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Iterative methods for variational inequality problems and fixed point problems of a countable family of strict pseudo-contractions in a q-uniformly smooth Banach space
Fixed Point Theory and Applications volume 2012, Article number: 65 (2012)
Abstract
In this article, we introduce iterative methods (implicit and explicit) for finding a common fixed point set of a countable family of strict pseudo-contractions, which is a unique solution of some variational inequality. Furthermore, we prove the strong convergence theorems of such iterative scheme in a q-uniformly smooth Banach space which admits a weakly sequentially continuous generalized duality mapping. The results presented in this article extend and generalize the corresponding results announced by Yamada and Ceng et al. from Hilbert spaces to Banach spaces.
Mathematics Subject Classification 2010: 47H09; 47J05; 47J20; 65J15.
1 Introduction
Let X be a real Banach space, and X* be its dual space. Let U = {x ∈ X: ∥x∥ = 1}. A Banach space X is said to be strictly convex if for all x, y ∈ X with ∥x∥ = ∥y∥ = 1 and x ≠ y. A Banach space X is called uniformly convex if for each ε > 0 there is a δ > 0 such that for x, y ∈ X with ∥x∥, ∥y∥ ≤ 1 and ∥x - y∥ ≥ ε, ∥x + y∥ ≤ 2(1 - δ) holds. The modulus of convexity of X defined by
for all ε ∈ [0,2]. X is uniformly convex if δ x (0) = 0 and δ x (ε) > 0 for all 0 < ε ≤ 2. It is know that every uniformly convex Banach space is strictly convex and reflexive (see [1]). The norm of X is said to be Gâteaux differentiable if the limit
exists for each x, y ∈ U. In this case X is smooth. Let ρ X : [0, ∞) → [0, ∞) be the modulus of smoothness of X defined by
A Banach space X is said to be uniformly smooth if as t → 0. Suppose that q > 1, then X is said to be q-uniformly smooth if there exists c > 0 such that ρ X (t) ≤ ctq. It is easy to see that if X is q-uniformly smooth, then q ≤ 2 and X is uniformly smooth. For q > 1, the generalized duality mapping is defined by
where 〈·,·〉 denotes the duality pairing between X and X*. In particular, J q = J2 is called the normalized duality mapping and J q (x) = ∥x∥q-2J2(x) for x ≠ 0. If X := H is a real Hilbert space, then J = I where I is the identity mapping. Further, we have the following properties of the generalized duality mapping J q :
-
(1)
J q (x) = ∥x∥q-2J2(x) for all x ∈ X with x ≠ 0.
-
(2)
J(tx) = tq-1J q (x) for all x ∈ X and t ∈ [0, ∞).
-
(3)
J q (-x) = -J q (x) for all x ∈ X.
It is well known that if X is smooth, then J q is single-valued, which is denoted by j q (see [1]). The duality mapping J q from a smooth Banach space X into X* is said to be weakly sequentially continuous generalized duality mapping if for all {x n } ⊂ X with x n ⇀ x implies J q (x n ) ⇀* J q (x).
Let C be a nonempty, closed and convex subset of X and T be a self-mapping of C. We denote the fixed points set of the mapping T by Fix(T) = {x ∈ C: Tx = x} and denote → and ⇀ by strong and weak convergence, respectively.
Definition 1.1. A mapping T: C → C is said to be:
-
(i)
λ-strictly pseudocontractive [2], if for all x, y ∈ C there exists λ > 0 and j q (x - y) ∈ J q (x - y) such that
or equivalently
-
(ii)
L-Lipschitzian if for all x, y ∈ C, there exists a constant L > 0 such that
If 0 < L < 1, then T is a contraction and if L = 1, then T is a nonexpansive mapping. By the definition, we know that every λ-strictly pseudocontractive mapping is -Lipschitzian (see [3]).
Remark 1.2. Let C be a nonempty subset of a real Hilbert space H and T: C → C be a mapping. Then T is said to be k-strictly pseudocontractive [2], if for all x, y ∈ C, there exists k ∈ [0,1) such that
It is well known that (1.1) is equivalent to the following:
Let C be a nonempty, closed and convex subset of X and Ψ: C → X be a nonlinear mapping. The variational inequality problem is to find u ∈ C such that
where j q (v - u) ∈ J q (v - u). The set of solution of variational inequality problem is denoted by VI(C, Ψ). If X := H is a real Hilbert space, the variational inequality problem reduces to find u ∈ C such that
Applications of variational inequalities span as diverse disciplines as differential equations, time-optimal control, optimization, mathematical programming, mechanics, finance and so on (see, e.g., [4, 5] for more details). Note that most of the variational problems, including minimization or maximization of functions, variational inequality problems, quasivariational inequality problems, decision and management sciences, and engineering sciences problems can be unified into form (1.2) and (1.3). For more details, we recommend the reader [6–11]. On the author hand, we note that iterative approximation of fixed points of nonexpansive mappings (and of common fixed points of nonexpansive semigroups) have recently been applied to image recovery and signal processing (see, e.g., [12–17]).
A mapping F: C → X is said to be accretive if for all x, y ∈ C there exists j q (x - y) ∈ J q (x - y) such that
For some η > 0, F: C → X is said to be strongly accretive if for all x, y ∈ C there exists j q (x -y) ∈ J q (x - y) such that
Remark 1.3. If X := H is a real Hilbert space, accretive and strongly accretive mappings coincide with monotone and strongly monotone mappings, respectively.
Let A be a strongly positive bounded linear operator on H, that is, there exists a constant such that
Remark 1.4. From the definition of operator A, we note that a strongly positive bounded linear operator A is a ∥A∥-Lipschitzian and η-strongly monotone operator.
A typical problem is to minimize a quadratic function over the set of the fixed points of a nonexpansive mapping on a real Hilbert space H:
where C is the fixed point set of a nonexpansive mapping T on H and u is a given point in H.
In 2006, Marino and Xu [18] introduced and considered the following general iterative method:
where A is a strongly positive bounded linear operator on a real Hilbert space H. They, proved that, if the sequence {α n } of parameters satisfies appropriate conditions, then the sequence {x n } generated by (1.6) converges strongly to the unique solution of the variational inequality
which is the optimality condition for the minimization problem
where C is the fixed point set of a nonexpansive mapping T and h is a potential function for γf (i.e., h'(x) = γf(x) for all x ∈ H).
On the other hand, Yamada [19] introduced a hybrid steepest descent method for a non-expansive mapping T as follows:
where F is a κ-Lipschitzian and η-strongly monotone operator with constants κ, η > 0 and . He proved that if {λ n } satisfying appropriate conditions, then the sequence {x n } generated by (1.9) converges strongly to the unique solution of variational inequality
In 2010, Tian [20] combined the iterative method (1.6) with the Yamada's method (1.9) and considered a general iterative method for a nonexpansive mapping T as follows:
Then he proved that the sequence {x n } generated by (1.11) converges strongly to the unique solution of variational inequality
Very recently, Ceng et al. [21] introduced implicit and explicit iterative schemes for finding the fixed points of a nonexpansive mapping T on a nonempty, closed and convex subset C in a real Hilbert space H as follows:
and
where V is an L-Lipschitzian mapping with a constant L ≥ 0 and F is a κ-Lipschitzian and η-strongly monotone operator with constants κ, η > 0 and . Then they proved that the sequences generated by (1.13) and (1.14) converge strongly to the unique solution of variational inequality
The following questions naturally arise in connection with above results:
Question 1.5. Can Theorem of Ceng et al. [21] be extend from a real Hilbert space to a general Banach space? such as q-uniformly smooth Banach space.
Question 1.6. Can we extend the iterative method of scheme (1.14) to a general iterative scheme define over the set of fixed points of a countable family of strict pseudo-contractions.
The purpose of this article is to give the affirmative answers to these questions mentioned above, motivated by Yamada [19], Tian [20] and Ceng et al. [21], we introduce a general iterative method for finding a common fixed point set of a countable family of strict pseudo-contractions, which is a unique solution of some variational inequality. Furthermore, we prove the strong convergence theorems of such iterative scheme in a q-uniformly smooth Banach space which admits a weakly sequentially continuous generalized duality mapping. The results presented in this article extend and generalize the corresponding results announced by Yamada [19] and Ceng et al. [21] and many others to Banach spaces.
2 Preliminaries
Let D be a nonempty subset of C. A mapping Q: C → D is said to be sunny if
whenever Qx + t(x - Qx) ∈ C for x ∈ C and t ≥ 0. A mapping Q: C → D is said to be retraction if Qx = x for all x ∈ D. Furthermore, Q is a sunny nonexpansive retraction from C onto D if Q is a retraction from C onto D which is also sunny and nonexpansive. A retraction Q is said to be orthogonal if for each x, x - Qx is normal to D in the sense of James (see [22]). A subset D of C is called a sunny nonexpansive retraction of C if there exists a sunny nonexpansive retraction from C onto D. It is well known that if X := H is a real Hilbert space, then a sunny nonexpansive retraction Q c is coincident with the metric projection from X onto C.
The following lemma concerns the sunny nonexpansive retraction.
Lemma 2.1. [23] Let C be a closed and convex subset of a real q-uniformly smooth Banach space X. Let Q: X → C be a retraction. Then, Q is an orthogonal retraction if and only if
Lemma 2.2. [24] Let X be a real q-uniformly smooth Banach space. Then the following inequality holds:
for all x, y ∈ X and for some C q > 0.
Lemma 2.3. [25] Suppose that q > 1. Then the following inequality holds:
for arbitrary positive real numbers a, b.
Lemma 2.4. [26] Let {s n } be a sequence of nonnegative real numbers, {a n } be a sequence of [0,1] with , {c n } be a sequence of nonnegative real number with and {b n } be a sequence of real numbers with lim supn→∞b n ≤ 0. Suppose that
for all n ∈ ℕ. Then, limn→∞s n = 0.
Definition 2.5. Let {T n } be a family of mappings from a subset C of a Banach space X into itself with . We say that {T n } satisfies the AKTT-condition (see [26]) if for each bounded subset B of C,
Lemma 2.6. [26] Suppose that {T n } satisfy the AKTT-condition such that
(i) For each x ∈ C, {T n } is converge strongly to some point in C.
(ii) Let the mapping T: C → C defined by Tx = limn→∞, T n x for all x ∈ C.
Then limn→∞supω∈B∥T ω - T n ω∥ = 0 for each bounded subset B of C.
Lemma 2.7. [27, 28] Let C be a closed and convex subset of a smooth Banach space X. Suppose that is a family of λ-strictly pseudocontractive mappings with and is a real sequence in (0,1) such that . Then the following conclusions hold:
(i) A mapping G: C → X defined by is a λ-strictly pseudocontractive mapping.
(ii) .
Lemma 2.8. [28] Let C be a closed and convex subset of a smooth Banach space X. Suppose that is a countable family of λ-strictly pseudocontractive mappings with . For all n ∈ ℕ, define T n : C → C by for all x ∈ C, where is a family of nonnegative numbers satisfying the following conditions:
(i) for all n ∈ ℕ;
(ii) for all k ∈ ℕ;
(iii) .
Then the following hold:
(1) Each T n is a λ-strictly pseudocontractive mapping.
(2) {T n } satisfies the AKTT-condition.
(3) If T: C → C is defined by for all x ∈ C, then Tx = limn→∞T n x and .
3 Main results
In order to prove our main result, the following lemmas are needed.
Lemma 3.1. Let C be a nonempty, closed and convex subset of a real q-uniformly smooth Banach space X. Let F: C → X be a κ-Lipschitzian and η-strongly accretive operator with constants κ, η > 0. Let and . Then for , the mapping S: C → X define by S := (I - tμF) is a contraction with constant 1-tτ.
Proof. Since with q > 1 and . This implies that 1 - tτ ∈ (0,1). From Lemma 2.2, for all x, y ∈ C, we have
It follows that
Hence, we have S := (I - tμF) is a contraction with a constant 1 - tτ.
Lemma 3.2. Let C be a nonempty, closed and convex subset of a real q-uniformly smooth Banach space X which admits weakly sequentially continuous generalized duality mapping j q from X into X*. Let T: C → C be a nonexpansive mapping. Then, for all {x n } ⊂ C, if x n ⇀ x and , then x = Tx.
Proof. From Lemma 2.2, for all x ∈ C, we have
Taking the limit as n → ∞ in both sides and noting that j q is weakly sequentially continuous generalized duality mapping. Then, ∥x - Tx∥q≤ 0, this implies that x = Tx.
3.1 Implicit iteration scheme
Let C be a nonempty, closed and convex subset of a real q-uniformly smooth Banach space X. Let Q C be a sunny nonexpansive retraction from X onto C. Let F: C → X be a κ-Lipschitzian and η-strongly accretive operator with constants κ, η > 0, V: C → X be an L-Lipschitzian mapping with a constant L ≥ 0 and T: C → C be a nonexpansive mapping such that Fix(T) ≠ ∅. Let and 0 ≤ γL < τ, where . For each , we define the mapping S t : C → C by
It is easy to see that S t is a contraction. Indeed, from Lemma 3.1, for all x, y ∈ C, we have
Hence, S t has a unique fixed point, denoted by x t , which uniquely solve the fixed point equation
The following proposition summarizes the properties of the net {x t }.
Proposition 3.3. Let C be a nonempty, closed and convex subset of a real q-uniformly smooth Banach space X. Let Q c be a sunny nonexpansive retraction from X onto C. Let F: C → X be a κ-Lipschitzian and η-strongly accretive operator with constants κ, η > 0, V: C → X be an L-Lipschitzian mapping with a constant L ≥ 0 and T: C → C be a nonexpansive mapping such that Fix(T) ≠ ∅. Let and 0 ≤ γL < τ, where .
then
(i) {x t } is bounded for each .
(ii) .
(iii) {x t } defines a continuous curve from into C.
Proof.
-
(i)
Taking . Then, we have
It follows that
Hence, {x t } is bounded, so are {Vx t } and {FTx t }.
-
(ii)
By definition of {x t }, we have
-
(iii)
Take t, . From Lemma 3.1, we have
It follows that
Since {Vx t } and {FTx t } is bounded. Hence, {x t } defines a continuous curve from into C.
Theorem 3.4. Let C be a nonempty, closed and convex subset of a real q-uniformly smooth Banach space X which admits a weakly sequentially continuous generalized duality mapping j q from X into X*. Let Q c be a sunny nonexpansive retraction such that Q c is an orthogonal from X onto C. Let F: C → X be a κ-Lipschitzian and η-strongly accretive operator with constants κ, η > 0, V: C → X be an L-Lipschitzian mapping with constant L ≥ 0 and T: C → C be a nonexpansive mapping such that Fix(T) ≠ ∅. Let and 0 ≤ γL < τ, where . For each , let {x t } defined by (3.1), then {x t } converges strongly to x* ∈ Fix(T) as t → 0, which x* is the unique solution of the variational inequality
Proof. We observe that
It follows that
First, we show the uniqueness of solution of the variational inequality (3.3). Suppose that are solutions of (3.3), then
and
Adding up (3.5) and (3.6), we have
Note that (3.4) implies that and the uniqueness is proves. Below, we use x* to denote the unique solution of the variational inequality (3.3).
Next, we show that x t → x* as t → 0. Setting , where . Then, we can rewrite (3.1) as x t = QCyt. Assume that {t n } ⊂ (0,1) is a sequence such that t n → 0 as n → ∞. Putting and . For z ∈ Fix(T), we note that
By Lemma 2.1, we have
It follows from (3.7) and (3.8) that
Thus, we have
which implies that
In particular, we have
By reflexivity of a Banach space X and boundedness of {x n }, there exists a subsequence of {x n } such that as i → ∞. Since Banach space X has a weakly sequentially continuous generalized duality mapping and by (3.9), we obtain . By Proposition 3.3 (ii), we have as n → ∞. Hence, it follows from Lemma 3.2 that .
Next, we show that solves the variational inequality (3.3). We note that
we derive that
Since I - T is accretive (i.e., 〈(I - T)x - (I - T)y, j q (x - y)〉 ≥ 0, for x, y ∈ C). For all z ∈ Fix(T), it follows from (3.10) and Lemma 2.1 that
where M > 0 is a constant such that M = sup{μκ∥x t - z∥q-1}, where . Now, replacing t in (3.11) with t n and taking the limit as n → ∞, we noticing that for , we obtain . That is is the solution of variational inequality (3.3). Consequently, by uniqueness. Therefore x t → x* as t → 0. This completes the proof.
3.2 Explicit iteration scheme
Theorem 3.5. Let C be a nonempty, closed and convex subset of a real q-uniformly smooth Banach space X which admits a weakly sequentially continuous generalized duality mapping j q from X into X*. Let Q c be a sunny nonexpansive retraction such that Q c is an orthogonal from X onto C. Let F: C → X be a κ-Lipschitzian and η-strongly accretive operator with constants κ, η > 0, V: C → X be an L-Lipschitzian mapping with constant L ≥ 0. Let and 0 ≤ γL < τ, where . Let be a family of λ-strict pseudo-contractions with 0 < λ < 1. Define a mapping S n x := (1 - γ n )x + γ n T n x for all x ∈ C and n ≥ 1. Assume that . Let {x n } be a sequence defined by x1 ∈ C and
where {α n } and {γ n } are sequences in (0,1) which satisfy the following conditions:
(C1) limn→∞α n = 0 and ;
(C2) either or ;
(C3) and .
Suppose in addition that satisfies the AKTT-condition. Let T: C → C be the mapping defined by Tx = limn→∞T n x for all x ∈ C and suppose that . Then the sequence {x n } defined by (3.12) converges strongly to x* ∈ Fix(T) as n → ∞, which x* is the unique solution of the variational inequality
Proof. From the condition (C 1), we may assume, without loss of generality, that for all n ∈ ℕ. First, we show that {x n } is bounded. From Lemma 2.2 and the condition (C 3), for all x, y ∈ C, we have
It follows that ∥S n x - S n y∥ ≤ ∥x - y∥, which implies that S n is nonexpansive and Fix(T n ) = Fix(S n ). Taking . Then we have
By induction, we have
Hence, {x n } is bounded, so are {Vx n } and {FS n x n }.
Next, we show that ∥xn+1- x n ∥ → 0 as n → ∞. Since
On the other hand, we have
Substituting (3.14) into (3.15), we obtain
where M1 = supn≥1{γ∥Vx n ∥, μ∥FS n x n ∥} and M2 = supn≥1{∥Tn+1x n - x n ∥}. It follows from the conditions (C 2), (C 3) and Lemma 2.4 that
Next, we show that ∥x n - Sx n ∥ → 0 as n → ∞. For any bounded subset B of C, we observe that
where M3 = supn≥1{∥ω∥, ∥T n ω∥}. From the condition (C 3) and {T n } satisfies the AKTT-condition, then we have
that is {S n } satisfies the AKTT-condition, we can define nonexpansive mapping S: C → C by Sx = limn→∞S n x for all x ∈ C. Since {γ n } is bounded, there exits a subsequence of {γ n } such that as i → ∞. It follows that
That is, Fix(S) = Fix(T). Hence, . We observe that
From the condition (C 1) and (3.16), we have
On the other hand, we observe that
which implies by Lemma 2.6 and (3.17) that
Next, we show that
where x* is the same as in Theorem 3.4. To show this, we take a subsequence of {x n } such that
By reflexivity of a Banach space X and boundedness of {x n }, there exists a subsequence of {x n } such that as i → ∞. It follows from (3.18) and Lemma 3.2 that z ∈ Ω. Since Banach space X has a weakly sequentially continuous generalized duality mapping, we obtain that
Finally, we show that x n → x* as n → ∞. Setting y n = α n γVx n +(I-α n μF)S n x n , ∀n ≥ 1. Then, we can rewrite (3.12) as xn+1= Qcy n . It follows from Lemmas 2.1 and 2.3 that
which implies that
Put a n = (τ - γL)α n and . Then (3.20) reduces to formula
It follows from the condition (C 1) and (3.19) that and lim supn→∞b n ≤ 0. From Lemma 2.4, we obtain that x n → x* as n → ∞. This completes the proof.
Remark 3.6. Note that Lemma 3.1 is quite similar to the result of Yamada [19] which is obtained in a real Hilbert space but we extended that result to a real q-uniformly smooth Banach space.
Remark 3.7. Theorems 3.4 and 3.8 extend and generalize the main result of Ceng et al. [21] in the following ways:
-
(i)
From a real Hilbert space to a real q-uniformly smooth Banach space which admits a weakly sequentially continuous generalized duality mapping.
-
(ii)
From a nonexpansive mapping to a countable family of a strict pseudo-contractions mapping.
From Lemmas 2.7, 2.8 and Theorem 3.8, we obtain the following result.
Theorem 3.8. Let C be a nonempty, closed and convex subset of a real q-uniformly smooth Banach space X which admits a weakly sequentially continuous generalized duality mapping j q from X into X*. Let Q c be a sunny nonexpansive retraction such that Q c is an orthogonal from X onto C. Let F: C → X be a κ-Lipschitzian and η-strongly accretive operator with constants κ, η > 0, V: C → X be an L-Lipschitzian mapping with a constant L ≥ 0. Let and 0 ≤ γL < τ, where . Let be a sequence of λ k -strict pseudo-contractions such that and λ := inf{λ k : k ∈ ℕ} > 0. Define a mapping for all x ∈ C and n ≥ 1. Let {x n } be a sequence defined by x1 ∈ C and
where {α n } and {γ n } are sequences in (0,1) which satisfy the conditions (C 1)-(C 3) of Theorem 3.8 and is a sequence which satisfies the conditions (i)-(iii) of Lemma 2.8. Let T: C → C be the mapping defined by for all x ∈ C. Then the sequence {x n } defined by (3.21) converges strongly to as n → ∞, which x* is the unique solution of the variational inequality
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Acknowledgements
This research was supported by the Centre of Excellence in Mathematics under the Commission on Higher Education, Thailand (under Grant No. RG-1-54-02-1). The first author was partially supported by the Centre of Excellence in Mathematics, the Commission on High Education, Thailand for the Ph.D. Program at King Mongkut's University of Technology Thonburi (KMUTT).
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Sunthrayuth, P., Kumam, P. Iterative methods for variational inequality problems and fixed point problems of a countable family of strict pseudo-contractions in a q-uniformly smooth Banach space. Fixed Point Theory Appl 2012, 65 (2012). https://doi.org/10.1186/1687-1812-2012-65
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DOI: https://doi.org/10.1186/1687-1812-2012-65