The symmetric Post Correspondence Problem, and errata for the freeness problem for matrix semigroups

12/04/2021

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by   J. C. Birget, et al.

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We define the symmetric Post Correspondence Problem (PCP) and prove that it is undecidable. As an application we show that the original proof of undecidability of the freeness problem for 3-by-3 integer matrix semigroups works for the symmetric PCP, but not for the PCP in general.