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The Post's Correspondence Problem or PCP is the following: given arbitrary lists A and B of r strings each in [{0, 1}.sup.+], say
The Post's Correspondence Problem is undecidable [Hopcroft and Ullman 1979].
The undecidability result is shown by a reduction from a variant of Post's Correspondence Problem (PCP).
An instance of Post's Correspondence Problem, or PCP, consists of two lists of strings, X and Y, where X and Y each consist of k strings in [{0, 1}.sup.+]: