The Contraction Property is Sufficient to Guarantee the Uniqueness of Fixed Points of Endofunctors in a Category of Complete Metric Spaces

In de Bakker and Zucker proposed to use complete metric spaces for the semantic definition of programming languages that allow for concurrency and synchronisation. The use of the tools of metric topology has been advocated by Nivat and his colleagues already in the seventies and metric topology was successfully applied to various problems (12, 13). Recently, the question under which circumstances fixed point equations involving complete metric spaces can be (uniquely) solved has attracted attention, e.g. (1,11). The solution of such equation provides the basis for the semantics of a given language and is hence of practical relevance. In (1), a criterion for the existence of a solution, namely that the respective functor is contracting, is provided. This property together with an additional criterion, namely that the respective functor is hom-contracting, was shown in (1) to guarantee uniqueness. In this paper we show that the contraction property is already sufficient to guarantee the uniqueness.