Generic properties of nonexpansive mappings on unbounded domains

TL;DR

This paper studies the typical behavior of nonexpansive mappings on unbounded hyperbolic metric spaces, revealing that most have unique fixed points locally but are not globally contractions, with properties depending on the convergence topology.

Contribution

It characterizes the generic properties of nonexpansive mappings on unbounded domains, including fixed point existence and contraction behavior under different metric topologies.

Findings

Typical nonexpansive mappings have unique fixed points on bounded subsets.

Most mappings are not global Rakotch contractions, having Lipschitz constant one on residual sets.

The set of Rakotch contractions is meagre under pointwise convergence topology.

Abstract

We investigate typical properties of nonexpansive mappings on unbounded complete hyperbolic metric spaces. For two families of metrics of uniform convergence on bounded sets, we show that the typical nonexpansive mapping is a Rakotch contraction on every bounded subset and that there is a bounded set which is mapped into itself by this mapping. In particular, we obtain that the typical nonexpansive mapping in this setting has a unique fixed point which can be reached by iterating the mapping. Nevertheless, it turns out that the typical mapping is not a Rakotch contraction on the whole space and that it has the maximal possible Lipschitz constant of one on a residual subset of its domain. By typical we mean that the complement of the set of mappings with this property is $\sigma$-$\phi$-porous, that is, small in a metric sense. For a metric of pointwise convergence, we show that the set of Rakotch contractions is meagre.