Uniqueness of Normal Forms for Shallow Term Rewrite Systems

Abstract: Uniqueness of normal forms ($UN^=$) is an important property of term rewrite systems. $UN^=$ is decidable for ground (i.e., variable-free) systems and undecidable in general. Recently it was shown to be decidable for linear, shallow systems. We generalize this previous result and show that this property is decidable for shallow rewrite systems, in contrast to confluence, reachability and other properties, which are all undecidable for flat systems. Our result is also optimal in some sense, since we prove that the $UN^=$ property is undecidable for two classes of linear rewrite systems: left-flat systems in which right-hand sides are of depth at most two and right-flat systems in which left-hand sides are of depth at most two.