For $f(n)\ge n$,

$$\mathsf{DSPACE}(f(n)) \subseteq \mathsf{DTIME}(2^{O(f(n))}).$$

Is there any function $f$ for which this containment is known to be proper?

  • 3
    $\begingroup$ Hmmm, $f(n) = O(1)$ :). $\endgroup$
    – R B
    Commented Jan 15, 2014 at 7:29
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    $\begingroup$ @RB The containment is not proper for $f(n)=O(1)$. In fact, there isn't containment at all: constant space includes all regular languages; constant time includes only those languages decidable by looking at a constant prefix of the string. $\endgroup$ Commented Jan 15, 2014 at 9:45
  • $\begingroup$ Yes, you're absolutely right, the containment is only valid for $f(n)\ge n$. I have corrected the question. Thanks! $\endgroup$
    – Siddharth
    Commented Jan 16, 2014 at 5:39
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    $\begingroup$ I'm pretty sure it's valid for $\:\: f(n) \: \geq \: c\hspace{-0.03 in}\cdot \hspace{-0.03 in}\log(n)\;\;$. $\;\;\;\;\;\;$ $\endgroup$
    – user6973
    Commented Jan 16, 2014 at 7:33
  • $\begingroup$ What precisely do you mean with $DTIME(2^{O(f(n))})$? Do you mean that for every $f$ there is some constant $C_f$ such that $DSPACE(f(n)) \subseteq DTIME(2^{C_f f(n)})$? $\endgroup$ Commented Jan 17, 2014 at 21:19

1 Answer 1


If $\mathsf{L} = \mathsf{P}$ then $\mathsf{DSPACE}(O(f(n))) = \mathsf{DTIME}(2^{O(f(n)})$ for all time-constructible functions $f(n) \geq \log n$ using standard padding arguments. I am not sure what happens if $f(n)$ is not time-constructible.

If $\mathsf{L} \neq \mathsf{P}$ then $f(n) = \log n$ will be an example. Therefore there is a time-constructable function $f(n)$ where the containment is proper iff the containment is proper for $f(n) = \log n$ .


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