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Levels of Undecidability in Rewriting
, 2011
"... Undecidability of various properties of first order term rewriting systems is wellknown. An undecidable property can be classified by the complexity of the formula defining it. This classification gives rise to a hierarchy of distinct levels of undecidability, starting from the arithmetical hierarc ..."
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Undecidability of various properties of first order term rewriting systems is wellknown. An undecidable property can be classified by the complexity of the formula defining it. This classification gives rise to a hierarchy of distinct levels of undecidability, starting from the arithmetical hierarchy classifying properties using first order arithmetical formulas, and continuing into the analytic hierarchy, where quantification over function variables is allowed. In this paper we give an overview of how the main properties of first order term rewriting systems are classified in these hierarchies. We consider properties related to normalization (strong normalization, weak normalization and dependency problems) and properties related to confluence (confluence, local confluence and the unique normal form property). For all of these we distinguish between the single term version and the uniform version. Where appropriate, we also distinguish between ground and open terms. Most uniform properties are Π 0 2complete. The particular problem of local confluence turns out to be Π 0 2complete for ground terms, but only Σ 0 1complete (and thereby recursively enumerable) for open terms. The most surprising result concerns dependency pair problems without minimality flag: we prove this problem to be Π 1 1complete, hence not in the arithmetical hierarchy, but properly in the analytic hierarchy. Some of our results are new or have appeared in our earlier publications [35, 7]. Others are based on folklore constructions, and are included for completeness as their precise classifications have hardly been noticed previously.
Degrees of Undecidability in Rewriting
, 902
"... Abstract. Undecidability of various properties of first order term rewriting systems is wellknown. An undecidable property can be classified by the complexity of the formula defining it. This gives rise to a hierarchy of distinct levels of undecidability, starting from the arithmetical hierarchy cl ..."
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Abstract. Undecidability of various properties of first order term rewriting systems is wellknown. An undecidable property can be classified by the complexity of the formula defining it. This gives rise to a hierarchy of distinct levels of undecidability, starting from the arithmetical hierarchy classifying properties using first order arithmetical formulas and continuing into the analytic hierarchy, where also quantification over function variables is allowed. In this paper we consider properties of first order term rewriting systems and classify them in this hierarchy. Weak and strong normalization for single terms turn out to be Σ 0 1complete, while their uniform versions as well as dependency pair problems with minimality flag are Π 0 2complete. We find that confluence is Π 0 2complete both for single terms and uniform. Unexpectedly weak confluence for ground terms turns out to be harder than weak confluence for open terms. The former property is Π 0 2complete while the latter is Σ 0 1complete (and thereby recursively enumerable). The most surprising result is on dependency pair problems without minimality flag: we prove this to be Π 1 1complete, which means that this property exceeds the arithmetical hierarchy and is essentially analytic. 1
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"... Abstract. As a window into the subject, we recount some of the history (and geography) of two mature, challenging, partially open, partially closed problems in the theory of rewriting (numbers 13 and 21 from the original RTA List of Open Problems). One problem deals with (criteria for leftlinear) c ..."
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Abstract. As a window into the subject, we recount some of the history (and geography) of two mature, challenging, partially open, partially closed problems in the theory of rewriting (numbers 13 and 21 from the original RTA List of Open Problems). One problem deals with (criteria for leftlinear) confluence and the other with termination (of one linear or string rule), the two paradigmatic properties of interest for rewrite systems of any flavor. Both problems were formulated a relatively long time ago, have seen considerable progress, but remain open. We also venture to contemplate the future evolution and impact of these investigations. 1