Modular Termination Proofs for Rewriting Using Dependency Pairs

Recently, Arts and Giesl developed the dependency pair approach which allows automated termination and innermost termination proofs for many term rewriting systems (TRSs) for which such proofs were not possible before. The motivation for this approach was that virtually all previous techniques for automated termination proofs of TRSs were based on simplification orderings. In practice, however, many rewrite systems are not simply terminating, i.e. their termination cannot be verified by any simplification ordering. In this paper we introduce a refinement of the dependency pair framework which further extends the class of TRSs for which termination or innermost termination can be shown automatically. By means of this refinement, one can now prove termination in a modular way. Thus, this refinement is inevitable in order to verify the termination of large rewrite systems occurring in practice. To be more precise, one may use several different orderings in one termination proof. Subsequently, we present several new modularity results based on dependency pairs. First, we show that the well-known modularity of simple termination for disjoint unions can be extended to DP quasi-simple termination, i.e. to the class of rewrite systems where termination can be shown automatically by the dependency pair technique in combination with quasi-simplification orderings. Under certain additional conditions, this new result also holds for constructor-sharing and composable systems. Second, the above-mentioned refinement of the dependency pair method yields new modularity criteria for innermost termination which extend previous results in this area considerably. In particular, existing results for modularity of innermost termination can easily be shown to be direct consequences of our new criteria.