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+19 votes

Let $L$ be a language and $\bar{L}$ be its complement. Which one of the following is NOT a viable possibility?

  1. Neither $L$ nor  $\bar{L}$ is recursively enumerable $(r.e.)$. 
  2. One of $L$ and $\bar{L}$ is r.e. but not recursive; the other is not r.e.
  3. Both $L$ and $\bar{L}$ are r.e. but not recursive.
  4. Both $L$ and $\bar{L}$ are recursive.
asked in Theory of Computation by Veteran (99.8k points)
edited by | 1.5k views

2 Answers

+21 votes
Best answer

(C) is not possible. If $L$ is re we have a TM that accepts string in $L$. If $L$' is re, we have a TM that accepts strings in $L$'. So, using both these TMs we can make a new TM M which accepts strings in $L$ and rejects strings in $L$' - that is $M$ decides $L$, making $L$ recursive.

answered by Veteran (355k points)
edited by
Can you give an example for option A?
@Shalini If $L$ itself is not RE then $L'$ is also not RE
IF L is recursive then L' is also Recursive
IF L is Recursively enumerable then L' is NOT recursively enumerable .

Correct me if wrong...
That seems correct
0 votes
A) It is possible if L itself is NOT RE. Then L' will also not be RE. B) Suppose there is a language such that turing machine halts on the input. The given language is RE but not recursive and its complement is NOT RE. C) This is not possible because if we can write enumeration procedure for both languages and it's complement, then the language becomes recursive. D) It is possible because L is closed under complement if it is recursive.   Thus, C is the correct choice.
answered by Loyal (8.3k points)

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