Abstract

We give simpler proof for the undecidability of the circular Post Correspondence Problem that was originally proved undecidable by Ruohonen (Acta Informatica 19:357---367, 1983). The key feature of our proof is the undecidability of the word problem of special semi-Thue systems where derivations are deterministic and reversible for words containing a single occurrence of a letter from a given set.

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