Constant runtime complexity of term rewriting is semi-decidable

We prove that it is semi-decidable whether the runtime complexity of a term rewrite system is constant. Our semi-decision procedure exploits that constant runtime complexity is equivalent to termination of a restricted form of narrowing, which can be examined by considering finitely many start terms. We implemented our semi-decision procedure in the tool AProVE to show its efficiency and its success for systems where state-of-the-art complexity analysis tools fail.