Extragradient Method for Solutions of Variational Inequality Problems in Banach Spaces
Abstract
We introduce an iterative process which converges strongly to solutions of a certain variational inequity problem for η-inverse strongly accretive mappings in the set of common fixed points of finite family of strictly pseudocontractive mappings in Banach spaces. Our theorems improve and unify most of the results that have been proved for this important class of nonlinear operators.
1. Introduction
Variational inequality theory has emerged as an important tool in studying a wide class of related problems arising in mathematical, physical, regional, engineering, and nonlinear optimization sciences (see, for instance, [3–12]).
We note that the above algorithms give strong convergence to a solution of the variational inequality (6). However, both algorithms fail, in general, to converge strongly in the setting of infinite-dimensional Hilbert spaces.
Question. Could we produce an iterative scheme which approximates a solution of variational inequality (5) for η-inverse strongly accretive mappings in Banach spaces?
In this paper, motivated by Yao et al. [8] and Yamada [7], it is our purpose to introduce an iterative scheme which converges strongly to a solution of the variational inequality (5) for η-inverse strongly accretive mapping in the set of common fixed points of finite family of strictly pseudocontractive mappings in a uniformly convex and q-uniformly smooth Banach space E possessing weakly sequentially continuous duality mapping. Our results complement or improve the results of Yao et al. [8], Aoyama et al. [13], and some authors.
2. Preliminaries
Let E be a real Banach space. The modulus of smoothness of E is the function ρE : [0, ∞) → [0, ∞) defined by ρE(τ) : = sup {(1/2)(∥x + y∥ + ∥x − y∥) − 1 : ∥x∥ = 1, ∥y∥ = τ}. If ρE(τ) > 0 for all τ > 0, then E is said to be smooth. If there exists a constant c > 0 and a real number 1 < q < ∞, such that ρE(τ) ≤ cτq, then E is said to be q-uniformly smooth.
Let C⊆E be closed convex and Q a mapping of E onto C. Then, Q is said to be sunny if Q(Q(x) + t(x − Q(x))) = Q(x) for all x ∈ E and t ≥ 0. A mapping Q of E into C is said to be a retraction if Q2 = Q. If a mapping Q is a retraction, then Q(z) = (z) for every z ∈ R(Q), range of Q. A subset C of E is said to be a sunny nonexpansive retract of E if there exists a sunny nonexpansive retraction of E onto C, and it is said to be a nonexpansive retract of E if there exists a nonexpansive retraction of E onto C. If E = H, the metric projection PC is a sunny nonexpansive retraction from H to any closed convex subset of H. Moreover, if C is a nonempty closed convex subset of a uniformly convex and uniformly smooth real Banach space E and T is a nonexpansive mapping of C into itself with F(T) ≠ ∅, then the set F(T) is a sunny nonexpansive retract of C.
In what follows, we will make use of the following lemmas.
Lemma 1 (see, e.g., [17]). Let E be a smooth Banach space, and let K be a nonempty subset of E. Let Q : E → K be a retraction, and let J be the normalized duality map on E. Then, the following are equivalent:
- (i)
Q is sunny nonexpansive,
- (ii)
〈x − Q(x), J(y − Q(x))〉 ≤ 0 for all x ∈ E and y ∈ K.
Lemma 2 (see [18].)Let {an} be a sequence of nonnegative real numbers satisfying the following relation:
Lemma 3 (see [13].)Let C be a nonempty closed convex subset of a smooth Banach space E. Let QC be a sunny nonexpansive retraction from E onto C, and let A be an accretive operator of C into E. Then, for all λ > 0,
Lemma 4 (see [17].)Let C be a nonempty bounded closed convex subset of a uniformly convex Banach space E, and let T be nonexpansive mapping of C into itself. If {xn} is a sequence of C such that xn → x weakly and xn − Txn → 0 strongly, then x is a fixed point of T.
Lemma 5 (see [19].)Let E be a real Banach space. Then, for any given x, y ∈ E, the following inequality holds:
Lemma 6 (see [20].)Let K be a nonempty closed convex subset of a strictly convex Banach space E. Let Ti : K → E, i = 1,2, …, r, be a family of nonexpansive mappings such that . Let α0, α1, α2, …, αr be real numbers in (0,1) such that , and let T : = α0I + α1T1 + ⋯+αrTr. Then, T is nonexpansive, and .
Lemma 7 (see [21].)Let C be a nonempty, closed and convex subset of a real uniformly convex and smooth Banach space E. Let Ti : C → E, i = 1, …, N, be λi-strictly pseudocontractive mappings such that . Let T : = θ1T1 + θ2T2 + ⋯+θNTN with θ1 + θ2 + ⋯+θN = 1. Then T is λ-strictly pseudocontractive with λ : = min {λi : i = 1, …, N} and .
Lemma 8 (see [22].)Let C be a nonempty closed and convex subset of a real q-uniformly smooth Banach space E for 1 < q < ∞. Let T : C → E be a λ-strictly pseudocontractive mapping. Then, for , where L is the Lipchitz constant of T and cq is a constant in (16), the mapping Tμx : = (1 − μ)x + μTx is nonexpansive, and F(Tμ) = F(T).
Lemma 9. Let C be a nonempty closed and convex subset of a a real q-uniformly smooth Banach space E for 1 < q < ∞. Let A : C → E be an η-inverse strongly accretive mapping. Then, for 0 < γ < (qη/cq) 1/(q−1), the mapping Aμx : = (x − γAx) is nonexpansive.
Proof. Now, using inequality (16), we get that
Lemma 10 (see [10].)Let E be a uniformly convex Banach space, and let BR(0) be a closed ball of E. Then, there exists a continuous strictly increasing convex function g : [0, ∞)→[0, ∞) with g(0) = 0 such that
Lemma 11 (see [5].)Let {an} be sequences of real numbers such that there exists a subsequence {ni} of {n} such that , for all i ∈ ℕ. Then, there exists a nondecreasing sequence {mk} ⊂ ℕ such that mk → ∞, and the following properties are satisfied by all (sufficiently large) numbers k ∈ ℕ:
3. Main Results
We now prove our main theorem.
Theorem 12. Let C be a nonempty closed convex subset of a real uniformly convex and q-uniformly smooth Banach space E possessing weakly sequentially continuous duality mapping. Let Ti : C → C, for i = 1, …, N, be λi-strictly pseudocontractive mappings, and let A : C → E be η-inverse strongly accretive mapping. Let {xn} be a sequence defined by (25). Assume that ℱ : = F∩VI(C, A) ≠ ∅, where . Then, {xn} converges strongly to Qℱ(0), where Qℱ is a sunny nonexpansive retraction of E onto ℱ, which is a solution of the variational inequality (24).
Proof. By Lemmas 7 and 8 we have that S is nonexpansive. In addition, by Lemma 9 we get that QC[I − γA] is nonexpansive. Let p ∈ ℱ and, let yn : = βnxn + (1 − βn)QC[I − γA]zn. Then from (25), Lemmas 8 and 9 we have that
Thus, from (25) and (27), we get that
Therefore, by induction,
Now, we prove that {xn} converges strongly to the point x* = Qℱ(0). Let tn = QC[I − γA]zn, and let dn = (1 − αn)yn. Then, since αn → 0, we have that
Now, following the method of proof of Lemma 3.2 of Maingé [5], we consider two cases.
Case 1. Suppose that there exists n0 ∈ ℕ such that {∥xn − x*∥} is decreasing for all n ≥ n0. Then, we get that {∥xn − x*∥} is convergent. Thus, from (36) and the fact that αn → 0, as n → ∞, we have that
In addition, since {dn} is bounded subset of a reflexive space E, we can choose a subsequence of {dn} such that and . This implies from (34) and (33) that. Then, from (39) and Lemma 4, we have that . Moreover, from (39) and Lemma 4, we have that z ∈ F(QC[I − γA]), and by Lemma 3, we get z ∈ VI(C, A), and hence z ∈ ℱ. Therefore, using the fact that E has a weakly sequentially continuous duality mapping and Lemma 1, we immediately obtain that
Case 2. Suppose that there exists a subsequence {ni} of {n} such that
If in Theorem 12, we consider that N = 1, we get the following corollary.
Corollary 13. Let C be a nonempty closed convex subset of a real uniformly convex and q-uniformly smooth Banach space E possessing weakly sequentially continuous duality mapping. Let T : C → C be a λ-strictly pseudocontractive mapping, and let A : C → E be an η-inverse strongly accretive mapping. Let {xn} be a sequence defined by (25), where S : = [(1 − μ)I + μT]. Assume that F : = F(T)∩VI(C, A) ≠ ∅. Then, {xn} converges strongly to QF(0) which is a solution of the variational inequality
If in Theorem 12, we assume that Ti, for i = 1,2, …, N, are nonexpansive, we get the following corollary.
Corollary 14. Let C be a nonempty closed convex subset of a real uniformly convex and q-uniformly smooth Banach space E possessing weakly sequentially continuous duality mapping. Let Ti : C → C, for i = 1,2, …, N, be nonexpansive mappings, and let A : C → E be an η-inverse strongly accretive mapping. For x0 ∈ C, let the sequence {xn} be generated iteratively by
If in Corollary 14, we consider that N = 1, we get the following corollary.
Corollary 15. Let C be a nonempty closed convex subset of a real uniformly convex and q-uniformly smooth Banach space E possessing weakly sequentially continuous duality mapping. Let T : C → C be a nonexpansive mapping, and let A : C → E be an η-inverse strongly accretive mapping. For x0 ∈ C, let the sequence {xn} be generated iteratively by
If in Corollary 14, we assume that T = T1 = T2 = ⋯ = TN = I, we obtain the following corollary.
Corollary 16. Let C be a nonempty closed convex subset of a real uniformly convex and q-uniformly smooth Banach space E possessing weakly sequentially continuous duality mapping. Let A : C → E be an η-inverse strongly accretive mapping. For x0 ∈ C, let the sequence {xn} be generated iteratively by
If in Theorem 12, we assume that A is an α-strongly accretive and L-Lipschitzian continuous mapping, we obtain the following corollary.
Corollary 17. Let C be a nonempty closed convex subset of a real uniformly convex and q-uniformly smooth Banach space E possessing weakly sequentially continuous duality mapping. Let Ti : C → C, for i = 1, …, N, be λi-strictly pseudocontractive mappings, and Let A : C → E be an α-strongly accretive and L-Lipschitzian continuous mapping. Let {xn} be a sequence defined by (25) for η = α/L2. Assume that ℱ : = F∩VI(C, A) ≠ ∅, where . Then, {xn} converges strongly to Qℱ(0), which is a solution of the variational inequality problem
Proof. We note that if A is an α-strongly accretive and L-Lipschitzian continuous mapping of C into E, then we have that
If E = H, a real Hilbert space, then E is a uniformly convex and q-uniformly smooth Banach space E for 1 < q < ∞ possessing weakly sequentially continuous duality mapping. In this case, we have that QC = PC, projection mapping from H onto C. Thus, we have the following corollary.
Corollary 18. Let C be a nonempty closed convex subset of a real Hilbert space H. Let Ti : C → C, for i = 1, …, N, be λi-strictly pseudocontractive mappings, and let A : C → E be an η-inverse strongly accretive mapping. For x0 ∈ C, let the sequence {xn} be generated iteratively by
Remark 19. Theorem 12 complements Theorem 3.2 of Yao et al. [8] in more general Banach spaces for η-inverse strongly accretive mappings. Moreover, Theorem 12 improves Theorem 3.1 of Aoyama et al. [13] and Theorem 3.7 of Saejung et al. [23] in the sense that our convergence is strong in the set of common fixed points of finite family of strictly pseudocontractive mappings.