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

  • Authors:
  • Alfons Geser;Dieter Hofbauer;Johannes Waldmann;Hans Zantema

  • Affiliations:
  • National Institute of Aerospace, Hampton, Virginia;Kassel, Germany;Fb IMN, Hochschule für Technik, Wirtschaft und Kultur (FH) Leipzig, Leipzig, Germany;Faculteit Wiskunde en Informatica, Technische Universiteit Eindhoven, Eindhoven, The Netherlands

  • Venue:
  • RTA'05 Proceedings of the 16th international conference on Term Rewriting and Applications
  • Year:
  • 2005

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Abstract

We present a new method for proving termination of term rewriting systems automatically. It is a generalization of the match bound method for string rewriting. To prove that a term rewriting system terminates on a given regular language of terms, 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 three methods to construct the enrichment: top heights, roof heights, and match heights. Top and roof heights work for left-linear systems, while match heights give a powerful method 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.