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Unformatted text preview: • If A ≤ T B and B is decidable, then A is decidable • If A ≤ T B and A is undecidable, then B is undecidable • If A ≤ T B and B is Turingrecognizable, then A is Turingrecognizable • If A ≤ T B and A is nonTuringrecognizable, then B is nonTuringrecognizable November 8, 2005 Today we will do examples • Hint for problem 5.20 • Show that L1 = {<M>  M is a TM and M halts on at least one input} is undecidable • Is L2 = {<M,w,t>  M is a TM that accepts w in at least t steps} decidable? Prove your answer • From Sipser – 5.13, 5.14, 5.15, 5.24, 5.30 c...
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 Spring '11
 sm
 Category theory, Halting problem, Sipser, Turing reductions

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