$ Fin = \{ L \in \Sigma^* : |L| $ is finite and greater than 0 $ \} $ Proof or disproof Fin = Fin-Complete Where Fin-Complete means that for every $ L_1,L_2 \in Fin $ there exist a valid reduction $ L_1 \leq L_2 $

my trial:

Consider two languages $ L_1 $ and $ L_2 $ where $ L_1 $ contains a single string s $ L_1 = \{ s \} \ s \in \Sigma^* $ and $ s \neq \epsilon $ a finite language of size 1. Let $ L_2 = \emptyset $ the empty language, which is also finite by definition as it contains zero strings. By the definition of reduction, $ L_1 \leq L_2 $ there must exist a computable function $ f $ such that for any string $ w \in L_1 $ if and only if $f(w) \in L_2 $ however since $ L_2 = \emptyset $ there are no strings in $ L_2 $ for $ f(x) $ to map to construct such a function f that satisfies the condition for all $ w\in L_1 $

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    $\begingroup$ By your definition Fin does not contain the empty set: $|L| > 0$. $\endgroup$ Feb 2 at 10:20
  • $\begingroup$ right i miss that thank u $\endgroup$
    – maya cohen
    Feb 2 at 10:58

1 Answer 1


The claim is correct. To see why, note that every language in $L\in Fin$ is non-trivial and regular. Indeed, $L$ is not empty, by the definition of $Fin$, and does not equal $\Sigma^*$ as then it would be infinite. Now the claim follows from the fact that every regular language is decidable, and the fact that every non-trivial language is $R$-hard. Specifically, for every two languages $L_1, L_2 \in Fin$, we have that $L_1 \in \text{REG} \subseteq \text{R}$ and $L_2 \notin \{ \emptyset, \Sigma^*\}$. Hence, we conclude that $L_1\leq_m L_2$.


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