15-453 Formal Languages, Automata, and Computation
Lecture 20: Some Undecidable Problems

We practice the method of reduction by showing that various problems in the theory of languages are undecidable. In particular, the verify that we cannot check in general if the set of words accepted by a Turing machine is empty. We also employ a reduction to show that this problem is not even Turing recognizable.

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Frank Pfenning