15-453 Formal Languages, Automata, and Computation
Lecture 23: Post Correspondence Problem

We complete the discussion of mapping reducibility by showing that there are languages which are neither Turing-recognizable nor co-Turing-recognizable. We then introduce the Post Correspondence Problem and prove that it is undecidable by a simple reduction from the acceptance problem for Turing machines, using computation histories.

• Reading: Ch. 5.2, pp. 183-189
• Previous lecture: Mapping Reduction
• Next lecture: Tiling

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Frank Pfenning