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Prove that the following language is recursively enumerable: L = {<M,x> | Turing machine M enters the same configuration twice on input x}

I have tried to construct a TM that maintains the current configuration which is of the form (u,q,v) where uv is the current tape content and the tape head is over the first symbol of v and q is the current state. However this would require infinite states to maintain the current tape content. I am not sure how to start this?

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  • $\begingroup$ Given input <M, x>, could you construct a Turing machine which halts iff M enters the same configuration twice on input x? $\endgroup$ Apr 28, 2023 at 3:41
  • $\begingroup$ @CommandMaster That is the idea but how do you do that ? $\endgroup$
    – revision
    Apr 28, 2023 at 14:29

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Big Hint: You can simulate infinite, random access memory (RAM) using a turing machine (this would not require an infinite number of states).

As you mentioned, you can encode the current configuration when running $M$ on $x$, so if you can store configurations in your unlimited memory, do you see how you can detect when $M$ enters the same configuration?

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