Convergence of Inexact Iterative Schemes for Nonexpansive Set-Valued Mappings
Taking into account possibly inexact data, we study iterative schemes for approximating fixed points and attractors of contractive and nonexpansive set-valued mappings, respectively. More precisely, we are concerned with the existence of convergent trajectories of nonstationary dynamical systems induced by approximations of a given set-valued mapping.
KeywordsNatural Number Nonexpansive Mapping Iterative Scheme Cauchy Sequence Contractive Mapping
The study of iterative schemes for various classes of nonexpansive mappings is a central topic in Nonlinear Functional Analysis. It began with the classical Banach theorem  on the existence of a unique fixed point for a strict contraction. This celebrated result also yields convergence of iterates to the unique fixed point. Since Banach's seminal result, many developments have taken place in this area. We mention, in particular, existence and approximation results regarding fixed points of those nonexpansive mappings which are not necessarily strictly contractive [2, 3]. Such results were obtained for general nonexpansive mappings in special Banach spaces, while for self-mappings of general complete metric spaces most of the results were established for several classes of contractive mappings . More recently, interesting developments have occurred for nonexpansive set-valued mappings, where the situation is more difficult and less understood. See, for instance, [5, 6, 7, 8] and the references cited therein. As we have already mentioned, one of the methods for proving the classical Banach result is to show the convergence of Picard iterations, which holds for any initial point. In the case of set-valued mappings, not all the trajectories of the dynamical system induced by the given mapping converge. Therefore, convergent trajectories have to be constructed in a special way. For example, in the setting of , if at the moment Open image in new window we reach a point Open image in new window , then the next iterate Open image in new window is an element of Open image in new window , where Open image in new window is the given mapping, which approximates the best approximation of Open image in new window in Open image in new window . Since Open image in new window is assumed to act on a general complete metric space, we cannot, in general, choose Open image in new window to be the best approximation of Open image in new window by elements of Open image in new window . Instead, we choose Open image in new window so that it provides an approximation up to a positive number Open image in new window , such that the sequence Open image in new window is summable. This method allowed Nadler  to obtain the existence of a fixed point of a strictly contractive set-valued mapping and the authors of  to obtain more general results.
In view of the above discussion, it is obviously important to study convergence properties of the iterates of (set-valued) nonexpansive mappings in the presence of errors and possibly inaccurate data. The present paper is a contribution in this direction. More precisely, we are concerned with the existence of convergent trajectories of nonstationary dynamical systems induced by approximations of a given set-valued mapping. In the second section of the paper, we consider an iterative scheme for approximating fixed points of closed-valued strict contractions in metric spaces and prove our first convergence theorem (see Theorem 2.1 below). Our second convergence theorem (Theorem 3.1) is established in the third section of our paper. We show there that if for any initial point, there exists a trajectory of the dynamical system induced by a nonexpansive set-valued mapping Open image in new window , which converges to a given invariant set Open image in new window , then a convergent trajectory also exists for a nonstationary dynamical system induced by approximations of Open image in new window .
2. Convergence to a Fixed Point of a Contractive Mapping
In this section we consider iterative schemes for approximating fixed points of closed-valued strict contractions in metric spaces.
We begin with a few notations.
Throughout this paper, Open image in new window is a complete metric space.
For each pair of nonempty Open image in new window , put
where Open image in new window is a constant.
In view of (2.8) and (2.10), inequality (2.11) holds for Open image in new window .
We claim that
as claimed. Theorem 2.1 is proved.
3. Convergence to an Attractor of a Nonexpansive Mapping
In this section we show that if for any initial point, there exists a trajectory of the dynamical system induced by a nonexpansive set-valued mapping Open image in new window , which converges to an invariant set Open image in new window , then a convergent trajectory also exists for a nonstationary dynamical system induced by approximations of Open image in new window .
We begin the proof of Theorem 3.1 with two lemmata.
We are now going to define by induction a sequence Open image in new window .
(Clearly, this assumption holds for Open image in new window .)
By (3.11) and (3.1),
Lemma 3.2 is proved.
Lemma 3.3 is proved.
Completion of the Proof of Theorem 3.1
Theorem 3.1 is proved.
This research was supported by the Israel Science Foundation (Grant no. 647/07), the Fund for the Promotion of Research at the Technion, and by the Technion President's Research Fund.
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