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Algorithms for treating equilibrium and fixed point problems
Fixed Point Theory and Applications volume 2013, Article number: 308 (2013)
Abstract
In this paper, a common solution problem is investigated based on a projection algorithm. Strong convergence theorems for common solutions of a system of equilibrium problems and a family of asymptotically quasi-ϕ-nonexpansive mappings are established in a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property.
1 Introduction
Bifunction equilibrium problems, which include many important problems in nonlinear analysis and optimization, such as the Nash equilibrium problem, variational inequalities, complementarity problems, vector optimization problems, fixed point problems, saddle point problems and game theory, recently have been studied as an effective and powerful tool for studying many real world problems which arise in economics, finance, image reconstruction, ecology, transportation, and network; see [1–23] and the references therein. The theory of fixed points as an important branch of functional analysis is a bridge between nonlinear functional analysis and optimization. Indeed, lots of problems arising in economics, engineering, and physics can be studied by fixed point techniques. The study of fixed point approximation algorithms for computing fixed points constitutes now a topic of intensive research efforts. Many well-known problems can be studied by using algorithms which are iterative in their nature. As an example, in computer tomography with limited data, each piece of information implies the existence of a convex set in which the required solution lies. The problem of finding a point in the intersection of these convex subsets is then of crucial interest, and it cannot be usually solved directly. Therefore, an iterative algorithm must be used to approximate such a point. The well known convex feasibility problem, which captures applications in various disciplines such as image restoration and radiation therapy treatment planning, is to find a point in the intersection of common fixed point sets of a family of nonlinear mappings. Krasnoselskii-Mann iteration, which is also known as a one-step iteration, is a classic algorithm to study fixed points of nonlinear operators. However, Krasnoselskii-Mann iteration only enjoys weak convergence for nonexpansive mappings; see [24] and the references therein. There are a lot of real world problems which exist in infinite dimension spaces. In such problems, strong convergence or norm convergence is often much more desirable than weak convergence. To guarantee the strong convergence of Krasnoselskii-Mann iteration, many authors use different regularization methods. The projection technique which was first introduced by Haugazeau [25] has been considered for the approximation of fixed points of nonexpansive mappings. The advantage of projection methods is that strong convergence of iterative sequences can be guaranteed without any compact assumptions.
In this paper, we study a common solution problem based on a projection algorithm. Strong convergence theorems of common solutions are established in a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property.
2 Preliminaries
Let E be a real Banach space, be the dual space of E and C be a nonempty subset of E. Let f be a bifunction from to ℝ, where ℝ denotes the set of real numbers. Recall the following equilibrium problem: find such that
We use to denote the solution set of equilibrium problem (2.1). That is,
Given a mapping , let
Then iff is a solution of the following variational inequality: find such that
In order to study the solution of problem (2.1), we assume that f satisfies the following conditions:
-
(A1)
, ;
-
(A2)
f is monotone, i.e., , ;
-
(A3)
-
(A4)
for each , is convex and weakly lower semi-continuous.
Recall that a Banach space E is said to be strictly convex iff for all with and . It is said to be uniformly convex iff for any two sequences and in E such that and . Let be the unit sphere of E. Then the Banach space E is said to be smooth iff
exists for each . It is also said to be uniformly smooth iff the above limit is attained uniformly for . It is well known that if E is uniformly smooth, then J is uniformly norm-to-norm continuous on each bounded subset of E. It is also well known that E is uniformly smooth if and only if is uniformly convex.
Recall that a Banach space E enjoys the Kadec-Klee property if for any sequence , and with , and , then as . It is well known that if E is a uniformly convex Banach space, then E enjoys the Kadec-Klee property. Let be a mapping. In this paper, we use to denote the fixed point set of T. T is said to be closed if for any sequence such that and , then . In this paper, we use → and ⇀ to denote strong convergence and weak convergence, respectively.
Recall that the normalized duality mapping J from E to is defined by
where denotes the generalized duality pairing. Next, we assume that E is a smooth Banach space. Consider the functional defined by
Observe that, in a Hilbert space H, the equality is reduced to , . As we all know, if C is a nonempty closed convex subset of a Hilbert space H and is the metric projection of H onto C, then is nonexpansive. This fact actually characterizes Hilbert spaces and, consequently, it is not available in more general Banach spaces. In this connection, Alber [26] recently introduced a generalized projection operator in a Banach space E which is an analogue of the metric projection in Hilbert spaces. Recall that the generalized projection is a map that assigns to an arbitrary point the minimum point of the functional , that is, , where is the solution to the minimization problem
Existence and uniqueness of the operator follows from the properties of the functional and strict monotonicity of the mapping J. In Hilbert spaces, . It is obvious from the definition of a function ϕ that
and
Remark 2.1 If E is a reflexive, strictly convex, and smooth Banach space, then if and only if .
Recall that a point p in C is said to be an asymptotic fixed point of a mapping T [27] iff C contains a sequence which converges weakly to p such that . The set of asymptotic fixed points of T will be denoted by .
Recall that a mapping T is said to be relatively nonexpansive iff
Recall that a mapping T is said to be relatively asymptotically nonexpansive iff
where is a sequence such that as .
Remark 2.2 The class of relatively asymptotically nonexpansive mappings was considered in [28] and [29]; see the references therein.
Recall that a mapping T is said to be quasi-ϕ-nonexpansive iff
Recall that a mapping T is said to be asymptotically quasi-ϕ-nonexpansive iff there exists a sequence with as such that
Remark 2.3 The class of asymptotically quasi-ϕ-nonexpansive mappings was considered in Zhou et al. [30] and Qin et al. [31]; see also [32].
Remark 2.4 The class of quasi-ϕ-nonexpansive mappings and the class of asymptotically quasi-ϕ-nonexpansive mappings are more general than the class of relatively nonexpansive mappings and the class of relatively asymptotically nonexpansive mappings. Quasi-ϕ-nonexpansive mappings and asymptotically quasi-ϕ-nonexpansive mappings do not require the restriction .
Remark 2.5 The class of quasi-ϕ-nonexpansive mappings and the class of asymptotically quasi-ϕ-nonexpansive mappings are generalizations of the class of quasi-nonexpansive mappings and the class of asymptotically quasi-nonexpansive mappings in Banach spaces.
In order to get our main results, we also need the following lemmas.
Lemma 2.6 [26]
Let E be a reflexive, strictly convex, and smooth Banach space, let C be a nonempty, closed, and convex subset of E, and let . Then
Lemma 2.7 [26]
Let C be a nonempty, closed, and convex subset of a smooth Banach space E, and let . Then if and only if
Lemma 2.8 [33]
Let E be a smooth and uniformly convex Banach space, and let . Then there exists a strictly increasing, continuous, and convex function such that and
for all and .
Lemma 2.9 Let C be a closed convex subset of a smooth, strictly convex, and reflexive Banach space E. Let f be a bifunction from to ℝ satisfying (A1)-(A4). Let and . Then
-
(a)
[1]There exists such that
-
(b)
Then the following conclusions hold:
-
(1)
is single-valued;
-
(2)
is a firmly nonexpansive-type mapping, i.e., for all ,
-
(3)
;
-
(4)
is quasi-ϕ-nonexpansive;
-
(5)
-
(6)
is closed and convex.
-
(1)
Lemma 2.10 [35]
Let E be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property. Let C be a nonempty closed and convex subset of E. Let be a closed asymptotically quasi-ϕ-nonexpansive mapping. Then is a closed convex subset of C.
3 Main results
Theorem 3.1 Let E be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property. Let C be a nonempty closed and convex subset of E. Let Λ be an index set. Let be a bifunction from to ℝ satisfying (A1)-(A4), and let be an asymptotically quasi-ϕ-nonexpansive mapping for every . Assume that is closed and uniformly asymptotically regular on C for every . Assume that is nonempty and bounded. Let be a sequence generated in the following manner:
where , is a real sequence in such that , and is a real sequence in , where is a positive real number sequence, for every . Then the sequence converges strongly to .
Proof We divide the proof into six steps.
Step 1. We prove that is closed and convex.
In the light of Lemma 2.9 and 2.10, we easily find the conclusion.
Step 2. We prove that is closed and convex.
To show that is closed and convex, it suffices to show that for each fixed but arbitrary , is closed and convex. This can be proved by induction on n. It is obvious that is closed and convex. Assume that is closed and convex for some . For , we see that . It follows that , where . Notice that
and
The above inequalities are equivalent to
and
Multiplying t and on the both sides of (3.1) and (3.2), respectively, yields that
That is,
where . This finds that is closed and convex. We conclude that is closed and convex. This in turn implies that is closed and convex. This implies that is well defined.
Step 3. We prove that .
is clear. Suppose that for some positive integer k. For any , we see that
which shows that . This implies that . This in turn implies that . This completes the proof that .
Step 4. We prove that the sequence is bounded.
In the light of the construction , we find from Lemma 2.7 that for any .
Since , we find that
On the other hand, we find from Lemma 2.6 that
This implies that the sequence is bounded. It follows from (2.3) that the sequence is also bounded. Since the space is reflexive, we may assume that .
Step 5. We prove that .
Since is closed and convex, we find that . This implies that . On the other hand, we see from the weak lower semicontinuity of the norm that
which implies that . Hence, we have . In view of the Kadec-Klee property of E, we find that as . Since , and , we find that . This shows that is nondecreasing. We find from its boundedness that exists. It follows that
This implies that
In the light of , we find that
This implies from (3.5) that
In view of (2.3), we see that . This implies that
That is,
This implies that is bounded. Note that both E and are reflexive. We may assume that . In view of the reflexivity of E, we see that . This shows that there exists an element such that . It follows that
Taking on the both sides of the equality above from (3.5) yields that
That is, , which in turn implies that . It follows that . Since enjoys the Kadec-Klee property, we obtain from (3.7) that . Since is demicontinuous and E enjoys the Kadec-Klee property, we obtain that , as . Note that . It follows that
On the other hand, we have
In view of (3.8), we find that
Since E is uniformly smooth, we know that is uniformly convex. In view of Lemma 2.8, we find that
This implies that
In view of the restrictions on the sequence , we find from (3.9) that
Notice that . It follows that
The demicontinuity of implies that . Note that
This implies from (3.10) that . Since E has the Kadec-Klee property, we obtain that . On the other hand, we have
It follows from the uniformly asymptotic regularity of that
That is, . From the closedness of , we find for each . This proves . Next, we show that . In view of Lemma 2.6, we find that
It follows from (3.9) that . This implies that . In view of (3.8), we see that as . This implies that , as . It follows that . This shows that is bounded. Since is reflexive, we may assume that . In view of , we see that there exists such that . It follows that
Taking on the both sides of the equality above yields that
That is, , which in turn implies that . It follows that . Since enjoys the Kadec-Klee property, we obtain that as . Note that is demicontinuous. It follows that . Since E enjoys the Kadec-Klee property, we obtain that as . Note that
This implies that
Since J is uniformly norm-to-norm continuous on any bounded sets, we have
From the assumption , we see that
In view of , we see that
It follows from (A2) that
In view of (A4), we find from (3.11) that
For and , define . It follows that , which yields that . It follows from the (A1) and (A4) that
That is,
Letting , we obtain from (A3) that , . This implies that for every . This shows that . This completes the proof that .
Step 6. We prove that .
Letting in (3.4), we see that
In view of Lemma 2.7, we find that . This completes the proof. □
Remark 3.2 Theorem 3.1 mainly improves Theorem 2.1 of Qin et al. [29] in the following aspects:
-
(1)
improves the mappings from a finite family of mappings to an infinite family of mappings;
-
(2)
extends the framework of the space from a uniformly smooth and uniformly convex space to a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property.
If T is a quasi-ϕ-nonexpansive mapping, we find from Theorem 2.1 the following.
Corollary 3.3 Let E be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property. Let C be a nonempty closed and convex subset of E. Let Λ be an index set. Let be a bifunction from to ℝ satisfying (A1)-(A4), and let be a closed quasi-ϕ-nonexpansive mapping for every . Assume that is nonempty. Let be a sequence generated in the following manner:
where is a real sequence in such that , and is a real sequence in , where is a positive real number sequence for every . Then the sequence converges strongly to .
Remark 3.4 Corollary 3.3 can be viewed as an extension of the corresponding results announced in [36] and [37].
Since every uniformly convex Banach space is a strictly convex Banach space which also enjoys the Kadec-Klee property, we see that Theorem 3.1 is still valid in a uniformly smooth and uniformly convex Banach space. Theorem 3.1 improves the corresponding results in Qin et al. [31].
For a single bifunction and mapping, we find from Theorem 3.1 the following.
Corollary 3.5 Let E be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property. Let C be a nonempty closed and convex subset of E. Let f be a bifunction from to ℝ satisfying (A1)-(A4), and let be an asymptotically quasi-ϕ-nonexpansive mapping. Assume that T is closed and uniformly asymptotically regular on C. Assume that is nonempty. Let be a sequence generated in the following manner:
where , is a real sequence in such that , and is a real sequence in , where a is a positive real number. Then the sequence converges strongly to .
If T is the identity mapping, then Theorem 3.1 is reduced to the following.
Corollary 3.6 Let E be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property. Let C be a nonempty closed and convex subset of E. Let Λ be an index set. Let be a bifunction from to ℝ satisfying (A1)-(A4) for every . Assume that is nonempty and bounded. Let be a sequence generated in the following manner:
where , is a real sequence in such that , and is a real sequence in , where is a positive real number sequence, for every . Then the sequence converges strongly to .
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Qin, X., Cho, S.Y. & Wang, L. Algorithms for treating equilibrium and fixed point problems. Fixed Point Theory Appl 2013, 308 (2013). https://doi.org/10.1186/1687-1812-2013-308
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DOI: https://doi.org/10.1186/1687-1812-2013-308