Is the language $S = \{\langle M \rangle \mid M \text{ is a Turing Machine and } L(M) = \{\langle M \rangle\}\,\}$ decidable, recognizable and/or co-recognizable?

I tried diagonalization but can only prove that $R = \{\langle M \rangle \mid M \text{ is a Turing Machine and } \langle M \rangle \notin L(M)\}$ is not recognizable, which does not seem to help in this case...

  • 1
    $\begingroup$ I'd try a few attempts exploiting the s-m-n theorem and the 2nd recursion theorem to build quine-like TMs. It might lead to a useful m-reduction. (Why is this marked complexity-theory ?) $\endgroup$
    – chi
    Dec 16, 2015 at 20:51

1 Answer 1


Using the recursion theorem, for any Turing machine $T$ you can construct a Turing machine $M$ such that on input $\langle M \rangle$, $M$ executes $T$ (on the empty tape), and otherwise $M$ immediately rejects. You can use this to show that $S$ is not decidable. Using the same ideas you can explore its recognizability and co-recognizability.

  • $\begingroup$ If I understand correctly: If S is decidable, there is a decider for $S$ denoted as $H$. To obtain a contradiction, we construct a TM $B$ such that on input $w$, $B$ ignores $w$ and obtains a description of itself $\langle B\rangle$. Then $B$ run $H$ on $\langle B\rangle$. For the output, $B$ reject if $H(\langle B\rangle)$ accept and accept if $H(\langle B\rangle)$ reject. So the output of $B$ is always in contradiction with itself? $\endgroup$ Dec 17, 2015 at 2:19
  • $\begingroup$ This is one way, though you can avoid diagonalization. $\endgroup$ Dec 17, 2015 at 5:10
  • $\begingroup$ Any thoughts on recognizability though? I suspect it is not recognizable but have no idea about its proof. $\endgroup$ Dec 17, 2015 at 5:43
  • 2
    $\begingroup$ It's your exercise. I won't give further hints. Sometimes solving an exercise takes time. Be patient. $\endgroup$ Dec 17, 2015 at 5:45

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