Strong Convergence to Solutions of Generalized Mixed Equilibrium Problems with Applications
Abstract
We introduce a Halpern-type iteration for a generalized mixed equilibrium problem in uniformly smooth and uniformly convex Banach spaces. Strong convergence theorems are also established in this paper. As applications, we apply our main result to mixed equilibrium, generalized equilibrium, and mixed variational inequality problems in Banach spaces. Finally, examples and numerical results are also given.
1. Introduction
Let E be a real Banach space, C a nonempty, closed, and convex subset of E, and E* the dual space of E. Let T : C → C be a nonlinear mapping. The fixed points set of T is denoted by F(T), that is, F(T) = {x ∈ C : x = Tx}.
Subsequently, motivated by Halpern [1], many mathematicians devoted time to study algorithm (1.1) in different styles. Several strong convergence results for nonlinear mappings were also continuously established in some certain Banach spaces (see also [2–9]).
The problem (1.2) is very general in the sense that it includes, as special cases, optimization problems, variational inequalities, minimax problems, the Nash equilibrium problem in noncooperative games, and others. For more details on these topics, see, for instance, [14–34].
- (A1)
f(x, x) = 0 for all x ∈ C;
- (A2)
f is monotone, that is, f(x, y) + f(y, x) ≤ 0 for all x, y ∈ C;
- (A3)
for all x, y, z ∈ C, limsup t↓0f(tz + (1 − t)x, y) ≤ f(x, y);
- (A4)
for all x ∈ C, f(x, ·) is convex and lower semicontinuous.
The purpose of this paper is to investigate strong convergence of Halpern-type iteration for a generalized mixed equilibrium problem in uniformly smooth and uniformly convex Banach spaces. As applications, our main result can be deduced to mixed equilibrium, generalized equilibrium, mixed variational inequality problems, and so on. Examples and numerical results are also given in the last section.
2. Preliminaries and Lemmas
In this section, we need the following preliminaries and lemmas which will be used in our main theorem.
Remark 2.1. We know the following: for any x, y, z ∈ E,
- (1)
(∥x∥−∥y∥) 2 ≤ ϕ(x, y)≤(∥x∥+∥y∥) 2;
- (2)
ϕ(x, y) = ϕ(x, z) + ϕ(z, y) + 2〈x − z, Jz − Jy〉;
- (3)
ϕ(x, y) = ∥x − y∥2 in a real Hilbert space.
Lemma 2.2 (see [36].)Let E be a uniformly convex and smooth Banach space and let {xn} and {yn} be sequences of E such that {xn} or {yn} is bounded and lim n→∞ϕ(xn, yn) = 0. Then lim n→∞∥xn − yn∥ = 0.
In fact, we have the following result.
Lemma 2.3 (see [37].)Let C be a nonempty, closed, and convex subset of a reflexive, strictly convex, and smooth Banach space E and let x ∈ E. Then there exists a unique element x0 ∈ C such that ϕ(x0, x) = min {ϕ(z, x) : z ∈ C}.
Lemma 2.4 (see [36], [37].)Let C be a nonempty closed and convex subset of a reflexive, strictly convex, and smooth Banach space E, x ∈ E, and z ∈ C. Then z = ΠCx if and only if
Lemma 2.5 (see [36], [37].)Let C be a nonempty closed and convex subset of a reflexive, strictly convex, and smooth Banach space E and let x ∈ E. Then
Lemma 2.6 (see [38].)Let E be a uniformly convex and uniformly smooth Banach space and C a nonempty, closed, and convex subset of E. Then ΠC is uniformly norm-to-norm continuous on every bounded set.
Lemma 2.7 (see [39].)Let E be a reflexive, strictly convex, smooth Banach space. Then
Lemma 2.8 (see [25].)Let C be a closed and convex subset of a smooth, strictly convex, and reflexive Banach space E, let f be a bifunction from C × C to ℝ which satisfies conditions (A1)–(A4), and let r > 0 and x ∈ E. Then there exists z ∈ C such that
Following [25, 40], we know the following lemma.
Lemma 2.9 (see [41].)Let C be a nonempty closed and convex subset of a smooth, strictly convex, and reflexive Banach space E. Let A : C → E* be a continuous and monotone mapping, let f be a bifunction from C × C to ℝ satisfying (A1)–(A4), and let φ be a lower semicontinuous and convex function from C to ℝ. For all r > 0 and x ∈ E, there exists z ∈ C such that
- (1)
Tr is single-valued;
- (2)
Tr is firmly nonexpansive-type mapping [42], that is, for all x, y ∈ E,
() - (3)
F(Tr) = GMEP (f, A, φ);
- (4)
GMEP (f, A, φ) is closed and convex.
Remark 2.10. It is known that T is of firmly nonexpansive type if and only if
The following lemmas give us some nice properties of real sequences.
Lemma 2.11 (see [43].)Assume that {an} is a sequence of nonnegative real numbers such that
- (a)
;
- (b)
limsup n→∞bn/αn ≤ 0 or .
Lemma 2.12 (see [44].)Let {γn} be a sequence of real numbers such that there exists a subsequence of {γn} such that for all j ≥ 1. Then there exists a nondecreasing sequence {mk} of ℕ such that lim k→∞mk = ∞ and the following properties are satisfied by all (sufficiently large) numbers k ≥ 1:
3. Main Results
In this section, we prove our main theorem in this paper. To this end, we need the following proposition.
Proposition 3.1. Let C be a nonempty closed and convex subset of a reflexive, strictly convex, and uniformly smooth Banach space E. Let f be a bifunction from C × C to ℝ satisfying (A1)–(A4), A : C → E* a continuous and monotone mapping, and φ a lower semicontinuous and convex function from C to ℝ such that GMEP (f, A, φ) ≠ ∅. Let {rn}⊂(0, ∞) be such that lim inf n→∞rn > 0. For each n ≥ 1, let be defined as in Lemma 2.9. Suppose that x ∈ C and {xn} is a bounded sequence in C such that . Then
Proof. Let x ∈ C and put p = ΠGMEP (f,A,φ)x. Since E is reflexive and {xn} is bounded, there exists a subsequence of {xn} such that and
Theorem 3.2. Let C be nonempty, closed, and convex subset of a uniformly smooth and uniformly convex Banach space E. Let f be a bifunction from C × C to ℝ satisfying (A1)–(A4), A : C → E* a continuous and monotone mapping, and φ a lower semicontinuous and convex function from C to ℝ such that GMEP (f, A, φ) ≠ ∅. Define the sequence {xn} as follows: x1 = x ∈ C and
- (a)
lim n→∞αn = 0;
- (b)
;
- (c)
lim inf n→∞rn > 0.
Proof. From Lemma 2.9(4), we know that GMEP (f, A, φ) is closed and convex. Let p = ΠGMEP (f,A,φ)x. Put and zn = J−1(αnJx + (1 − αn)Jyn) for all n ∈ ℕ. So, by Lemma 2.5, we have
We next show that if there exists a subsequence of {xn} such that
We next consider the following two cases.
Case 1. ϕ(p, xn+1) ≤ ϕ(p, xn) for all sufficiently large n. Hence the sequence {ϕ(p, xn)} is bounded and nonincreasing. So lim n→∞ϕ(p, xn) exists. This shows that lim n→∞(ϕ(p, xn+1) − ϕ(p, xn)) = 0 and hence
Finally, we show that xn → p. Using Lemma 2.7, we see that
Case 2. There exists a subsequence of {ϕ(p, xn)} such that for all j ∈ ℕ. By Lemma 2.12, there exists a strictly increasing sequence {mk} of positive integers such that the following properties are satisfied by all numbers k ∈ ℕ:
As a direct consequence of Theorem 3.2, we obtain the following results.
Corollary 3.3. Let C be nonempty closed and convex subset of a uniformly smooth and uniformly convex Banach space E. Let f be a bifunction from C × C to ℝ satisfying (A1)–(A4) and φ a lower semicontinuous and convex function from C to ℝ such that MEP (f, φ) ≠ ∅. Define the sequence {xn} as follows: x1 = x ∈ C and
- (a)
lim n→∞αn = 0;
- (b)
;
- (c)
lim inf n→∞rn > 0.
Corollary 3.4. Let C be nonempty, closed, and convex subset of a uniformly smooth and uniformly convex Banach space E. Let f be a bifunction from C × C to ℝ satisfying (A1)–(A4), and A : C → E* a continuous and monotone mapping such that GEP (f, A) ≠ ∅. Define the sequence {xn} as follows: x1 = x ∈ C and
- (a)
lim n→∞αn = 0;
- (b)
;
- (c)
lim inf n→∞rn > 0.
Corollary 3.5. Let C be nonempty, closed, and convex subset of a uniformly smooth and uniformly convex Banach space E. Let A : C → E* be a continuous and monotone mapping, and φ a lower semicontinuous and convex function from C to ℝ such that VI (C, A, φ) ≠ ∅. Define the sequence {xn} as follows: x1 = x ∈ C and
- (a)
lim n→∞αn = 0;
- (b)
;
- (c)
lim inf n→∞rn > 0.
4. Examples and Numerical Results
In this section, we give examples and numerical results for our main theorem.
Example 4.1. Let E = ℝ and C = [−1,1]. Let f(x, y) = −9x2 + xy + 8y2, φ(x) = 3x2, and Ax = 2x. Find such that
Solution 4. It is easy to check that f, φ, and A satisfy all conditions in Theorem 3.2. For each r > 0 and x ∈ [−1,1], Lemma 2.9 ensures that there exists z ∈ [−1,1] such that, for any y ∈ [−1,1],
Let be the sequence generated by x1 = x ∈ [−1,1] and
We next give two numerical results for algorithm (4.5).
Algorithm 4.2. Let αn = 1/80n and rn = n/(n + 1). Choose x1 = x = 1. Then algorithm (4.5) becomes
Numerical Result I See Table 1.
n | xn |
---|---|
1 | 1.0000 |
2 | 0.0856 |
3 | 0.0111 |
4 | 0.0047 |
5 | 0.0033 |
⋮ | ⋮ |
261 | 0.0001 |
262 | 0.0000 |
Algorithm 4.3. Let αn = 1/100n and rn = (n + 1)/2n. Choose x1 = x = −1. Then algorithm (4.5) becomes
Numerical Result II See Table 2.
n | xn |
---|---|
1 | −1.0000 |
2 | −0.0481 |
3 | −0.0074 |
4 | −0.0038 |
5 | −0.0027 |
⋮ | ⋮ |
217 | −0.0001 |
218 | 0.0000 |
5. Conclusion
Tables 1 and 2 show that the sequence {xn} converges to 0 which solves the generalized mixed equilibrium problem. On the other hand, using Lemma 2.9(3), we can check that GMEP (f, A, φ) = F(Tr) = {0}.
Remark 5.1. In the view of computation, our algorithm is simple in order to get strong convergence for generalized mixed equilibrium problems.
Acknowledgments
The first and the second authors wish to thank the Thailand Research Fund and the Centre of Excellence in Mathematics, the Commission on Higher Education, Thailand. The third author was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant no. 2011-0021821).