I think I'm able to prove NTIME($n^\alpha$) $\subset$ EXPTIME for arbitrary $\alpha$.

  • Is this a new result?
  • If it was, would there be a way to deduce NP $\subset$ EXPTIME from it?
  • $\begingroup$ It’s knows that $\mathsf{NTIME}(f(n)) \subseteq \mathsf{DTIME}(2^{f(n)})$, and this implies that $\mathsf{NP} \subseteq \mathsf{EXPTIME}$. $\endgroup$ Commented Jan 9, 2019 at 6:06
  • $\begingroup$ With "$\subset$" do you mean a proper inclusion or is equality allowed? (In the latter case, see Yuval Filmus' comment.) $\endgroup$
    – dkaeae
    Commented Jan 9, 2019 at 8:43
  • $\begingroup$ Assuming strict inclusion, I'd lean towards "no" for the second question. From $A \subset X$ and $B \subset X$ we can only conclude the loose inclusion $A \cup B \subseteq X$, and not the stricter $A \cup B \subset X$, in general. $\endgroup$
    – chi
    Commented Jan 9, 2019 at 13:08
  • $\begingroup$ Yes, I meant proper inclusion. $\endgroup$ Commented Jan 9, 2019 at 22:54
  • $\begingroup$ $\subset$ is ambiguous: it's better to use $\subseteq$ for the "maybe equal" version and $\subsetneq$ (\subsetneq) for the "definitely not equal" version. $\endgroup$ Commented Jan 10, 2019 at 0:03

2 Answers 2


Upon reflection this is an easy consequence of the Time Hierarchy Theorem.

  • NTIME$(n^\alpha) \subseteq$ TIME$\left(2^{n^\alpha}\right)$ by brute force
  • TIME$\left(2^{n^\alpha}\right) \subset$ TIME$\left(2^{n^{2\alpha}}\right)$ by the Time Hierarchy Theorem
  • TIME$\left(2^{n^{2\alpha}}\right) \subseteq$ EXPTIME obviously

So there's nothing new here.


As you've already shown, $\mathrm{NTIME}[n^\alpha]\subsetneq\mathrm{EXPTIME}$ isn't new, as it follows easily from the time hierarchy theorem.

Furthermore, the fact that, for some class $X$, $\mathrm{NTIME}[n^\alpha]\subsetneq X$ for all $\alpha$ doesn't imply that $\mathrm{NP}\subsetneq X$. For example, the time hierarchy theorem tells us $\mathrm{NTIME}[n^\alpha]\subsetneq\mathrm{NP}$ for all $\alpha$ but that certainly doesn't imply that $\mathrm{NP}\subsetneq\mathrm{NP}\,$!


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