Hindawi Publishing Corporation Fixed Point Theory and Applications Volume 2011, Article ID 276859, potx

Hindawi Publishing Corporation Fixed Point Theory and Applications Volume 2011, Article ID 276859, 10 pages doi:10.1155/2011/276859 Research Article An Implicit Iteration Method for Variational Inequalities over the Set of Common Fixed Points for a Finite Family of Nonexpansive Mappings in Hilbert Spaces Nguyen Buong 1 and Nguyen Thi Quynh Anh 2 1 Vietnamese Academy of Science and Technology, Institute of Information Technology, 18, Hoang Quoc Viet, Cau Giay, Ha Noi 122100, Vietnam 2 Department of Information Technology, Thai Nguyen National University, Thainguye 842803, Vietnam Correspondence should be addressed to Nguyen Buong, nbuong@ioit.ac.vn Received 17 December 2010; Accepted 7 March 2011 Academic Editor: Jong Kim Copyright q 2011 N. Buong and N. T. Quynh Anh. 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. We introduce a new implicit iteration method for finding a solution for a variational inequality involving Lipschitz continuous and strongly monotone mapping over the set of common fixed points for a finite family of nonexpansive mappings on Hilbert spaces. 1. Introduction Let C be a nonempty closed and convex subset of a real Hilbert space H with inner product ·, · and norm ·,andletF : H → H be a nonlinear mapping. The variational inequality problem is formulated as finding a point p ∗ ∈ C such that  F  p ∗  ,p− p ∗  ≥ 0, ∀p ∈ C. 1.1 Variational inequalities were initially studied by Kinderlehrer and Stampacchia in 1 and ever since have been widely investigated, since they cover as diverse disciplines as partial differential equations, optimal control, optimization, mathematical programming, mechanics, and finance see 1–3. 2 Fixed Point Theory and Applications It is well known that if F is an L-Lipschitz continuous and η-strongly monotone, that is, F satisfies the following conditions:   F  x  − F  y    ≤ L   x − y   ,  F  x  − F  y  ,x− y  ≥ η   x − y   2 , 1.2 where L and η are fixed positive numbers, then 1.1 has a unique solution. It is also known that 1.1 is equivalent to the fixed-point equation p  P C  p − μF  p  , 1.3 where P C denotes the metric projection from x ∈ H onto C and μ is an arbitrarily fixed positive constant. Let {T i } N i1 be a finite family of nonexpansive self-mappings of C. For finding an element p ∈∩ N i1 FixT i , Xu and Ori introduced in 4 the following implicit iteration process. For x 0 ∈ C and {β k } ∞ k1 ⊂ 0, 1, the sequence {x k } is generated as follows: x 1  β 1 x 0   1 − β 1  T 1 x 1 , x 2  β 2 x 1   1 − β 2  T 2 x 2 , . . . x N  β N x N−1   1 − β N  T N x N , x N1  β N1 x N   1 − β N1  T 1 x N1 , . . . 1.4 The compact expression of the method is the form x k  β k x k−1   1 − β k  T k x k ,k≥ 1, 1.5 where T n  T n mod N , for integer n ≥ 1, with the mod function taking values in the set {1, 2, ,N}. They proved the following result. Theorem 1.1. Let H be a real Hilbert space and C a nonempty closed convex subset of H.Let{T i } N i1 be N nonexpansive self-maps of C such that ∩ N i1 FixT i  /  ∅,whereFixT i {x ∈ C : T i x  x}.Let x 0 ∈ C and {β k } ∞ k1 be a sequence in 0, 1 such that lim k →∞ β k  0. Then, the sequence {x k } defined implicitly by 1.5 converges weakly to a common fixed point of the mappings {T i } N i1 . Fixed Point Theory and Applications 3 Further, Zeng and Yao introduced in 5 the following implicit method. For an arbitrary initial point x 0 ∈ H, the sequence {x k } ∞ k1 is generated as follows: x 1  β 1 x 0   1 − β 1  T 1 x 1 − λ 1 μF  T 1 x 1   , x 2  β 2 x 1   1 − β 2  T 2 x 2 − λ 2 μF  T 2 x 2   , . . . x N  β N x N−1   1 − β N  T N x N − λ N μF  T N x N   , x N1  β N1 x N   1 − β N1  T 1 x N1 − λ N1 μF  T 1 x N1   , . . . 1.6 The scheme is written in a compact form as x k  β k x k−1   1 − β k  T k x k − λ k μF  T k x k  ,k≥ 1. 1.7 They proved the following result. Theorem 1.2. Let H be a real Hilbert space and F : H → H a mapping such that for some constants L, η > 0, F is L-Lipschitz continuous and η-strongly monotone. Let {T i } N i1 be N nonexpansive self- maps of H such that C  ∩ N i1 FixT i  /  ∅.Letμ ∈ 0, 2η/L 2 , and let x 0 ∈ H, with {λ k } ∞ k1 ⊂ 0, 1 and {β k } ∞ k1 ⊂ 0, 1 satisfying the conditions:  ∞ k1 λ k < ∞ and α ≤ β k ≤ β, k ≥ 1,forsome α, β ∈ 0, 1. Then, the sequence {x k } defined by 1.7 converges weakly to a common fixed point of the mappings {T i } N i1 . The convergence is strong if and only if lim inf k →∞ dx k ,C0. Recently, Ceng et al. 6 extended the above result to a finite family of asymptotically self-maps. Clearly, from  ∞ k1 λ k < ∞ we have that λ k → 0ask →∞. To obtain strong convergence without the condition  ∞ k1 λ k < ∞, in this paper we propose the following implicit algorithm: x t  T t x t ,T t : T t 0 T t N ···T t 1 ,t∈  0, 1  , 1.8 where T t i are defined by T t i x   1 − β i t  x  β i t T i x, i  1, ,N, T t 0 y   I − λ t μF  y, x, y ∈ H, 1.9 I denotes the identity mapping of H, and the parameters {λ t }, {β i t }⊂0, 1 for all t ∈ 0, 1 satisfy the following conditions: λ t → 0ast → 0and0< lim inf t → 0 β i t ≤ lim sup t → 0 β i t < 1,i 1, ,N. 4 Fixed Point Theory and Applications 2. Main Result We formulate the following facts for the proof of our results. Lemma 2.1 see 7. i x  y 2 ≤x 2  2y, x  y and for any fixed t ∈ 0, 1, ii 1 − tx  ty 2 1 − tx 2  ty 2 − 1 − ttx − y 2 , for all x, y ∈ H. Put T λ x  Tx − λμFTx,x∈ H, λ ∈ 0, 1; for any nonexpansive mapping T of H, we have the following lemma. Lemma 2.2 see 8. T λ x − T λ y≤1 − λτx − y, for all x, y ∈ H and for a fixed number μ ∈ 0, 2η/L 2 ,whereτ  1 −  1 − μ2η − μL 2  ∈ 0, 1. Lemma 2.3 Demiclosedness Principle 9. Assume that T is a nonexpansive self-mapping of a closed convex subset K of a Hibert space H.IfT has a fixed point, then I − T is demiclosed; that is, whenever {x k } is a sequence in K weakly converging to s ome x ∈ K and the sequence {I − Tx k } strongly converges to some y, it follows that I − Tx  y. Now, we are in a position to prove the following result. Theorem 2.4. Let H be a real Hilbert space and F : H → H a mapping such that for some constants L, η > 0, F is L-Lipschitz continuous and η-strongly monotone. Let {T i } N i1 be N nonexpansive self- maps of H such that C  ∩ N i1 FixT i  /  ∅.Letμ ∈ 0, 2η/L 2  and let t ∈ 0, 1, {λ t }, {β i t }⊂0, 1, such that λ t −→ 0, as t −→ 0, 0 < lim inf t → 0 β i t ≤ lim sup t → 0 β i t < 1,i 1, ,N. 2.1 Then, the net {x t } defined by 1.8-1.9 converges strongly to the unique element p ∗ in 1.1. Proof. By using Lemma 2.2 with T λ  T t 0 ,thatis,T  I, we have that   T t x − T t y   ≤  1 − λ t τ    T t N ···T t 1 x − T t N ···T t 1 y   . . . ≤  1 − λ t τ    T t i ···T t 1 x − T t i ···T t 1 y   . . . ≤  1 − λ t τ    T t 1 x − T t 1 y   ≤  1 − λ t τ    x − y   ∀x, y ∈ H. 2.2 So, T t is a contraction in H. By Banach’s Contraction Principle, there exists a unique element x t ∈ H such that x t  T t x t for all t ∈ 0, 1. Fixed Point Theory and Applications 5 Next, we show that {x t } is bounded. Indeed, for a fixed point p ∈ C, we have that T t i p  p for i  1, ,N, and hence   x t − p      T t x t − p      T t x t − T t N ···T t 1 p       I − λ t μF  T t N ···T t 1 x t −  I − λ t μF  T t N ···T t 1 p − λ t μF  p    ≤  1 − λ t τ    T t N ···T t 1 x t − T t N ···T t 1 p    λ t μ   F  p    ≤  1 − λ t τ    T t N−1 ···T t 1 x t − T t N−1 ···T t 1 p    λ t μ   F  p    . . . ≤  1 − λ t τ    T t i ···T t 1 x t − T t i ···T t 1 p    λ t μ   F  p    . . . ≤  1 − λ t τ    T t 1 x t − T t 1 p    λ t μ   F  p    ≤  1 − λ t τ    x t − p    λ t μ   F  p    . 2.3 Therefore,   x t − p   ≤ μ τ   F  p    2.4 that implies the boundedness of {x t }. So, are the nets {Fy N t }, {y i t },i 1, ,N. Put y 1 t   1 − β 1 t  x t  β 1 t T 1 x t , y 2 t   1 − β 2 t  y 1 t  β 2 t T 2 y 1 t , . . . y i t   1 − β i t  y i−1 t  β i t T i y i−1 t , . . . y N t   1 − β N t  y N−1 t  β N t T N y N−1 t . 2.5 Then, x t   I − λ t μF  y N t . 2.6 6 Fixed Point Theory and Applications Moreover,   x t − p   2      I − λ t μF  y N t − p    2     y N t − p    2 − 2λ t μ  F  y N t  ,y N t − p   λ 2 t μ 2    F  y N t     2 ≤    y N−1 t − p    2 − 2λ t μ  F  y N t  ,y N t − p   λ 2 t μ 2    F  y N t     2 . . . ≤    y 1 t − p    2 − 2λ t μ  F  y N t  ,y N t − p   λ 2 t μ 2    F  y N t     2 ≤   x t − p   2 − 2λ t μ  F  y N t  ,y N t − p   λ 2 t μ 2    F  y N t     2 . 2.7 Thus, η    y N t − p    2   F  p  ,y N t − p  ≤ λ t μ 2    F  y N t     2 . 2.8 Further, for the sake of simplicity, we put y 0 t  x t and prove that    y i−1 t − T i y i−1 t    −→ 0, 2.9 as t → 0fori  1, ,N. Let {t k }⊂0, 1 be an arbitrary sequence converging to zero as k →∞and x k : x t k . We have to prove that y i−1 k − T i y i−1 k →0, where y i k are defined by 2.5 with t  t k and y i k  y i t k .Let{x l } be a subsequence of {x k } such that lim sup k →∞    y i−1 k − T i y i−1 k     lim l →∞    y i−1 l − T i y i−1 l    . 2.10 Let {x k j } be a subsequence of {x l } such that lim sup k →∞   x k − p    lim j →∞    x k j − p    . 2.11 Fixed Point Theory and Applications 7 From 2.6 and Lemma 2.1, it implies that    x k j − p    2      I − λ k j μF  y N k j − p    2 ≤    y N k j − p    2 − 2λ k j μ  F  y N k j  ,x k j − p       1 − β N k j  y N−1 k j − p   β N k j  T N y N−1 k j − T N p     2 − 2λ k j μ  F  y N k j  ,x k j − p  ≤  1 − β N k j     y N−1 k j − p    2  β N k j    T N y N−1 k j − T N p    2 − 2λ k j μ  F  y N k j  ,x k j − p  ≤    y N−1 k j − p    2 − 2λ k j μ  F  y N k j  ,x k j − p  ≤···≤    y 1 k j − p    2 − 2λ k j μ  F  y N k j  ,x k j − p  ≤    x k j − p    2 − 2λ k j μ  F  y N k j  ,x k j − p  . 2.12 Hence, lim j →∞    x k j − p     lim j →∞    y i k j − p    ,i 1, ,N. 2.13 By Lemma 2.1,    y i k j − p    2   1 − β i k j     y i−1 k j − p    2  β i k j    T i y i−1 k j − p    2 − β i k j  1 − β i k j     y i−1 k j − T i y i−1 k j    2 ≤  1 − β i k j     y i−1 k j − p    2  β i k j    y i−1 k j − p    2 − β i k j  1 − β i k j     y i−1 k j − T i y i−1 k j    2     y i−1 k j − p    2 − β i k j  1 − β i k j     y i−1 k j − T i y i−1 k j    2 ≤···    y 0 k j − p    2 − β i k j  1 − β i k j     y i−1 k j − T i y i−1 k j    2     x k j − p    2 − β i k j  1 − β i k j     y i−1 k j − T i y i−1 k j    2 ,i 1, ,N. 2.14 8 Fixed Point Theory and Applications Without loss of generality, we can assume that α ≤ β i t ≤ β for some α, β ∈ 0, 1. Then, we have α  1 − β     y i−1 k j − T i y i−1 k j    2 ≤    x k j − p    2 −    y i k j − p    2 . 2.15 This together with 2.13 implies that lim j →∞    y i−1 k j − T i y i−1 k j    2  0,i 1, ,N. 2.16 It means that y i−1 t − T i y i−1 t →0ast → 0fori  1, ,N. Next, we show that x t − T i x t →0ast → 0. In fact, in the case that i  1 we have y 0 t  x t .So,x t − T 1 x t →0ast → 0. Further, since    y 1 t − T 1 x t      1 − β 1 t   x t − T 1 x t  , 2.17 and x t − T 1 x t →0, we have that y 1 t − T 1 x t →0. Therefore, from    x t − y 1 t    ≤  x t − T 1 x t      T 1 x t − y 1 t    , 2.18 it follows that x t − y 1 t →0ast → 0. On the other hand, since    y 2 t − T 2 y 1 t      1 − β 2 t     y 1 t − T 2 y 1 t    −→ 0,    y 2 t − x t    ≤  1 − β 2 t     y 1 t − x t     β 2 t    T 2 y 1 t − x t    ≤  1 − β 2 t     y 1 t − x t     β 2 t    T 2 y 1 t − y 1 t        y 1 t − x t    , 2.19 we obtain that y 2 t − x t →0ast → 0. Now, from  x t − T 2 x t  ≤    x t − y 2 t        y 2 t − T 2 y 1 t        T 2 y 1 t − T 2 x t    ≤    x t − y 2 t        y 2 t − T 2 y 1 t        y 1 t − x t    , 2.20 and x t − y 2 t , y 2 t − T 2 y 1 t , y 1 t − x t →0, it follows that x t − T 2 x t →0. Similarly, we obtain that x t − T i x t →0, for i  1, ,N and y N t − x t →0ast → 0. Let {x k } be any sequence of {x t } converging weakly to p as k →∞. Then, x k − T i x k →0, for i  1, ,N and {y N k } also converges weakly to p.ByLemma 2.3, we have p ∈ C  ∩ N i1 FixT i  and from 2.8, it follows that  F  p  ,p− p  ≥ 0 ∀p ∈ C. 2.21 Fixed Point Theory and Applications 9 Since p, p ∈ C, by replacing p by tp 1 − tp in the last inequality, dividing by t and taking t → 0 in the just obtained inequality, we obtain  F  p  ,p− p  ≥ 0 ∀p ∈ C. 2.22 The uniqueness of p ∗ in 1.1 guarantees that p  p ∗ . Again, replacing p in 2.8 by p ∗ ,we obtain the strong convergence for {x t }. This completes the proof. 3. Application Recall that a mapping S : H → H is called a γ-strictly pseudocontractive if there exists a constant γ ∈ 0, 1 such that   Sx − Sy   2 ≤   x − y   2  γ    I − S  x −  I − S  y   2 , ∀x, y ∈ H. 3.1 It is well known 10  that a mapping T : H → H by Tx  αx 1−αSx with a fixed α ∈ γ,1 for all x ∈ H is a nonexpansive mapping and FixTFixS. Using this fact, we can extend our result to the case C  ∩ N i1 FixS i , where S i is γ i -strictly pseudocontractive as follows. Let α i ∈ γ i , 1 be fixed numbers. Then, C  ∩ N i1 Fix  T i  with  T i y  α i y 1 − α i S i y,a nonexpansive mapping, for i  1, ,N, and hence  T t i y   1 − β i t  y  β i t  T i y   1 − β i t  1 − α i   y  β i t  1 − α i  S i y, i  1, ,N. 3.2 So, we have the following result. Theorem 3.1. Let H be a real Hilbert space and F : H → H a mapping such that for some constants L, η > 0, F is L-Lipschitz continuous and η-strongly monotone. Let {S i } N i1 be Nγ i -strictly pseudocontractive self-maps of H such that C  ∩ N i1 FixS i  /  ∅.Letα i ∈ γ i , 1,μ∈ 0, 2η/L 2  and let t ∈ 0, 1, {λ t }, {β i t }⊂0, 1, such that λ t −→ 0, as t −→ 0, 0 < lim inf t → 0 β i t ≤ lim sup t → 0 β i t < 1,i 1, ,N. 3.3 Then, the net {x t } defined by x t   T t x t ,  T t : T t 0  T t N ···  T t 1 ,t∈  0, 1  , 3.4 where  T t i ,fori  1, ,N, are defined by 3.2 and T t 0 x I − λ t μFx, converges strongly to the unique element p ∗ in 1.1. It is known in 11 that Fix  SC where  S   N i1 ξ i S i with ξ i > 0and  N i1 ξ i  1 for Nγ i -strictly pseudocontractions {S i } N i1 . Moreover,  S is γ-strictly pseudocontractive with γ  max{γ i :1≤ i ≤ N}. So, we also have the following result. 10 Fixed Point Theory and Applications Theorem 3.2. Let H be a real Hilbert space and F : H → H a mapping such that for some constants L, η > 0, F is L-Lipschitz continuous and η-strongly monotone. Let {S i } N i1 be Nγ i - strictly pseudocontractive self-maps of H such that C  ∩ N i1 FixS i  /  ∅.Letα ∈ γ,1,where γ  max{γ i :1≤ i ≤ N}, μ ∈ 0, 2η/L 2 , and let t ∈ 0, 1, {λ t }, {β t }⊂0, 1, such that λ t −→ 0, as t −→ 0, 0 < lim inf t → 0 β t ≤ lim sup t → 0 β t < 1. 3.5 Then, the net {x t }, defined by x t   T t x t ,  T t : T t 0   1 − β t  1 − α   I  β t  1 − α  N  i1 ξ i S i  ,t∈  0, 1  , 3.6 where T t 0 I − λ t μF, ξ i > 0, and  N i1 ξ i  1, converges strongly to the unique element p ∗ in 1.1. Acknowledgment This work was supported by the Vietnamese National Foundation of Science and Technology Development. References 1 D. Kinderlehrer and G. Stampacchia, An Introduction to Variational Inequalities and Their Applications, vol. 88 of Pure and Applied Mathematics, Academic Press, New York, NY, USA, 1980. 2 R. 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Xu, “Iterative methods for strict pseudo-contractions in Hilbert spaces,” Nonlinear Analysis. Theory, Methods & Applications, vol. 67, no. 7, pp. 2258–2271, 2007. . Hindawi Publishing Corporation Fixed Point Theory and Applications Volume 2011, Article ID 276859, 10 pages doi:10.1155/2011/276859 Research Article An Implicit Iteration. common fixed point of the mappings {T i } N i1 . Fixed Point Theory and Applications 3 Further, Zeng and Yao introduced in 5 the following implicit method. For an arbitrary initial point x 0 ∈. in Feasibility and Optimization and Their Applications, D. Butnariu, Y. Censor, and S. Reich, Eds., pp. 473– 504, North-Holland, Amsterdam, Holland, 2001. 9 K. Goebel and W. A. Kirk, Topics in Metric Fixed