Finite-Step Relaxed Hybrid Steepest-Descent Methods for Variational Inequalities
© Yen-Cherng Lin. 2008
Received: 22 August 2007
Accepted: 13 March 2008
Published: 8 April 2008
The classical variational inequality problem with a Lipschitzian and strongly monotone operator on a nonempty closed convex subset in a real Hilbert space was studied. A new finite-step relaxed hybrid steepest-descent method for this class of variational inequalities was introduced. Strong convergence of this method was established under suitable assumptions imposed on the algorithm parameters.
where is the (nearest point) projection from onto , that is, for each and where is an arbitrarily fixed constant. If is strongly monotone and Lipschitzian on and is small enough, then the mapping determined by the right-hand side of this equation is a contraction. Hence the Banach contraction principle guarantees that the Picard iterates converge in norm to the unique solution of the . Such a method is called the projection method. However, Zeng and Yao  point out that the fixed-point equation involves the projection which may not be easy to compute due to the complexity of the convex set . To reduce the complexity problem probably caused by the projection , a class of hybrid steepest-descent methods for solving VI( has been introduced and studied recently by many authors (see, e.g., [3, 4]). Zeng and Yao  have established the method of two-step relaxed hybrid steepest-descent for variational inequalities. A natural arising problem is whether there exists a general relaxed hybrid steepest-descent algorithm that is more than two steps for finding approximate solutions of VI( or not. Motivated and inspired by the recent research work in this direction, we introduce the following finite step relaxed hybrid steepest-descent algorithm for finding approximate solutions of VI( and aim to unify the convergence results of this kind of methods.
We will prove a strong convergence result for Algorithm 1.1 under suitable restrictions imposed on the parameters.
The following lemmas will be used for proving the main result of the paper in next section.
Lemma 2.1 (See ).
Lemma 2.2 (See ).
Demiclosedness principle: assume that is a nonexpansive self-mapping on a nonempty closed convex subset of a Hilbert space If has a fixed point, then is demiclosed; that is, whenever is a sequence in weakly converging to some and the sequence strongly converges to some , it follows that Here is the identity operator of
The following lemma is an immediate consequence of an inner product.
3. Convergence Theorem
respectively. Since F is -strongly monotone, the variational inequality problem has a unique solution (see, e.g., ).
Lemma 3.1 (See ).
We now state and prove the main result of this paper.
Let be a real Hilbert space and let be a nonempty closed convex subset of Let be an operator such that for some constants is -Lipschitzian and -strongly monotone on . Assume that is a nonexpansive mapping with the fixed points set the real sequences , for , in Algorithm 1.1 satisfy the following conditions:
Since is -strongly monotone, by , the has the unique solution . Next we divide the rest of the proof into several steps.
This research was partially supported by Grant no. NSC95-2115-M-039-001- from the National Science Council of Taiwan.
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