For $L_S=\{\langle M\rangle : L(M)\in S \}$ what is known about $S$ in case of:

  1. $L_S\in RE$

  2. $L_S\in R$


2 Answers 2


Rice's theorem states that if $L_S$ is not trivial (i.e., is not $\varnothing$ nor all languages) then $L_S$ can't be decided (it might be computably enumerable, though).

An extension to Rice's theorem states that $L_S$ is computably enumerable if and only if after replacing $S$ with $S\cap RE$, all the following hold.

  1. For all $L_1, L_2$ computably enumerable, if $L_1 \in S$ and $L_1 \subseteq L_2$, then $L_2 \in S$.

  2. If $L \in S$, there is a finite subset $L' \subseteq L$ so that $L' \in S$.

  3. The set of finite languages in $S$ is computably enumerable.

Proofs are given at here.


Take a look at Rice's theorem and its extensions.

Basically, Rice's theorem and its extension state:

  1. If $\emptyset\neq S\subset RE$ then $L_S\notin R$
  2. If $\emptyset \neq S\subset RE$ and $\Sigma^*\notin S$ (please fix me if this is wrong, this is what I remember) then $L_S\notin RE$
  • 1
    $\begingroup$ I believe this is a correct answer. Can you explain why the downvote? Maybe consider adding a comment so that I can fix the answer if it isn't correct. $\endgroup$
    – nir shahar
    Commented Jul 12, 2021 at 6:03

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