Undecidability of Confluence for Binary Length-Reducing Cycle Rewriting

Abstract: Confluence guarantees that diverging rewrite choices can always be rejoined. For finite terminating string- and term-rewriting systems, confluence is decidable by critical-pair analysis, and in polynomial time for length-reducing strings. For finite terminating (hyper)graph transformation systems, in contrast, confluence is undecidable. We show that undecidability already appears for words on a circle, that is, strings up to rotation. Confluence of finite cycle-rewriting systems over the fixed alphabet ${0,1}$ is undecidable, indeed $Π<sup>0_1$-complete, even when every rule has a nonempty right-hand side and strictly reduces length. Under this restriction termination is syntactically evident, and derivations from a nonempty length-$n$ cycle have fewer than $n$ steps. The same holds over every fixed alphabet with at least two letters, while the one-letter case is decidable. Rotation alone separates cyclic from string rewriting. The proof compiles a deterministic verifier into a weighted cycle system with one controlled branch, then into a binary length-reducing system via a run-length code whose cleanup rules send every reducible malformed cycle to one error normal form.