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.