Abstract
We give new and simpler proof for the undecidability of the n-permutation Post Correspondence Problem that was originally proved by K. Ruohonen [10]. Our proof uses a recent result on deterministic semi-Thue systems according to which it is undecidable for a given deterministic semi-Thue system T and a word u whether or not there exists a nonempty cyclic derivation u→T+u in T.
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