
LetCbe a nonempty closed convex subset of a strictly convex Banach spaceX. Letbe a sequence of nonexpansive mappings onC. Suppose thatis nonempty. Letbe a sequence of positive numbers with. Then a mappingSonCdefined byforis well defined, nonexpansive andholds.
Lemma 2.9LetCbe a nonempty closed convex subset of a smooth Banach spaceXand let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Then, for, we have
for. In particular, if, thenis nonexpansive for.
Proof Taking into account the -strictly pseudocontractivity of , we derive for every
which implies that
Hence,
Utilizing the -strong accretivity and -strict pseudocontractivity of , we get
So, we have
Therefore, for we have
Since , it follows immediately that
This implies that is nonexpansive for . □
Lemma 2.10LetCbe a nonempty closed convex subset of a smooth Banach spaceX. Letbe a sunny nonexpansive retraction fromXontoC, and let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a mapping defined by
If, thenis nonexpansive.
Proof According to Lemma 2.9, we know that is nonexpansive for . Hence, for all , we have
This shows that is nonexpansive. This completes the proof. □
Lemma 2.11LetCbe a nonempty closed convex subset of a smooth Banach spaceX. Letbe a sunny nonexpansive retraction fromXontoC, and let the mappingbe-strictly pseudocontractive and-strongly accretive for. For given, is a solution of GSVI (1.4) if and only if, where.
Proof We can rewrite GSVI (1.4) as
which is obviously equivalent to
because of Lemma 2.4. This completes the proof. □
Remark 2.1 By Lemma 2.11, we observe that
which implies that is a fixed point of the mapping .
3 Two-step implicit iterative algorithm
In this section, we introduce our two-step implicit iterative algorithm and show the strong convergence of the purposed algorithm.
Theorem 3.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. Letbe an infinite family of nonexpansive mappings ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , andare the sequences insatisfying the following conditions:
-
(i)
and;
-
(ii)
, , and;
-
(iii)
;
-
(iv)
or;
-
(v)
or;
-
(vi)
or.
Assume thatfor any bounded subsetDofC, and letSbe a mapping ofCinto itself defined byfor all, and suppose that. Thenconverges strongly to, which solves the following VIP:
Proof Take a fixed arbitrarily. Then by Lemma 2.11 we know that . Moreover, by Lemma 2.10 we have
From (3.3) we obtain
which together with (3.3) implies that
which implies that is bounded. By Lemma 2.10 we know from (3.3) that and both are bounded.
Let us show that and as . As a matter of fact, from (3.1) we have
It follows that
Taking into consideration that and , we may assume that and for some . First, we write , , where . It follows that for all ,
This together with (3.4) implies that
where and for some .
Now we observe that
and
Hence from (3.6) it follows that
where for some . Therefore, we get
Utilizing Lemma 2.1, from conditions (i), (iv)-(vi) and the assumption on , we deduce that
Also, we note that for ,
Since and both are bounded, by Lemma 2.5 there exists a continuous strictly increasing function , such that
which immediately yields
Since , , and , we get and hence
In the meantime, according to condition (iii), we have
Thus, from (3.7) and (3.9) it follows that
That is,
This together with (3.1) leads to
That is,
On the other hand, we observe that
which together with implies that
We note that
From (3.7), (3.9), (3.11) and the assumption on , we obtain that
By (3.13) and Lemma 2.6, we have
In terms of (3.11) and (3.14), we have
Define a mapping , where G is defined by (1.7), is a constant. Then by Lemma 2.8 we have that . We observe that
From (3.11) and (3.15), we obtain
Now, we claim that
where with being the fixed point of the contraction
Then solves the fixed point equation . Thus we have
By Lemma 2.2 we conclude that
where
It follows from (3.18) that
Letting in (3.20) and noticing (3.19), we derive
where is a constant such that for all and . Taking in (3.21), we have
On the other hand, we have
It follows that
Taking into account that as , we have from (3.22)
Since X has a uniformly Gâteaux differentiable norm, the duality mapping J is norm-to-weak∗ uniformly continuous on bounded subsets of X. Consequently, the two limits are interchangeable and hence (3.17) holds. From (3.10) we get . Noticing that J is norm-to-weak∗ uniformly continuous on bounded subsets of X, we deduce from (3.17) that
Finally, let us show that as . We observe that
which implies that
Also, by the convexity of and (3.1), we get
which together with (3.24) leads to
Applying Lemma 2.1 to (3.25), we obtain that as . This completes the proof. □
Corollary 3.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. LetSbe a nonexpansive mapping ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , andare the sequences insatisfying the following conditions:
-
(i)
and;
-
(ii)
, , and;
-
(iii)
;
-
(iv)
or;
-
(v)
or;
-
(vi)
or.
Thenconverges strongly to, which solves the following VIP:
4 Three-step implicit iterative algorithm
In this section, we introduce our three-step implicit iterative algorithm and show strong convergence of the purposed algorithm.
Theorem 4.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. Letbe an infinite family of nonexpansive mappings ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , , andare the sequences insatisfying the following conditions:
-
(i)
;
-
(ii)
and;
-
(iii)
, , and;
-
(iv)
;
-
(v)
or;
-
(vi)
or;
-
(vii)
or;
-
(viii)
or.
Assume thatfor any bounded subsetDofC, and letSbe a mapping ofCinto itself defined byfor alland suppose that. Thenconverges strongly to, which solves the following VIP:
Proof Take a fixed arbitrarily. Then by Lemma 2.11 we know that and for all . Moreover, by Lemma 2.10 we have
and
From (4.1) we obtain
which together with (4.4) implies that
which implies that is bounded. By Lemma 2.10 we know from (4.3) and (4.4) that , , and are bounded.
Let us show that and as . As a matter of fact, from (4.1) we have
and
It follows that
and
Taking into consideration that and , we may assume that and for some . Now, we write , , where . It follows that for all ,
This together with (4.5) implies that
where and for some .
Now we observe that
and
Hence from (4.7) it follows that
where for some . Therefore, we get
Utilizing Lemma 2.1, from conditions (ii), (v)-(viii) and the assumption on , we deduce that
Also, taking into account the boundedness of and , by Lemma 2.5 there exists a continuous strictly increasing function , such that for ,
and hence
Since and both are bounded, by Lemma 2.5 there exists a continuous strictly increasing function , such that
which immediately yields
According to condition (iv), we have
Since , , and , we obtain from condition (i) that
Utilizing the properties of and , we have
Thus, from (4.8) and (4.10) it follows that
That is,
Since it follows from (4.1) that
we conclude from (4.10) and that
That is,
This together with (4.11) leads to
By (4.10) and Lemma 2.6, we have
In terms of (4.13) and (4.14), we have
Define a mapping , where G is defined by (1.7), is a constant. Then by Lemma 2.8 we have that . We observe that
From (4.13) and (4.15), we obtain
Utilizing the arguments similar to those of (3.17) in the proof of Theorem 3.1, we can obtain
where with being the fixed point of the contraction ; that is, solves the fixed point equation . Noticing that J is norm-to-weak∗ uniformly continuous on bounded subsets of X, we deduce from (4.11) and (4.17) that
Finally, let us show that as . We observe that
which implies that
Also, by (4.1) and the convexity of , we get
which together with (4.19) implies that
Applying Lemma 2.1 to (4.20), we conclude that as . This completes the proof. □
Corollary 4.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. LetSbe a nonexpansive mapping ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , , andare the sequences insatisfying the following conditions:
-
(i)
;
-
(ii)
and;
-
(iii)
, , and;
-
(iv)
;
-
(v)
or;
-
(vi)
or;
-
(vii)
or;
-
(viii)
or.
Thenconverges strongly to, which solves the following VIP:
5 Two-step explicit iterative algorithm
In this section, we introduce our two-step explicit iterative algorithm and show strong convergence of the purposed algorithm.
Theorem 5.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. Letbe an infinite family of nonexpansive mappings ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , andare the sequences insatisfying the following conditions:
-
(i)
and;
-
(ii)
, , and;
-
(iii)
;
-
(iv)
.
Assume thatfor any bounded subsetDofCand letSbe a mapping ofCinto itself defined byfor alland suppose that. Thenconverges strongly to, which solves the following VIP:
Proof Take a fixed arbitrarily. Then by Lemma 2.11 we know that . Moreover, by Lemma 2.10 we have
From (5.3) we obtain
which implies that is bounded. By Lemma 2.10 we know from (5.3) that and both are bounded.
Let us show that and as . As a matter of fact, from (5.1) we have
It follows that
Now, we write , , where . It follows that for all ,
This together with (5.4) implies that
where for some . Since , and the assumption on , we have
Utilizing Lemma 2.3, from condition (iii) we obtain that
So, we get
Also, we note that for ,
Since and both are bounded, by Lemma 2.5 there exists a continuous strictly increasing function , such that
which immediately yields
Since , , and , we get and hence
In the meantime, according to condition (iii), we have
Thus, from (5.7) and (5.9) it follows that
That is,
This together with (5.1) leads to
That is,
On the other hand, we observe that
which together with implies that
We note that
From (5.9), (5.11) and (5.12), we obtain
By (5.13) and Lemma 2.6, we have
In terms of (5.11) and (5.14), we have
Define a mapping , where G is defined by (1.7), is a constant. Then by Lemma 2.8 we have that . We observe that
From (5.11) and (5.15), we obtain
Utilizing the arguments similar to those of (3.17) in the proof of Theorem 3.1, we can obtain
where with being the fixed point of the contraction ; that is, solves the fixed point equation . Noticing that J is norm-to-weak∗ uniformly continuous on bounded subsets of X, we deduce from (5.10) and (5.17) that
Finally, let us show that as . We observe that
which implies that
Also, by the convexity of and (5.1), we get
It follows from (5.19) that
Applying Lemma 2.1 to (5.20), we obtain that as . This completes the proof. □
Corollary 5.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. LetSbe a nonexpansive mapping ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , andare the sequences insatisfying the following conditions:
-
(i)
and;
-
(ii)
, , and;
-
(iii)
;
-
(iv)
.
Thenconverges strongly to, which solves the following VIP:
6 Three-step explicit iterative algorithm
In this section, we introduce our three-step explicit iterative algorithm and show strong convergence of the proposed algorithm.
Theorem 6.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. Letbe an infinite family of nonexpansive mappings ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , , andare the sequences insatisfying the following conditions:
-
(i)
;
-
(ii)
and;
-
(iii)
, , and;
-
(iv)
;
-
(v)
and.
Assume thatfor any bounded subsetDofC, and letSbe a mapping ofCinto itself defined byfor alland suppose that. Thenconverges strongly to, which solves the following VIP:
Proof Take a fixed arbitrarily. Then by Lemma 2.11 we know that and for all . Moreover, by Lemma 2.10 we have
and
From (6.1) and (6.4) we obtain
which implies that is bounded. By Lemma 2.10 we know from (6.3) and (6.4) that , , and are bounded.
Let us show that and as . As a matter of fact, from (6.1), we have
and
It follows that
and
Now, we write , , where . It follows that for all ,
This together with (6.5) implies that
where for some . Since , , and the assumption on , we have
Utilizing Lemma 2.3, from condition (iv) we obtain that
So, we get
Also, taking into account the boundedness of and , by Lemma 2.5 there exists a continuous strictly increasing function , such that for ,
and hence
Since and both are bounded, by Lemma 2.5 there exists a continuous strictly increasing function , such that
which immediately yields
Since , and , we deduce from conditions (i) and (iv) that
Utilizing the properties of and , we have
In the meantime, according to condition (iv), we have
Thus, from (6.8) and (6.10) it follows that
That is,
Since it follows from (6.1) that
we obtain from (6.10) and that
which together with (6.11) leads to
By (6.10) and Lemma 2.6, we have
In terms of (6.13) and (6.14), we have
Define a mapping , where G is defined by (1.7) and is a constant. Then by Lemma 2.8 we have that . We observe that
From (6.13) and (6.15), we obtain
Utilizing the arguments similar to those of (3.17) in the proof of Theorem 3.1, we can obtain
where with being the fixed point of the contraction ; that is, solves the fixed point equation . Noticing that J is norm-to-weak∗ uniformly continuous on bounded subsets of X, we deduce from (6.11) and (6.17) that
Finally, let us show that as . We observe that
which implies that
Also, by (6.1) and the convexity of , we get
It follows from (6.19) that
Applying Lemma 2.1 to (6.20), we conclude that as . This completes the proof. □
Corollary 6.1LetCbe a nonempty closed convex subset of a uniformly convex Banach spaceXwhich has a uniformly Gâteaux differentiable norm. Letbe a sunny nonexpansive retraction fromXontoC. Let the mappingbe-strictly pseudocontractive and-strongly accretive withfor. Letbe a contraction with coefficient. LetSbe a nonexpansive mapping ofCinto itself such that, whereΩis the fixed point set of the mapping. For arbitrarily given, letbe the sequence generated by
wherefor. Suppose that, , , andare the sequences insatisfying the following conditions:
-
(i)
;
-
(ii)
and;
-
(iii)
, , and;
-
(iv)
;
-
(v)
and.
Thenconverges strongly to, which solves the following VIP:
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Acknowledgements
This article was partially funded by the Deanship of Scientific Research (DSR), King Abdulaziz University. Therefore, the second, third and forth authors gratefully acknowledge with thanks DSR for financial support. The research of the first author was partially supported by the National Science Foundation of China (11071169), Innovation Program of Shanghai Municipal Education Commission (09ZZ133) and Ph.D. Program Foundation of Ministry of Education of China (20123127110002). The authors thank the referees for their valuable comments and suggestions.
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Ceng, LC., Hussain, N., Latif, A. et al. Strong convergence for solving a general system of variational inequalities and fixed point problems in Banach spaces. J Inequal Appl 2013, 334 (2013). https://doi.org/10.1186/1029-242X-2013-334
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DOI: https://doi.org/10.1186/1029-242X-2013-334