Complexity of Normal Form Properties and Reductions for Term Rewriting Problems Complexity of Normal Form Properties and Reductions for Term Rewriting Problems

  • Authors:
  • Rakesh Verma

  • Affiliations:
  • (Correspd.) Computer Science Department, University of Houston, Houston, TX, 77204-3010, USA. rverma@uh.edu

  • Venue:
  • Fundamenta Informaticae
  • Year:
  • 2009

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Abstract

We present several new and some significantly improved polynomial-time reductions between basic decision problems of term rewriting systems. We prove two theorems that imply tighter upper bounds for deciding the uniqueness of normal forms (UN$^{=}$) and unique normalization (UN$^{→}$) properties under certain conditions. From these theorems we derive a new and simpler polynomial-time algorithm for the UN$^{=}$ property of ground rewrite systems, and explicit upper bounds for both UN$^{=}$ and UN$^{→}$ properties of left-linear right-ground systems. We also show that both properties are undecidable for right-ground systems. It was already known that these properties are undecidable for linear systems. Hence, in a sense the decidability results are "close" to optimal.