I came across following problem:

  1. $L=\{M$ is a turing machine $M$ accepts two strings of different length $\}$
  2. $L=\{M$ is a turing machine $M$ accepts atleast two strings of different length $\}$

Which one Recursively Enumerable (RecEn)? Which one Recursive (Rec)?

I feel both are not recursively enumerable and hence not recursive. We may list strings in some order and run Turing machine on them. But we may never come across two strings of same length accepted by same TM. So we cannot answer both questions "accepts two strings of different length" and "acceptes at least two strings of different length". Am I correct? Also is my approach correct?

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1 Answer 1


If (1) means exactly two strings of different lengths, not r.e. (2) is r.e.: guess two strings, check their lengths are different, verify that $M$ accepts both.

Use the extension to Rice's theorem to prove (1) isn't r.e.


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