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Convergence of iterative sequences for fixed points of an infinite family of nonexpansive mappings based on a hybrid steepest descent methods
Journal of Inequalities and Applications volume 2012, Article number: 101 (2012)
Abstract
The propose of this article is to consider the strong convergence of an iterative sequences for finding a common element of the set of fixed points of an infinite family of nonexpansive mappings, the set of solutions of the variational inequalities for inverse strongly monotone mappings, and the set of solutions of system of equilibrium problems in Hilbert spaces by using a hybrid steepest descent methods. Our results improve and generalize many known corresponding results.
AMS (2000) Subject Classification: 46C05; 47H09; 47H10.
1. Introduction
Let H be a real Hilbert space whose inner product and norm are denoted by 〈·,·〉 and ||·||, respectively. Let C be a nonempty closed convex subset of H and let F: C × C → ℝ be a bifunction, where ℝ is the set of real numbers. The equilibrium problem for F: C × C → ℝ is to find x* ∈ C such that
The set of solutions of (1.1) is denoted by EP(F).
Let {F i , i = 1, 2,..., N} be a finite family of bifunctions from C × C into ℝ, where ℝ is the set of real numbers. The system of equilibrium problems for {F1, F2,..., F N } is to find a common element x* ∈ C such that
We denote the set of solutions of (1.2) by , where SEP(F i ) is the set of solutions to the equilibrium problems, that is,
If N = 1, then the problem (1.2) is reduced to the equilibrium problems.
If N = 1 and F(x*,y) = 〈Bx*, y - x*〉, then the problem (1.2) is reduced to the variational inequality problems of finding x* ∈ C such that
The set of solutions of (1.4) is denoted by VI(C, B).
Many problems in applied sciences, such as monotone inclusion problems, saddle point problems, optimization problems, variational inequality problems, Nash equilibrium problems, and equilibrium problems as special cases. Some methods have been proposed to solve VI(C, B), EP(F), and SEP(F i ); see, for example [1–22] and references therein. The above formulations (1.2) extends this formulism to such problems, covering in particular various forms of feasibility problems [23, 24].
Let P C be the metric projection of H onto the closed convex subset C. Let S: C → C be a nonexpansive mapping, that is, ||Sx - Sy|| ≤ ||x - y|| for all x, y ∈ C. The set of fixed points of S is denoted by F(S) = {x ∈ C: Sx = x}. If C ⊂ H is nonempty, bounded, closed and convex and S is a nonexpansive mapping of C into itself, then F(S) is nonempty; see, for example, [25, 26]. A mapping f: C → C is a contraction on C if there exists a constant η ∈ (0,1) such that ||f(x) - f(y)|| ≤ η||x - y|| for all x, y ∈ C.
Definition 1.1. Let B: C → H be nonlinear mappings. Then B is called
-
(1)
monotone if 〈Bx - By, x - y〉 ≥ 0, ∀x, y ∈ C,
-
(2)
ξ-inverse-strongly monotone if there exists a constant ξ > 0 such that
-
(3)
A set-valued mapping Q: H → 2His called monotone if for all x, y ∈ H, f ∈ Qx and g ∈ Qy imply 〈x - y, f - g〉 ≥ 0. A monotone mapping Q: H → 2His called maximal monotone, if it is monotone and if for any (x, f) ∈ H × H
(the graph of mapping Q) implies that f ∈ Qx.
A typical problem is to minimize a quadratic function over the set of fixed points of a nonexpansive mapping defined on a real Hilbert space H:
where F is the fixed point set of a nonexpansive mapping S defined on H and b is a given point in H.
A linear bounded operator A is strong positive if there exists a constant with the property
Marino and Xu [27] introduced a new iterative scheme by the viscosity approximation method:
They proved that the sequences {x n } generated by (1.5) converges strongly to the unique solution of the variational inequality:
which is the optimality condition for the minimization problem:
where h is a potential function for γf.
In order to find a common element of the set of fixed points of a nonexpansive mapping and the set of solutions of variational inequalities for a ξ-inverse-strongly monotone mapping, Takahashi and Toyoda [28] introduced the following iterative scheme:
where B is a ξ-inverse-strongly monotone mapping, {γ n } is a sequence in (0, 1), and {α n } is a sequence in (0,2ξ). They showed that if F(S) ⋂ VI(C,B) is nonempty, then the sequence {x n } generated by (1.7) converges weakly to some z ∈ F(S) ⋂ VI(C, B).
In order to find a common element of F(S) ⋂ VI(C, B), let S: H → H be a nonexpansive mapping, Yamada [29] introduced the following iterative scheme called the hybrid steepest descent method:
where x1 = x ∈ H, {α n } ⊂ (0,1), let B: H → H be a strongly monotone and Lipschitz continuous mapping and μ is a positive real number. He proved that the sequence {x n } generated by (1.8) converges strongly to the unique solution of the F(S) ⋂ VI(C, B).
Let C be a nonempty closed convex subset of H. Given r > 0 the operators defined by
is called the resolvent of F (see [3]). It is shown in [3] that under suitable hypotheses on F (to be stated precisely in Section 2), is single-valued and firmly nonexpansive and satisfied
Using the result, in 2009, Colao et al. [10] introduced and considered an implicit iterative scheme for finding a common element of the set of solutions of the system equilibrium problems and the set of common fixed points of an infinite family of nonexpansive mappings on C. Starting with an arbitrary initial x0 ∈ C and defining a sequence {z n } recursively by
where {ϵ n } be a sequences in (0,1). It is proved [10] that under certain appropriate conditions imposed on {ϵ n } and {r n }, the sequence {x n } generated by (1.9) converges strongly to , where z is the unique solution of the variational inequality and which is the optimality condition for the minimization problem.
In 2010, Colao and Marino [30] introduced the following explicit viscosity scheme with respect to W-mappings for an infinite family of nonexpansive mappings
They prove that sequence {x n } and converge strongly to , where z is an equilibrium point for F and is the unique solution of the variational inequality:
or, equivalently, the unique solution of the minimization problem
where h is a potential function for γf. Recently, Chantarangsi et al. [11] introduced some iterative processes based on the viscosity hybrid steepest descent method for finding a common solutions of a generalized mixed equilibrium problem, the set of fixed points of a nonexpansive mapping and the set of solutions of variational inequality problem in a real Hilbert space.
In this article, motivated by above results, we introduce an iterative scheme for finding a common element of the set of solutions of system of equilibrium problems, the set of fixed points of an infinite family of nonexpansive mapping, and the set of solutions of variational inequality problems for inverse strongly monotone mapping in a real Hilbert space by using a new hybrid steepest descent methods. The results shown in this article improve and extend the recent ones announced by many others.
2. Preliminaries
Let H be a real Hilbert space, when {x n } is a sequence in H, we denote strong convergence of {x n } to x ∈ H by x n → x and weak convergence by x n ⇀ x. Let C be nonempty closed convex subset of H. The nearest point projection P C : H → C defined from H onto C is the function which assigns to each x ∈ H its nearest point denoted by P C x in C. Thus, P C x is the unique point in C such that ||x - P C x|| ≤ ||x - y||, ∀y ∈ C. It easy to see that P C is nonexpansive and
Lemma 2.1. [26] Let H be a Hilbert space, let C be a nonempty closed convex subset of H. Let ξ > 0 and let A: C → H be ξ-inverse strongly monotone. If 0 < ϱ ≤ 2ξ, then I - ϱB is a nonexpansive mapping of C into H.
Lemma 2.2. [26] Let H be a real Hilbert spaces, there hold the following identities:
-
(i)
for each x ∈ H and x* ∈ C, x* = P C x ⇔ 〈x - x*, y - x*〉 ≤ 0 for all y ∈ C;
-
(ii)
P C : H → C is nonexpansive, that is, ||P C x - P C y|| ≤ ||x - y|| for all x, y ∈ H;
-
(iii)
P C is firmly nonexpansive, that is, ||P C x - P C y||2 ≤ 〈P C x - P C y, x - y〉 for all x,y ∈ H;
-
(iv)
||tx + (1 - t)y||2 = t||x||2 + (1 - t)||y||2 - t(1 - t)||x - y||2, ∀t ∈ [0,1], for all x,y ∈ H;
-
(v)
||x + y||2 ≤ ||x||2 + 2〈y,x + y〉.
Lemma 2.3. [31] Each Hilbert space H satisfies Opial's condition, that is, for any sequence {x n } ⊂ H with x n ⇀ x, the inequality
hold for each y ∈ H with y ≠ x.
Lemma 2.4. [27] Let C be a nonempty closed convex subset of H and let f be a contraction of H into itself with η ∈ (0,1), and A be a strongly positive linear bounded operator on H with coefficient . Then, for ,
That is, A - γf is a strongly monotone with coefficient .
Lemma 2.5. [27] Assume A be a strongly positive linear bounded operator on H with coefficient and 0 < ρ ≤ ||A||-1. Then .
Throughout this article, we assume that a bifunction F : C × C → ℝ satisfies the following conditions:
(A1) F(x, x) = 0 for all x ∈ C;
(A2) F is monotone, i.e., F(x, y) + F(y, x) ≤ 0 for all x, y ∈ C;
(A3) for each x, y, z ∈ C, limt↓0F(tz + (1 - t)x, y) ≤ F(x, y);
(A4) for each x ∈ C, y ↦ F(x, y) is convex and lower semicontinuous.
Then, we have the following lemmas.
Lemma 2.6. [1] Let C be a nonempty closed convex subset of H and let F be a bifunction of C × C into ℝ satisfying (A1)-(A4). Let r > 0 and x ∈ H. Then, there exists z ∈ C such that
Lemma 2.7. [3] Assume that F : C × C → ℝ satisfies (A1)-(A4). For r > 0 and x ∈ H, define a mapping as follows:
for all z ∈ H. Then, the following hold:
-
(1)
is single-valued;
-
(2)
is firmly nonexpansive, that is, for any x,y ∈ H,
-
(3)
;
-
(4)
EP(F) is closed and convex.
Lemma 2.8. [32] Let {x n } and {l n } be bounded sequences in a Banach space X and let {β n } be a sequence in [0,1] with 0 < lim infn→∞β n ≤ lim supn→∞β n < 1. Suppose xn+1= (1 - β n )l n + β n x n for all integers n ≥ 0 and lim supn→∞(||ln+1- l n || - ||xn+1- x n ||) ≤ 0. Then, limn→∞||l n - x n || = 0.
Lemma 2.9. [33] Assume {a n } is a sequence of nonnegative real numbers such that
where {b n } is a sequence in (0,1) and {c n } is a sequence in ℝ such that
-
(1)
,
-
(2)
or ,
Then, limn→∞a n = 0.
3. Main results
Let C be a nonempty closed convex subset of a real Hilbert space H. Let be a family of infinitely of nonexpansive mappings of C into itself and let be a sequence of nonnegative numbers in [0,1]. For any n ≥ 1, define a mapping W n : C → C as follows:
such a mappings W n is nonexpansive from C to C and it is called the W-mapping generated by T1,T2,...,T n and μ1, μ2, ..., μ n (see [34]).
Lemma 3.1. [34, 35] Let C be a nonempty closed convex subset of a real Hilbert space H. Let T1,T2,..., be an infinite family of nonexpansive mappings of C into itself such that , let μ1, μ2, ... be real numbers such that 0 ≤ μ n ≤ b < 1 for every n ≥ 1. Then,
-
(1)
for every x ∈ C and k ∈ ℕ, the limit limn→∞U n,k x exists;
-
(2)
the mapping W of C into itself as follows:
(3.2)
is a nonexpansive mapping satisfying , which it is called the W-mapping generated by T1, T2, ... and μ1, μ2, ...;
-
(3)
, for each n ≥ 1;
-
(4)
If E is any bounded subset of C, then .
Theorem 3.2. Let C be a nonempty closed convex subset of a real Hilbert space H, let F k , k ∈ {1, 2, 3,..., M} be a bifunction from C × C to ℝ satisfying (A1)-(A4), let {T n } be an infinite family of nonexpansive mappings of C into itself and let B be ξ-inverse strongly monotone such that
Let f be a contraction of H into itself with η ∈ (0,1) and let A be a strongly positive linear bounded operator on H with coefficient and . Let {x n }, {y n } and {u n } be sequences generated by
where {W n } is the sequence generated by (3.1) and {ϵ n }, {β n } are two sequences in (0,1) and {r k,n }, k ∈ {1,2,3,..., M} are a real sequence in (0, ∞) satisfy the following conditions:
(C1) limn→∞ϵ n = 0 and ,
(C2) 0 < lim infn→∞β n ≤ lim supn→∞β n < 1,
(C3) {α n }, {λ n } ⊂ [e, g] ⊂ (0, 2ξ), limn→∞α n = 0 and limn→∞λ n = 0,
(C4) {δ n } ⊂ [0, b], for some b ∈ (0,1) and limn→∞|δn+1- δ n | = 0,
(C5) lim infn→∞r k,n > 0 and limn→∞|rk,n+1- r k,n | = 0 for each k ∈ {1, 2, 3,..., M}.
Then, {x n } and {u n } converge strongly to a point z ∈ Θ, which is the unique solution of the variational inequality
Equivalently, we have z = P Θ (I - A + γf)(z).
Proof. From the restrictions on control sequence, without loss of generality, that ϵ n ≤ (1 - β n )||A||-1 for all n ≥ 1. From Lemma 2.5, we know that if 0 ≤ ρ ≤ ||A||-1, then . We will assume that . Since A is a strongly positive bounded linear operator on H, we have
Observe that
this show that (1 - β n )I - ϵ n A is positive. It follows that
We divide the proof of Theorem 3.2 into seven steps.
Step 1. We show that the mapping PΘ(γf + (I - A)) has a unique fixed point.
Since f be a contraction of C into itself with coefficient η ∈ (0,1). Then, we have
Since , it follows that PΘ (γf + (I - A)) is a contraction of C into itself. Therefore, by the Banach Contraction Mapping Principle, has a unique fixed point, say z ∈ C, that is,
Step 2. We show that W n - λ n BW n is nonexpansive.
For all x, y ∈ C, let W n is the sequence defined by (3.1) and λ n ∈ (0, 2ξ), we obtain W n - λ n BW n is a nonexpansive. Indeed,
which implies that W n - λ n BW n is a nonexpansive.
Step 3. We show that the sequence {x n } is bounded.
In fact, let , then
Setting v n = P C (x n - α n Bx n ) and I - α n B is a nonexpansive mapping (Lemma 2.1), we obtain
and
Let for k ∈ {1, 2, 3,..., M} and for all n. Because is nonexpansive for each k = 1, 2, 3,..., M, and (3.7), we note that . It follows that
Let e n = P C (W n u n - λ n BW n u n ), we can prove that
which yields that
By induction, we have
This implies that {x n } is bounded, and hence so are {u n }, {e n }, {y n }, {BW n u n }, {Bx n }, {Ae n }, {v n - x n }, and {f(u n )}.
Step 4. We show that .
We claim that if ω n be a bounded sequence in C, then
for every k ∈ {1, 2, 3,..., M}. From Step 2 of the proof of Theorem 3.1 in [10], we have that for k ∈ {1,2,3,...,M},
Note that for every k ∈ {1,2,3,...,M}, we obtain
Thus,
Now, apply (3.12) to conclude (3.11).
Since T n and U n,n are nonexpansive, we have
where M1 ≥ 0 is an appropriate constant such that ||Un+1,n+1x n - Un,n+1x n || ≤ M1 for all n ≥ 0. From I - α n B is nonexpansive, we have
From (3.3) and (3.15), we have
Now, we compute ||un+1- u n || and ||en+1- e n ||. Consider the following computation:
and
Setting
we have xn+1= (1 - β n )l n + β n x n , n ≥ 1. It follows that
It follows from (3.18) and (3.19) that
This together with conditions (C1)-(C4) and (3.11) imply that
By Lemma 2.8, we obtain
Consequently,
Applying (3.11), (3.21) and conditions (C3), (C4) to (3.15) and (3.17), we obtain that
Step 5. We show that .
For any and (3.5), we obtain
By Lemma 2.2(iv) and (3.23), we have
So, from (3.8) and (3.24), we derive
From (3.3), we have
It follows that
where
By conditions (C1), (C2) and (3.21), we obtain
Since P C is firmly nonexpansive mapping, we have
Hence, we have
and so
Using (3.26) and (3.28), we also have
It follow that
From conditions (C1), C(4), (3.21) and (3.27), we obtain
Observe also that if e n = P C (W n u n - λ n BW n u n ), then
Substituting (3.30) in (3.26), we have
It follows that
Since ||xn+1- x n || → 0 (n → ∞) and conditions (C1) and (C2), we obtain
Since P C is firmly nonexpansive (Lemma 2.2 (iii)), we have
Hence, we have
Using (3.26) and (3.32), we also have
It follow that
From conditions (C1), (3.21), and (3.31), we obtain
For any , note that is firmly nonexpansive (Lemma 2.7(2)) for k ∈ {1, 2, 3,..., M}, then we have
So, we obtain
which implies that for each k ∈ {1, 2, 3,..., M - 1},
Consequently, from (3.26) we derive that
Thus, we have
By as n → ∞ and (3.21), so we deduce that
that is,
Therefore, we have
From (3.34), we have
Since xn+1= ϵ n γf(u n ) + β n x n + ((1 - β n )I - ϵ n A)e n , we have
that is,
By conditions (C1), (C2), and (3.21) it follows that
On the other hand, from (3.3), we have
Since , we get
We observe that
Consequently, we obtain
Let W be the mapping defined by (3.2). Since {e n } is bounded, applying Lemma 3.1(4) and (3.38), we have
Step 6. We show that q ∈ Θ, where .
Since {x n } is bounded, we see that there exits a subsequence of {x n } which converges weakly to q. It follows from (3.37) and (3.36) that and . From (3.35), we obtain that for k = 1, 2,...,M.
First, we show that . Since for k = 1, 2, 3,..., M, we also have
If follows from (A2) that,
Replacing n by n i , we have
Since and , it follows by (A4) that
for each k = 1,2,3, ...,M.
For t with 0 < t ≤ 1 and y ∈ H, let y t = ty + (1 - t)q. Since y ∈ C and q ∈ C, we have y t ∈ C and hence F k (y t , q) ≤ 0. So, from (A1) and (A4) we have
and hence F k (y t ,y) ≥ 0. From (A3), we have F k (q,y) ≥ 0 for all y ∈ C and hence q ∈ SEP(F k ) for k = 1, 2, 3,..., M, that is, .
Next, we show that . By Lemma 3.1(2), we have . Assume q ∉ F(W). Since and q ≠ Wq, it follows by the Opial's condition (Lemma 2.3) that
which derives a contradiction. Thus, we have .
Finally, now we prove that q ∈ VI(C, B).
We define the maximal monotone operator
For any given (q1, q2) ∈ G(Q), hence q2 - Bq1 ∈ N C q1. Since e n ∈ C we see from the definition of N C that
On the other hand, from e n = P C (W n u n - α n BW n u n ), we have
that is
Therefore, we obtain
Since as i → ∞ and B is Lipschitz continuous we obtain that
Notice that Q is maximal monotone, we obtain that q ∈ Q-10 and hence q ∈ VI(C,B). This implies q ∈ Θ. Since z = PΘ(γf + (I - A))(z), we have
On the other hand, we have
From (3.36) and (3.41), we obtain that
Step 7. Finally, we show that {x n } converges strongly to z = PΘ(I - A + γf)(z). Indeed, from (3.3), we have
where K is an appropriate constant such that
Set and c n = ϵ n {2〈e n - z,γf(z) - Az〉 + ϵ n K}. Then we have
From the conditions (C1) and (3.42), we see that
Therefore, applying Lemma 2.8 to (3.43), we get that {x n } converges strongly to z ∈ Θ. This completes the proof.
Corollary 3.3. Let C be a nonempty closed convex subset of a real Hilbert space H, let F k , k ∈ {1, 2, 3,..., M} be a bifunction from C × C to ℝ satisfying (A1)-(A4) and let B be ξ-inverse strongly monotone such that
Let f be a contraction of H into itself with η ∈ (0,1). Let {x n }, {y n } and {u n } be sequences generated by
where {ϵ n }, {β n } are two sequences in (0,1) and {r k,n }, k ∈ {1,2,3,...,M} are a real sequence in (0, ∞) satisfy the following conditions:
(C1) limn→∞ϵ n = 0 and ,
(C2) 0 < lim infn→∞β n ≤ lim supn→∞β n < 1,
(C3) {α n }, {λ n } ⊂ [e, g] ⊂ (0, 2ξ), limn→∞α n = 0 and limn→∞λ n = 0,
(C4) {δ n } ⊂ [0, b], for some b ∈ (0,1) and limn→∞|δn+1- δ n | = 0,
(C5) lim infn→∞r k,n > 0 and limn→∞|rk,n+1- r k,n | = 0 for each k ∈ {1, 2, 3,..., M},
Then, {x n } and {u n } converge strongly to a point z ∈ Θ which is the unique solution of the variational inequality
Equivalently, we have z = PΘf(z).
Proof. Put T n ≡ I for all n ≥ 1 and for all x ∈ C. Then W n = I, A = I and γ = 1. The conclusion follows from Theorem 3.2. This completes the proof.
If δ n = 0 and M = 1, in Theorem 3.2, then we can obtain the following result immediately.
Corollary 3.4. Let C be a nonempty closed convex subset of a real Hilbert space H, let F k , k ∈ {1, 2, 3,..., M} be a bifunction from C × C to ℝ satisfying (A1)-(A4), let {T n } be an infinite family of nonexpansive mappings of C into itself and let B be ξ-inverse strongly monotone such that
Let f be a contraction of H into itself with η ∈ (0,1) and let A be a strongly positive linear bounded operator on H with coefficient and . Let {x n }, {y n } and {u n } be sequences generated by
where {W n } is the sequence generated by (3.1) and {ϵ n }, {β n } are two sequences in (0,1) and {r n } are a real sequence in (0,∞) satisfy the following conditions:
(C1) limn→∞ϵ n = 0 and ,
(C2) 0 < lim infn→∞β n ≤ lim supn→∞β n < 1,
(C3) {λ n } ⊂ [e, g] ⊂ (0, 2ξ) and limn→∞λ n = 0,
(C4) lim infn→∞r n > 0 and limn→∞|rn+1- r n | = 0.
Then, {x n } and {u n } converge strongly to a point z ∈ Θ which is the unique solution of the variational inequality
Equivalently, we have z = P Θ (I - A + γf)(z).
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Acknowledgements
The authors were supported by the Centre of Excellence in Mathematics, the Commission on Higher Education, Thailand. CJ was partially supported by Rajamangala University of Technology Rattanakosin Research and Development Institute, the Thailand Research Fund and the Commission on Higher Education under Grant No. MRG5480206. The authors thank the referees for their careful reading of the article and useful comments.
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Onjai-uea, N., Jaiboon, C., Kumam, P. et al. Convergence of iterative sequences for fixed points of an infinite family of nonexpansive mappings based on a hybrid steepest descent methods. J Inequal Appl 2012, 101 (2012). https://doi.org/10.1186/1029-242X-2012-101
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DOI: https://doi.org/10.1186/1029-242X-2012-101