Volume 2022, Issue 1 4035695
Research Article
Open Access

Extragradient Methods for Solving Split Feasibility Problem and General Equilibrium Problem and Resolvent Operators in Banach Spaces

Mostafa Ghadampour

Corresponding Author

Mostafa Ghadampour

Department of Mathematics, Payame Noor University, P. O. Box 19395-4697, Tehran, Iran pnu.ac.ir

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First published: 18 July 2022
Academic Editor: Giovanni Di Fratta

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

Let C be a nonempty closed convex subset of a real Banach space X with norm ‖.‖ and X be the dual of X. We consider the following variational inequality problem (VI), which consists in finding a point xC such that
(1)
where is a mapping and 〈..〉 denotes the duality pairing. The solution set of the variational inequality problem is denoted by VI(C, A).
The operator is called
  • (i)

    Monotone if

(2)
  • (ii)

    α-inverse strongly monotone if there exists a constant α > 0 such that

(3)
  • (iii)

    Demiclosed if for all {xn} ⊂ X with xnx in X, and ynAxn with yny in X, we have xX and yAx

A monotone mapping B is said to be maximal if its graph G(B) = {(x, Bx): xD(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, 〈xy, xy〉 ≥ 0 for all (y, y) ∈ G(B), then it is implied that xBx.

Assume that is a nonlinear mapping and f : C × C is a bifunction. The equilibrium problem (EP) is as follows: find xC such that
(4)

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 [14]). 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 [69]). Then, the authors proved the strong convergence of the algorithms in a uniformly convex and uniformly smooth Banach space (see [10]).

Suppose that C and D are nonempty, closed, and convex subsets of real Banach spaces X1 and X2, respectively. The split feasibility problem (SFP) is to find a point
(5)
which A : X1X2 is a bounded linear operator. The solution set of (5) is denoted by Ω.
In 1994, the split feasibility problem was first studied by Censor and Elfving [11] in finite dimensional Hilbert spaces. In solving (SFP), Schöpfer et al. [12] proposed the next algorithm in p-uniformly convex real Banach spaces: x1X1 is chosen arbitrarily and for n ≥ 1,
(6)
where J is the duality mapping, ΠC denotes the Bregman projection, A is a bounded linear operator, and A is the adjoint of A. Also, they have proven the generated sequence {xn} by algorithm (6) is weakly convergent under suitable conditions. The split feasibility problems were studied extensively by many authors [13, 14].

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

Let X be a real smooth Banach space with norm ‖.‖ and let X be the dual space of X. We denote the strong convergence and the weak convergence {xn} to x in X by xnx and xnx, respectively. A function δ : [0, 2]⟶[0, 1] is said to be the modulus of convexity of X as follows:
(7)
for every ε ∈ [0, 2]. A Banach space X is said to be uniformly convex if and only if δ(ε) > 0 for all ε > 0. It is known that a uniformly convex Banach space has the Kadec-Klee property, that is, xnu and ‖xn‖⟶‖u‖ imply that xnu (see [15]). Let p be a fixed real number with p ≥ 2. A Banach space X is called p-uniformly convex [16], if there exists a constant c > 0 such that δcϵp for all ϵ ∈ [0, 2]. Let S(E) = {xX : ‖x‖ = 1}. A Banach space X is said to be smooth if for all xS(X), there exists a unique functional jxX such that 〈x, jx〉 = ‖x‖ and ‖jx‖ = 1 (see [17]).
The norm of X is said to be differentiable if for all x, yS(X), the limit
(8)
exists. In this case, X is said to be smooth, and X is called uniformly smooth if the limit (8) is attained uniformly for all x, yS(X) [18]. If a Banach space X is uniformly convex, then X is reflexive and strictly convex, and X is uniformly smooth [17]. The duality mapping on X is defined by
(9)
for every xX. If X is a p-uniformly convex and uniformly smooth, then is single valued, one-to-one and satisfies , where is the duality mapping of X (see [19]). If p = 2, then JX = J2 = J is the normalized duality mapping. It is well known that if X is a reflexive, strictly convex, and smooth Banach space and is the duality mapping on X, then . If X is a uniformly smooth and uniformly convex Banach space, then JX is uniformly norm to norm continuous on bounded sets of X, and is also uniformly norm to norm continuous on bounded sets of X. Let X be a smooth Banach space and let JX be the duality mapping on X. The function ϕ : X × X is defined by
(10)
Clearly, from (10), we can conclude that
(11)
If X is a reflexive, strictly convex, and smooth Banach space, then for all x, yX
(12)
Also, it is clear from the definition of the function ϕ that the following condition holds for all x, yX,
(13)
Now, the function V : X × X is defined as follows:
(14)
for all xX and xX. Moreover, for all xX and xX. If X is a reflexive strictly convex and smooth Banach space with X as its dual, then
(15)
for all xX and all x, yX [20].

An operator A : CX is hemicontinuous at x0C, 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 xC.

The generalized projection ΠC : XC is a mapping that assigns to an arbitrary point xX, the minimum point of the functional ϕ(y, x); that is, ΠCx = x0, where x0 is the solution of the minimization problem
(16)

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) = {xC : xTx}. A point pC is called an asymptotically fixed point of T if C contains a sequence {xn} which converges weakly to p such that Txnxn⟶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 xC and pF(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 xnyn⟶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 yX. Then,

(17)

Lemma 3. (see [21].)Let C be a nonempty closed convex subset of a smooth, strictly convex, and reflexive Banach space X, let xX, and let zC. Then,

(18)

Lemma 4. (see [25].)Let X be a 2-uniformly convex and smooth Banach space. Then, for all x, yX, we have that

(19)
where 1/c(0 ≤ c ≤ 1) is the 2-uniformly convex constant of X.

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

(20)
for all x, yBr(0) = {zX : ‖z‖ ≤ r} and t ∈ [0, 1].

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

(21)
for all x, yBr(0) = {zX : ‖z‖ ≤ r}.

Lemma 7. (see [25].)Let x, yX. If X is p-uniformly smooth, then there is a c > 0 so that

(22)

Throughout this paper, we assume that f : C × C is a bifunction satisfying the following conditions

(A1) f(x, x) = 0 for all xC

(A2) f is monotone, i.e., f(x, y) + f(y, x) ≤ 0, for all x, yC

(A3) limt↓0f(tz + (1 − t)x, y) ≤ f(x, y), for all x, y, zC

(A4) For each xC, yf(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 : CX be an α-inverse-strongly monotone operator and f be a bifunction from C × C to satisfying (A1) − (A4). Then, for all r > 0 the following hold

  • (i)

    For xX, there exists uC such that

(23)
  • (ii)

    If X is additionally uniformly smooth and Kr : XC is defined as

(24)
then, the following conditions hold:

Kr is single-valued

Kr is firmly nonexpansive, i.e., for all x, yX,

(25)

EP is a closed convex subset of C.

(26)

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 xX.

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 xX, yM−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 which satisfies the conditions A1-A4, A : X1X2 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 : CX 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 v1C and

(27)
where rn ∈ [a, ∞) for some a > 0, {sn} and {βn} are real sequences in [a, b] ⊂ (0, 1), and τ and satisfy the following conditions:
  • (i)

    , , liminfn⟶∞αn,1αn,2 > 0, and liminfn⟶∞αn,3 > 0

  • (ii)

    τ is real number such that 0 < τ < 2/cA2, where c depends on 2-uniformly smoothness of

Then, {xn} converges strongly to .

Proof. Let . By (10), Lemma 2 and the convexity of ‖.‖2, we have

(28)

Now, it follows from Lemma 11 and the above that
(29)
(30)
(31)
Let kn = AunPDAun. From (10) and Lemmas 2 and 7, we have that
(32)
Since for each yD and for each xX2, we have that
(33)
From (32), our assumptions, and the above, we conclude that
(34)
In a similar way as above, we obtain that
(35)
It follows from (10), (29), (34), (35), Lemma 11, and the convexity of ‖.‖2 that
(36)
(37)
(38)
(39)
By (10), (29), (37), Lemmas 2, 8, the condition (i), the convexity of ‖.‖2, and the relatively nonexpansiveness of f, we have that
(40)

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.

Next, by (28), (37), (40), and Lemma 11, we conclude that
(41)
(42)
(43)
Now, from (11) and (41), we have the following inequalities:
(44)
Now, since is convergent, it follows from (44), the conditions (i), and our assumptions that
(45)
Therefore, from Lemma 1, we have that
(46)
Then,
(47)
From (13), (47), the boundedness of the sequences {xn}, , and using uniformly norm-to-norm continuity of J on bounded sets, it is clear that
(48)
By (29), (34), (35), (36), (40), and Lemma 11, we conclude that
(49)
Hence, from (11), the above, and our assumptions, we obtain the following results
(50)
(51)
Since is convergent, we conclude from (i), (50), (51), and our assumptions that
(52)
(53)
Now, from (29), (34), (35), (37), and (40), we have
(54)
Hence, it follows from (54) that
(55)
Then, it follows from (i) and our assumptions that
(56)
From (29), (34), (35), (37), and (40), we have
(57)
So,
(58)
Therefore, it follows from (i) and our assumptions that
(59)
Suppose that r1 = supn{‖f(xn)‖, ‖un‖}. Therefore, from Lemma 5, there exists a continuous strictly increasing convex function g1 : [0, 2r1]⟶[0, ∞) such that g1(0) = 0 and using (29), (37), Lemmas 2 and 8, the convexity of ‖.‖2, and the condition relatively nonexpansiveness of f, we have that
(60)
So,
(61)
Since exists. Therefore, it follows from the condition (i) that
(62)
Because g1 is continues function, we conclude that
(63)
Therefore,
(64)
Since is uniformly norm-to-norm continuous on bounded sets, it imply that
(65)
Using (13), (65), the uniformly norm-to-norm continuity of on bounded sets, and the boundedness of the sequences {f(xn)} and {un}, we conclude that
(66)
By (48) and using our assumptions, we obtain that
(67)
Then, it follows from Lemma 1 that
(68)
Now, by (15), (56), and Lemmas 2 and 4, we conclude that
(69)
Then, using Lemma 1, we obtain
(70)
Also, from (15), (59), and Lemma 2, and the same way used for proving (70), we can conclude that
(71)
Then, using Lemma 1, we get
(72)
Now, it follows from (13), (52), and Lemma 1 that
(73)
Similarly, from (13), (53), and Lemma 1, we have
(74)
then, by (72), we obtain that
(75)
Now, by (13), (73), (75), and using uniformly norm-to-norm continuity of on bounded sets, it is implied that
(76)
It follows from (10), (76), Lemma 2, and the convexity of ‖.‖2 that
(77)
Now, by Lemma 1, we have limn⟶∞znwn‖ = 0. Therefore, we obtain from (70) that limn⟶∞unwn‖ = 0, then by (13), we conclude that
(78)
From (66), (78), Lemma 2, and our assumptions, it implied that
(79)
Therefore, by Lemma 1, we have
(80)
Let r2 = supn{‖vn‖, ‖xn‖}. Therefore, by Lemma 6, there exists a continuous, convex, and strictly increasing function g2 : [0, 2r2]⟶[0, ∞) such that g2(0) = 0 and
(81)
It follows from (40), (81), Lemma 8, and the fact that , we conclude that
(82)
Therefore,
(83)
because g2 is a continuous strictly increasing convex function. Now, by (80) and (83), we have
(84)
From (68) and (84), we obtain that
(85)

This shows that {xn} is a Cauchy sequence, so {xn} converges strongly to a point qC. Therefore, by (68), (70), and (72), we imply that {un}, {yn}, and {zn} converge strongly to q.

Next, we prove that . It follows from (46) and uniformly continuity of on bounded subset of X1 that as n⟶∞. Get ; hence, by Definition 9, we have . Therefore, there exists hnM1ηn such that
(86)

So, by the above observation, hn⟶0 as n⟶∞. On the other hand, since xnq, we can conclude from (47) that ηnq. 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, .

Next, we show that qF(f). From (65), (68), and the triangle inequality, we conclude that
(87)

Hence, q is an asymptotic fixed point of f. Then, because f is a relatively nonexpansive mapping. Hence, qF(f).

Now, we prove that qEP(g). Since is uniformly norm-to-norm continuous on bounded sets, it follows from (83) that
(88)
By , we conclude that for all yC. Moreover, by the condition A2, g(y, xn) ≤ −g(xn, y) for all yC. Therefore,
(89)
for all yC. Using (88), the condition A4, and letting n⟶∞, we have that
(90)
for all yC. Let yλ = λy + (1 − λ)q for all yC and λ ∈ (0, 1). It follows from (90), the conditions A1, A4, and the monotonicity of B that
(91)
for all yC. Therefore, 0 ≤ g(yλ, y) + 〈Byλ, yyλ〉. Using the condition A3 and letting λ⟶0, we obtain that 0 ≤ g(q, y) + 〈Bq, yq〉 for all yC. Then, qEP(g).

Finally, we prove that qΩ. From (56), we have that ‖PDAqAq‖ = limn⟶∞PDAunAun‖ = 0. Therefore, AqD, i.e., qΩ. Hence, , and this completed the proof.

Conflicts of Interest

This work does not have any conflicts of interest.

Data Availability

No data were used to support the study.

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