I would like to determine if the following language is decidable or not. L = { w $\in$ $\Sigma^*$ | $T(M_w)$ is recognized by a Turing machine with at most 42 states}.

I know that every finite language is decidable, but I am not sure if this has anything to do with this particular problem.

Every help is appreciated, I am a little lost.

Thank you

  • 2
    $\begingroup$ What is T(M)? Is it the language generated from the turing machine M? $\endgroup$
    – nir shahar
    May 28, 2020 at 14:23

1 Answer 1


Define C = { $L | L$ is decidable by a turing machine with at most 42 states }

Notice that there is a finite amount of such languages, as there are a finite amount of turing machines with 42 states. for that reason, we have $C\ne\emptyset, \sum^*$

By Rice's theorem, the language $S=$ { $w | L(M_w) \in C$ } is undecidable. This language is precisely the language in question

  • $\begingroup$ Rice's theorem does not apply here. $\endgroup$
    – cody
    Jun 28, 2020 at 3:39
  • $\begingroup$ Why? The conditions to it are obviously met $\endgroup$
    – nir shahar
    Jun 28, 2020 at 6:30

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