On tree automata that certify termination of left-linear term rewriting systems

AbstractWe present a new method for automatically proving termination of left-linear term rewriting systems on a given regular language of terms. It is a generalization of the match bound method for string rewriting. To prove that a term rewriting system terminates we first construct an enriched system over a new signature that simulates the original derivations. The enriched system is an infinite system over an infinite signature, but it is locally terminating: every restriction of the enriched system to a finite signature is terminating. We then construct iteratively a finite tree automaton that accepts the enriched given regular language and is closed under rewriting modulo the enriched system. If this procedure stops, then the enriched system is compact: every enriched derivation involves only a finite signature. Therefore, the original system terminates. We present two methods to construct the enrichment: roof heights for left-linear systems, and match heights for linear systems. For linear systems, the method is strengthened further by a forward closure construction. Using these methods, we give examples for automated termination proofs that cannot be obtained by standard methods