Extragradient Methods for Solving Split Feasibility Problem and General Equilibrium Problem and Resolvent Operators in Banach Spaces
Abstract
In this paper, we introduce a new extragradient algorithm by using generalized metric projection. We prove a strong convergence theorem for finding a common element of the solution set of split feasibility problem and the set of fixed points of relatively nonexpansive mapping and a finite family of resolvent operator and the set of solutions of an equilibrium problem.
1. Introduction
- (i)
Monotone if
- (ii)
α-inverse strongly monotone if there exists a constant α > 0 such that
- (iii)
Demiclosed if for all {xn} ⊂ X with xn⇀x in X, and yn ∈ Axn with yn⟶y in X∗, we have x ∈ X and y ∈ Ax
A monotone mapping B is said to be maximal if its graph G(B) = {(x, Bx): x ∈ D(B)} is not properly contained in the graph of any other monotone mapping. Obviously, the monotone mapping B is maximal if and only if for (x, x∗) ∈ X × X∗, 〈x − y, x∗ − y∗〉 ≥ 0 for all (y, y∗) ∈ G(B), then it is implied that x∗ ∈ Bx.
The solution set of (4) is denoted by EP(f). The equilibrium problem is very general because it includes many well-known problems such as variational inequality problems and saddle point problems (see [1–4]). Several methods have been proposed to solve the equilibrium problem in Hilbert space (see [5]), and some authors obtained weak and strong convergence algorithms for finding a common element of the set of solutions of an equilibrium problem and the set of fixed points of a nonexpansive mapping in a Hilbert space (see [6–9]). Then, the authors proved the strong convergence of the algorithms in a uniformly convex and uniformly smooth Banach space (see [10]).
In this paper, motivated by Schöpfer et al. [12], we present a new hybrid algorithm using the inverse strongly monotone operation and a finite family of resolvent operator. Then, we show that our generated sequence is strongly converges to a common point, the set of solution set of split feasibility problem, and the fixed point of relatively nonexpansive mapping and the fixed point of resolvent operator.
2. Preliminaries
An operator A : C⟶X∗ is hemicontinuous at x0 ∈ C, if for any sequence {xn} converging to x0 along a line implies that the sequence {Axn} is weakly convergent to Ax0, i.e., Axn = A(x0 + tnx)⇀Ax0 as tn⟶0 for all x ∈ C.
The existence and uniqueness of the operator ΠC follows from the properties of the functional ϕ(x, y) and strict monotonicity of the mapping J [21]. Suppose that C is a nonempty closed convex subset of X and T is a self mapping on C. We denote the set of fixed points of T by F(T), that is F(T) = {x ∈ C : x ∈ Tx}. A point p ∈ C is called an asymptotically fixed point of T if C contains a sequence {xn} which converges weakly to p such that Txn − xn⟶0 [17]. The set of asymptotical fixed points of T will be denoted by . A mapping T from C into itself is said to be relatively nonexpansive if and ϕ(p, Tx) ≤ ϕ(p, x) for all x ∈ C and p ∈ F(T). The asymptotic behavior of a relatively nonexpansive mapping was studied in [22, 23].
We need the following lemmas for proving our main results.
Lemma 1. (see [24].)Let X be a smooth and uniformly convex Banach space and let {xn} and {yn} be two sequences of X. If ϕ(xn, yn)⟶0 and either {xn} or {yn} is bounded, then xn − yn⟶0.
Lemma 2. (see [21].)Let C be a nonempty closed convex subset of a smooth, strictly convex, and reflexive Banach space X and let y ∈ X. Then,
Lemma 3. (see [21].)Let C be a nonempty closed convex subset of a smooth, strictly convex, and reflexive Banach space X, let x ∈ X, and let z ∈ C. Then,
Lemma 4. (see [25].)Let X be a 2-uniformly convex and smooth Banach space. Then, for all x, y ∈ X, we have that
Lemma 5. (see [25].)Let X be a uniformly convex Banach space and r > 0. Then, there exists a continuous strictly increasing convex function g : [0, 2r]⟶[0, ∞) such that g(0) = 0 and
Lemma 6. (see [24].)Let X be a uniformly convex Banach space and r > 0. Then, there exists a continuous strictly increasing convex function g : [0, 2r]⟶[0, ∞) such that g(0) = 0 and
Lemma 7. (see [25].)Let x, y ∈ X. If X is p-uniformly smooth, then there is a c > 0 so that
Throughout this paper, we assume that f : C × C⟶ℝ is a bifunction satisfying 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) limt↓0f(tz + (1 − t)x, y) ≤ f(x, y), for all x, y, z ∈ C
(A4) For each x ∈ C, y ↦ f(x, y) is convex and lower semicontinuous.
Lemma 8. (see [26].)Let C be a nonempty closed convex subset of a smooth, strictly convex and reflexive Banach space X. Let A : C⟶X∗ be an α-inverse-strongly monotone operator and f be a bifunction from C × C to ℝ satisfying (A1) − (A4). Then, for all r > 0 the following hold
- (i)
For x ∈ X, there exists u ∈ C such that
- (ii)
If X is additionally uniformly smooth and Kr : X⟶C is defined as
Kr is single-valued
Kr is firmly nonexpansive, i.e., for all x, y ∈ X,
EP is a closed convex subset of C.
Definition 9.
Let X be a real smooth and uniformly convex Banach space and let be a maximal monotone operator. For all ι > 0, define the operator by for all x ∈ X.
Lemma 10. (see [18].)Let X be a real smooth and uniformly convex Banach space, and let be a maximal monotone operator. Then, M−10 is a closed and convex subset of X, and the graph G(M) of M is demiclosed.
Lemma 11. Let X be a real reflexive, strictly convex, and let smooth Banach space and be a maximal monotone operator with M−10 ≠ ∅. Then, for all x ∈ X, y ∈ M−10 and ι > 0, then .
3. Main Results
In this section, we introduce our new extragradient algorithm.
Theorem 12. Let X1 and X2 are real 2-uniformly convex and uniformly smooth Banach spaces. Suppose that C and D are nonempty closed and convex subsets of X1 and X2, respectively. Suppose that g is a bifunction from C × C to ℝ which satisfies the conditions A1-A4, A : X1⟶X2 is a bounded linear operator and is the adjoint of A. Let be a maximal monotone operator with for all i = 1, 2, ⋯, k. Assume that B : C⟶X∗ is an α-inverse strongly monotone operator, and f is a relatively nonexpansive mappings from C into itself and . Let {xn} is a sequence generated by v1 ∈ C and
- (i)
, , liminfn⟶∞αn,1αn,2 > 0, and liminfn⟶∞αn,3 > 0
- (ii)
τ is real number such that 0 < τ < 2/c‖A‖2, where c depends on 2-uniformly smoothness of
Then, {xn} converges strongly to .
Therefore, is bounded, and exists. Now, by (11), we conclude that {xn} is bounded. It follows from (29), (34), (35), (37), (40), and relatively nonexpansiveness of f that the sequences {un}, {zn}, {yn}, {wn}, {vn}, and {f(xn)} are bounded.
This shows that {xn} is a Cauchy sequence, so {xn} converges strongly to a point q ∈ C. Therefore, by (68), (70), and (72), we imply that {un}, {yn}, and {zn} converge strongly to q.
So, by the above observation, hn⟶0 as n⟶∞. On the other hand, since xn⇀q, we can conclude from (47) that ηn⇀q. Then, from Lemma 10, 0 ∈ M1q, i.e., . Similar to the above, by using (46), we can also prove that for all i = 2, 3, ⋯k. Hence, .
Hence, q is an asymptotic fixed point of f. Then, because f is a relatively nonexpansive mapping. Hence, q ∈ F(f).
Finally, we prove that q ∈ Ω. From (56), we have that ‖PDAq − Aq‖ = limn⟶∞‖PDAun − Aun‖ = 0. Therefore, Aq ∈ D, i.e., q ∈ Ω. Hence, , and this completed the proof.
Conflicts of Interest
This work does not have any conflicts of interest.
Open Research
Data Availability
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