A Characterization of Weakly Church-Rosser not Church-Rosser Abstract Reduction Systems

ABSTRACT. Basic properties of Rewriting Systems can be stated in the framework of Abstract Reduction Systems (ARS). Properties like confluence (or Church-Rosser, CR) and weak confluence (or weak Church-Rosser, WCR) and their relationships can be studied in this setting: as a matter of fact, well-known counterexamples to the implication WCR $\Rightarrow$ CR have been formulated as ARS. In this paper, starting from the observation that such counterexamples are structurally similar, we set out a graph--theoretic characterization of WCR ARS which are not CR in terms of a suitable class of reduction graphs, such that in every WCR not CR ARS, we can embed at least one element of this class. Moreover, we give a tighter characterization for a restricted class of ARS enjoying a suitable regularity condition. Finally, as a consequence of our approach, we prove some interesting results about ARS using the mathematical tools developed. In particular, we prove an extension of Newman's lemma and we find out conditions that, once assumed together with WCR property, ensure the unique normal form property. The Appendix treats two interesting examples, both generated by graph-rewriting rules, with specific combinatorial properties.