$NP = \cup_{k \in \mathbb{N}} NTIME(n^k)$

$P = \cup_{k \in \mathbb{N}} TIME(n^k)$

Can we show that $NTIME(n^k) = TIME(n^k)$ for a specific $k$?

For how large of a $k$ can we show the above statement to be true?

  • 4
    $\begingroup$ What have you tried and where did you get stuck? (Come on people, how did this get six upvotes? It's a problem dump like any other.) $\endgroup$
    – Raphael
    Nov 5, 2015 at 20:31
  • 1
    $\begingroup$ Well, $k=0$ seems like an easy place to start... $\endgroup$ Nov 5, 2015 at 20:59
  • $\begingroup$ @Raphael I just learned about this today. This is not part of a homework problem (if that's what you mean by "problem dump"). I barely have the mathematical background at this point to solve this - I was just curious. $\endgroup$
    – pushkin
    Nov 5, 2015 at 21:07
  • 3
    $\begingroup$ See this prior answer, which is essentially a duplicate of the current Question, ( cstheory.stackexchange.com/questions/1079 ) for $k=1$. $\endgroup$ Nov 5, 2015 at 21:10
  • 4
    $\begingroup$ Our policy generally doesn't depend on whether it was assigned to you as a homework. "Problem dump" means that the question provides only the statement of a problem, without showing anything about what you've tried or where you got stuck and without formulating a specific question about some aspect of the problem. Such questions are often discouraged, for reasons explained here. It's often better to make a serious attempt before asking, and show us what approaches you tried. $\endgroup$
    – D.W.
    Nov 6, 2015 at 0:31

1 Answer 1


If $\mathsf{NTIME}(n^k) \subseteq \mathsf{TIME}(n^\ell)$ for any $k,\ell$ then $\mathsf{P} = \mathsf{NP}$. Indeed, any problem $L \in \mathsf{NP}$ can be solved in non-deterministic time $O(n^r)$ for some $r$. Consider now the problem $L' = \{0^{|x|^{r/k}}1x : x \in L\}$. Clearly this problem is still in $\mathsf{NP}$, and furthermore the previous algorithm solves it in non-deterministic time $O(n^k)$. Therefore $L'$ has a deterministic algorithm running in time $O(n^\ell)$, implying that $L$ has a deterministic algorithm running in time $O(n^{r(\ell/k)})$.

This trick is known as padding.


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