Let $L$ be a language over a finite alphabet $A$. A language $L'$ over the same alphabet $A$ is called a sublanguage of $L$ if $L' \subset L$. Assume that $L$ is Turing-decidable. Does it follow that all of its sublanguages $L'$ are likewise Turing-decidable?
2026-03-26 01:07:25.1774487245
If a language is Turing decidable, does that make the sublanguages also Turing decidable
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Take $L$ to be the language with every single possible word. It is trivially Turing-decidable (just take a machine that always says "yes").
But obviously some sublanguages are not decidable, since the sublanguages of $L$ are all languages.