Consider a language $L$ such that $L \subseteq \Sigma^*$, where the cardinality of $\Sigma$ is $1$ (i.e. the alphabet has only one symbol). E.g. $L \subseteq \{a\}^*$.

Can anything be said about the time complexity of $L$? Is it the case that $L \in P$?

I can't come up with any example of a language with a single symbol that is not in $P$.


1 Answer 1


The language $\{a^n : \text{ the $n$th Turing machine halts on the empty string} \}$ is undecidable.

In fact, a random unary language is undecidable almost surely, for the simple reason that there are only countably many computable languages.

  • $\begingroup$ Thanks, this makes sense. Would anything change if we restricted $L$ to be decidable? $\endgroup$
    – denidare
    Commented Nov 1, 2018 at 21:38
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    $\begingroup$ Perhaps you can search a bit on the Internet. You might find things like "if there exists a unary language that is NP-complete, then P = NP" and other interesting observations. $\endgroup$
    – Pål GD
    Commented Nov 1, 2018 at 22:03
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    $\begingroup$ Not really. You can ask whether the $n$th Turing machine halts within $T(n)$ steps on the empty string, which should be hard to determine given much less than $T(n)$ time. $\endgroup$ Commented Nov 1, 2018 at 22:33

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