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I know that if L is decidable and L' is acceptable, than L is not-acceptable. Is it also true to say that if L is decidable and acceptable, than L' is not-acceptable? I'm sorry if this question sounds dumb, but I have a test on these stuff in 2 days and I just wanna be 100% sure.

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Both statements are wrong. True is this: If L is acceptable but not decidable, then L' is not acceptable. –  Raphael Jan 29 '12 at 15:26
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1 Answer 1

Your initial statement

if L is decidable and L' is acceptable, then L is not-acceptable

Is incorrect. If L is decidable, then L' is also decidable, because the decidable languages are closed under complement. Since every decidable language is also acceptable, the above statement is incorrect.

Consequently, the statement

if L is decidable and acceptable, then L' is not-acceptable

Is also incorrect, because if L is decidable, so is L'. Consequently, L' is acceptable as well.

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