Research Article | Open Access

J. Vahidi, A. Latif, M. Eslamian, "Strong Convergence Results for Equilibrium Problems and Fixed Point Problems for Multivalued Mappings", *Abstract and Applied Analysis*, vol. 2013, Article ID 825130, 8 pages, 2013. https://doi.org/10.1155/2013/825130

# Strong Convergence Results for Equilibrium Problems and Fixed Point Problems for Multivalued Mappings

**Academic Editor:**Mohamed Amine Khamsi

#### Abstract

Using viscosity approximation method, we study strong convergence to a common element of the set of solutions of an equilibrium problem and the set of common fixed points of a finite family of multivalued mappings satisfying the condition () in the setting of Hilbert space. Our results improve and extend some recent results in the literature.

#### 1. Introduction

Let be a real Hilbert space with inner product and norm . Let be a nonempty closed convex subset . A subset is called proximal if, for each , there exists an element such that

A single-valued mapping is said to be nonexpansive, if

Let be a nearest point projection of into ; that is, for , is a unique nearest point in with the property

We denote by , , and the collection of all nonempty closed bounded subsets, nonempty compact subsets, and nonempty proximal bounded subsets of respectively. The Hausdorff metric on is defined by for all .

Let be a multivalued mapping. An element is said to be a fixed point of , if and the set of fixed points of is denoted by .

A multivalued mapping is called (i)nonexpansive if (ii)quasi-nonexpansive if and for all and all .

Recently, García-Falset et al. [1] introduced a new condition on single-valued mappings, called condition , which is weaker than nonexpansiveness.

*Definition 1. * A mapping is said to satisfy condition provided that

We say that satisfies condition whenever satisfies for some .

Recently, Abkar and Eslamian [2, 3] generalized this condition for multivalued mappings as follows.

*Definition 2. * A multivalued mapping is said to satisfy condition provided that
for some .

It is obvious that every nonexpansive multivalued mapping satisfies the condition , and every mapping which satisfies the condition with nonempty fixed point set is quasi-nonexpansive.

*Example 3. * Let us define a mapping on by
It is easy to see that satisfies the condition but is not nonexpansive. Indeed, for , . Let and . Then . If and , then, we have and ; hence
Thus, satisfies the condition . However, is not nonexpansive; indeed for and , .

Let be a bifunction. The equilibrium problem associated with the bifunction and the set is:
Such a point is called the solution of the equilibrium problem. The set of solutions is denoted by .

A broad class of problems in optimization theory, such as variational inequality, convex minimization, and fixed point problems, can be formulated as an equilibrium problem; see [4, 5]. In the literature, many techniques and algorithms have been proposed to analyze the existence and approximation of a solution to equilibrium problem; see [6]. Many researchers have studied various iteration processes for finding a common element of the set of solutions of the equilibrium problems and the set of fixed points of a class of nonlinear mappings. For example, see [7–22].

Fixed points and fixed point iteration process for nonexpansive mappings have been studied extensively by many authors to solve nonlinear operator equations, as well as variational inequalities; see, for example, [23–28]. In the recent years, fixed point theory for multivalued mappings has been studied by many authors; see [29–40] and the references therein.

In this paper, using viscosity approximation method, we study strong convergence to a common element of the set of solutions of an equilibrium problem and the set of common fixed points of a finite family of multivalued mappings satisfying the condition in the setting of Hilbert space. Our results improve and extend some recent results in the literature.

#### 2. Preliminaries

For solving the equilibrium problem, we assume that the bifunction satisfies the following conditions: (A1) for any ; (A2) is monotone; that is, for any ;(A3) is upper-hemicontinuous; that is, for each , (A4) is convex and lower semicontinuous for each .

Lemma 4 (see [4]). *Let be a nonempty closed convex subset of and let be a bifunction of into satisfying . Let and . Then, there exists such that
*

Lemma 5 (see [6]). *Assume that satisfies . For and , define a mapping as follows:**
Then, the following hold: *(i)* is single valued; *(ii)* is firmly nonexpansive; that is, for any ,
*(iii)*;
*(iv)* is closed and convex. *

Lemma 6 (see [41]). *Let be a real Hilbert space. Then, for all and with one has
*

Lemma 7. *For every and in a Hilbert space , the following inequality holds:
*

Lemma 8 (see [42]). *Let be a sequence of nonnegative real numbers, a sequence in with , a sequence of nonnegative real numbers with , and a sequence of real numbers with . Suppose that the following inequality holds:
**
Then, .*

Lemma 9 (see [43]). *Let be a sequence of real numbers that does not decrease at infinity, in the sense that there exists a subsequence of such that for all . For every sufficiently large number , define an integer sequence as
**
Then, as and for all ,
*

Lemma 10 (see [20]). *Let be a closed convex subset of a real Hilbert space . Let be a quasi-nonexpansive multivalued mapping. If and for all . Then is closed and convex.*

Lemma 11 (see [20]). *Let be a closed convex subset of a real Hilbert space . Let be a multivalued mapping such that is quasi-nonexpansive and , where . Then, is closed and convex.*

Lemma 12 (see [16, 20]). *Let be a nonempty closed convex subset of a real Hilbert space . Let be a multivalued mapping satisfying the condition . If converges weakly to and , then .*

#### 3. A Strong Convergence Theorem

Theorem 13. *Let be a nonempty closed convex subset of a real Hilbert space and a bifunction of into satisfying . Let () be a finite family of multivalued mappings, each satisfying condition . Assume further that and for each . Let be a -contraction of into itself. Let and be sequences generated the following algorithm:
**
where , for , and , , , , and satisfy the following conditions: *(i)*,
*(ii)*, , , *(iii)*, and .** Then, the sequences and converge strongly to , where .*

* Proof. *Let . It is easy to see that is a contraction. By Banach contraction principle, there exists a such that . From Lemma 5 for all , we have
We show that is bounded. Since, for each , satisfies the condition and we have

By continuing this process, we obtain

This implies that
By induction, we get
for all . This implies that is bounded and we also obtain that , and are bounded. Next, we show that for each . By Lemma 6, we have
Applying Lemma 6 once more, we have
By continuing this process we have
which implies that
Therefore, we have that
In order to prove that as , we consider the following two cases.*Case 1.* Suppose that there exists such that is nonincreasing, for all . Boundedness of implies that is convergent. From (31) and conditions (i), (ii) we have that
By a similar argument, for , we obtain that
Hence,
Therefore, we have
For , we have
Using the previous inequality for , we have
Next, we show that
where . To show this inequality, we choose a subsequence of such that
Since is bounded, there exists a subsequence of which converges weakly to . Without loss of generality, we can assume that converges weakly to . Since , we have converges weakly to . We show that . Let us show . Since , we have
From (A2), we have
Replacing with , we have
From (A4), we have
For and , let . Since , and is convex, we have and hence . So, from (A1) and (A4) we have
which gives . Letting , we have, for each , Also, since and , by Lemma 12 we have . Hence, . Since and , it follows that
By using Lemma 7 and inequality (31) we have

This implies that
From Lemma 8, we conclude that the sequence converges strongly to .*Case 2.* Assume that there exists a subsequence of such that
for all . In this case, from Lemma 9, there exists a nondecreasing sequence of for all (for some large enough) such that as and the following inequalities hold for all
From (31) we obtain , and . Following an argument similar to that in Case 1, we have
Thus, by Lemma 9 we have
Therefore, converges strongly to . This completes the proof.

Now, we remove the condition that for all , and state the following theorem.

Theorem 14. *Let be a nonempty closed convex subset of a real Hilbert space and a bifunction of into satisfying . Let, for each , be multivalued mappings such that satisfies the condition . Assume that . Let be a -contraction of into itself. Let and be sequences generated the following algorithm:
**
where , for , and , , , and, satisfy the following conditions: *(i)* ,,
*(ii)

*, , ,*(iii)

*, and .*

*Then, the sequences and converge strongly to , where .*

* Proof. *Let ; then . Now by substituting instead of , and using a similar argument as in the proof of Theorem 13, the desired result follows.

As a corollary for single-valued mappings, we obtain the following result.

Corollary 15. *Let be a nonempty closed convex subset of a real Hilbert space and a bifunction of into satisfying . Let, for each , be a finite family of mappings satisfying condition . Assume that . Let be a -contraction of into itself. Let and be sequences generated the following algorithm:
**
where , ,, , and satisfy the following conditions: *(i)*, ,*(ii)*, , *(iii)*, and .** Then, the sequences and converge strongly to , where .*

*Remark 16. * Our results generalize the corresponding results of S. Takahashi and W. Takahashi [9] from a single valued nonexpansive mapping to a finite family of multivalued mappings satisfying the condition . Our results also improve the recent results of Eslamian [16].

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgments

This paper was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah. The authors therefore, acknowledge with thanks DSR for technical and financial support. The authors are also thankful to the referees for their valuable suggestion/comments.

#### References

- J. García-Falset, E. Llorens-Fuster, and T. Suzuki, “Fixed point theory for a class of generalized nonexpansive mappings,”
*Journal of Mathematical Analysis and Applications*, vol. 375, no. 1, pp. 185–195, 2011. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - A. Abkar and M. Eslamian, “Common fixed point results in CAT(0) spaces,”
*Nonlinear Analysis: Theory, Methods & Applications*, vol. 74, no. 5, pp. 1835–1840, 2011. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - M. Eslamian and A. Abkar, “Fixed point theorems for suzuki generalized nonexpansive multivalued mappings in Banach spaces,”
*Fixed Point Theory and Applications*, vol. 2010, Article ID 457935, 10 pages, 2010. View at: Publisher Site | Google Scholar - E. Blum and W. Oettli, “From optimization and variational inequalities to equilibrium problems,”
*The Mathematics Student*, vol. 63, no. 1-4, pp. 123–145, 1994. View at: Google Scholar | Zentralblatt MATH | MathSciNet - S. D. Flåm and A. S. Antipin, “Equilibrium programming using proximal-like algorithms,”
*Mathematical Programming*, vol. 78, no. 1, Ser. A, pp. 29–41, 1997. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - P. L. Combettes and S. A. Hirstoaga, “Equilibrium programming in Hilbert spaces,”
*Journal of Nonlinear and Convex Analysis*, vol. 6, no. 1, pp. 117–136, 2005. View at: Google Scholar | Zentralblatt MATH | MathSciNet - A. Latif, L.-C. Ceng, and Q. H. Ansari, “Multi-step hybrid viscosity method for systems of variational inequalities defined over sets of solutions of an equilibrium problem and fixed point problems,”
*Fixed Point Theory and Applications*, vol. 2012, article 186, 26 pages, 2012. View at: Publisher Site | Google Scholar | MathSciNet - A. Tada and W. Takahashi, “Weak and strong convergence theorems for a nonexpansive mapping and an equilibrium problem,”
*Journal of Optimization Theory and Applications*, vol. 133, no. 3, pp. 359–370, 2007. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - S. Takahashi and W. Takahashi, “Viscosity approximation methods for equilibrium problems and fixed point problems in Hilbert spaces,”
*Journal of Mathematical Analysis and Applications*, vol. 331, no. 1, pp. 506–515, 2007. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - L.-C. Ceng, S. Al-Homidan, Q. H. Ansari, and J.-C. Yao, “An iterative scheme for equilibrium problems and fixed point problems of strict pseudo-contraction mappings,”
*Journal of Computational and Applied Mathematics*, vol. 223, no. 2, pp. 967–974, 2009. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - L.-C. Ceng, Q. H. Ansari, and J.-C. Yao, “Viscosity approximation methods for generalized equilibrium problems and fixed point problems,”
*Journal of Global Optimization*, vol. 43, no. 4, pp. 487–502, 2009. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - L.-C. Zeng, Q. H. Ansari, and S. Al-Homidan, “Hybrid proximal-type algorithms for generalized equilibrium problems, maximal monotone operators, and relatively nonexpansive mappings,”
*Fixed Point Theory and Applications*, vol. 2011, Article ID 973028, 23 pages, 2011. View at: Google Scholar | Zentralblatt MATH | MathSciNet - S. Plubtieng and R. Punpaeng, “A general iterative method for equilibrium problems and fixed point problems in Hilbert spaces,”
*Journal of Mathematical Analysis and Applications*, vol. 336, no. 1, pp. 455–469, 2007. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - L.-C. Ceng, N. Hadjisavvas, and N.-C. Wong, “Strong convergence theorem by a hybrid extragradient-like approximation method for variational inequalities and fixed point problems,”
*Journal of Global Optimization*, vol. 46, no. 4, pp. 635–646, 2010. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - F. Cianciaruso, G. Marino, and L. Muglia, “Iterative methods for equilibrium and fixed point problems for nonexpansive semigroups in Hilbert spaces,”
*Journal of Optimization Theory and Applications*, vol. 146, no. 2, pp. 491–509, 2010. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - M. Eslamian, “Convergence theorems for nonspreading mappings and nonexpansive multivalued mappings and equilibrium problems,”
*Optimization Letters*, vol. 7, no. 3, pp. 547–557, 2013. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - M. Eslamian, “Hybrid method for equilibrium problems and fixed point problems of finite families of nonexpansive semigroups,”
*Revista de la Real Academia de Ciencias Exactas, Fisicas y Naturales. Serie A*, vol. 107, no. 2, pp. 299–307, 2013. View at: Publisher Site | Google Scholar - A. Moudafi, “Weak convergence theorems for nonexpansive mappings and equilibrium problems,”
*Journal of Nonlinear and Convex Analysis*, vol. 9, no. 1, pp. 37–43, 2008. View at: Google Scholar | Zentralblatt MATH | MathSciNet - U. Kamraksa and R. Wangkeeree, “Generalized equilibrium problems and fixed point problems for nonexpansive semigroups in Hilbert spaces,”
*Journal of Global Optimization*, vol. 51, no. 4, pp. 689–714, 2011. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - A. Abkar and M. Eslamian, “Strong convergence theorems for equilibrium problems and fixed point problem of multivalued nonexpansive mappings via hybrid projection method,”
*Journal of Inequalities and Applications*, vol. 2012, article 164, 13 pages, 2012. View at: Publisher Site | Google Scholar | MathSciNet - S. Wang, G. Marino, and Y.-C. Liou, “Strong convergence theorems for variational inequality, equilibrium and fixed point problems with applications,”
*Journal of Global Optimization*, vol. 54, no. 1, pp. 155–171, 2012. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - X. Qin, Y. J. Cho, and S. M. Kang, “Viscosity approximation methods for generalized equilibrium problems and fixed point problems with applications,”
*Nonlinear Analysis: Theory, Methods & Applications*, vol. 72, no. 1, pp. 99–112, 2010. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - C. Byrne, “A unified treatment of some iterative algorithms in signal processing and image reconstruction,”
*Inverse Problems*, vol. 20, no. 1, pp. 103–120, 2004. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - C. I. Podilchuk and R. J. Mammone, “Image recovery by convex projections using a least-squares constraint,”
*Journal of the Optical Society of America A*, vol. 7, pp. 517–521, 1990. View at: Google Scholar - D. C. Youla, “On deterministic convergence of iterations of relaxed projection operators,”
*Journal of Visual Communication and Image Representation*, vol. 1, no. 1, pp. 12–20, 1990. View at: Google Scholar - W. R. Mann, “Mean value methods in iteration,”
*Proceedings of the American Mathematical Society*, vol. 4, pp. 506–510, 1953. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - S. Ishikawa, “Fixed points by a new iteration method,”
*Proceedings of the American Mathematical Society*, vol. 44, pp. 147–150, 1974. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - R. P. Agarwal, D. O'Regan, and D. R. Sahu, “Iterative construction of fixed points of nearly asymptotically nonexpansive mappings,”
*Journal of Nonlinear and Convex Analysis*, vol. 8, no. 1, pp. 61–79, 2007. View at: Google Scholar | Zentralblatt MATH | MathSciNet - E. Lami Dozo, “Multivalued nonexpansive mappings and Opial's condition,”
*Proceedings of the American Mathematical Society*, vol. 38, pp. 286–292, 1973. View at: Google Scholar | Zentralblatt MATH | MathSciNet - T. C. Lim, “A fixed point theorem for multivalued nonexpansive mappings in a uniformly convex Banach space,”
*Bulletin of the American Mathematical Society*, vol. 80, pp. 1123–1126, 1974. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - S. Dhompongsa, A. Kaewkhao, and B. Panyanak, “Browder's convergence theorem for multivalued mappings without endpoint condition,”
*Topology and Its Applications*, vol. 159, no. 10-11, pp. 2757–2763, 2012. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - M. A. Khamsi and W. A. Kirk, “On uniformly Lipschitzian multivalued mappings in Banach and metric spaces,”
*Nonlinear Analysis: Theory, Methods & Applications*, vol. 72, no. 3-4, pp. 2080–2085, 2010. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - J. Garcia-Falset, E. Llorens-Fuster, and E. Moreno-Galvez, “Fixed point theory for multivalued generalized nonexpansive mappings,”
*Applicable Analysis and Discrete Mathematics*, vol. 6, no. 2, pp. 265–286, 2012. View at: Publisher Site | Google Scholar - A. Latif, T. Husain, and I. Beg, “Fixed point of nonexpansive type and $K$-multivalued maps,”
*International Journal of Mathematics and Mathematical Sciences*, vol. 17, no. 3, pp. 429–435, 1994. View at: Publisher Site | Google Scholar | MathSciNet - S. Dhompongsa, W. A. Kirk, and B. Panyanak, “Nonexpansive set-valued mappings in metric and Banach spaces,”
*Journal of Nonlinear and Convex Analysis*, vol. 8, no. 1, pp. 35–45, 2007. View at: Google Scholar | Zentralblatt MATH | MathSciNet - A. Abkar and M. Eslamian, “Fixed point theorems for Suzuki generalized nonexpansive multivalued mappings in Banach spaces,”
*Fixed Point Theory and Applications*, vol. 2010, Article ID 457935, 10 pages, 2010. View at: Google Scholar | Zentralblatt MATH | MathSciNet - Y. Song and H. Wang, “Convergence of iterative algorithms for multivalued mappings in Banach spaces,”
*Nonlinear Analysis: Theory, Methods & Applications*, vol. 70, no. 4, pp. 1547–1556, 2009. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - A. Latif and I. Beg, “Geometric fixed points for single and multivalued mappings,”
*Demonstratio Mathematica*, vol. 30, no. 4, pp. 791–800, 1997. View at: Google Scholar | Zentralblatt MATH | MathSciNet - M. Eslamian and A. Abkar, “One-step iterative process for a finite family of multivalued mappings,”
*Mathematical and Computer Modelling*, vol. 54, no. 1-2, pp. 105–111, 2011. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - S. Dhompongsa, A. Kaewkhao, and B. Panyanak, “On Kirk's strong convergence theorem for multivalued nonexpansive mappings on CAT(0) spaces,”
*Nonlinear Analysis: Theory, Methods & Applications*, vol. 75, no. 2, pp. 459–468, 2012. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - M. O. Osilike and D. I. Igbokwe, “Weak and strong convergence theorems for fixed points of pseudocontractions and solutions of monotone type operator equations,”
*Computers & Mathematics with Applications*, vol. 40, no. 4-5, pp. 559–567, 2000. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - H. K. Xu, “An iterative approach to quadratic optimization,”
*Journal of Optimization Theory and Applications*, vol. 116, no. 3, pp. 659–678, 2003. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet - P.-E. Maingé, “Strong convergence of projected subgradient methods for nonsmooth and nonstrictly convex minimization,”
*Set-Valued Analysis*, vol. 16, no. 7-8, pp. 899–912, 2008. View at: Publisher Site | Google Scholar | Zentralblatt MATH | MathSciNet

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Copyright © 2013 J. Vahidi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.