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This question most likely has a simple answer; however, I do not see it.

Let $g:\mathbb{N} \rightarrow \mathbb{N}$ be an uncomputable function and $c$ a positive real number. Can there be a computable function $f : \mathbb{N} \rightarrow \mathbb{N}$ such that, for all $n$ large enough: $g(n) \leq f(n) \leq c \cdot g(n)$ (that is $f(n) = \Theta(g(n)$)?

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  • $\begingroup$ I assume that (i) we are to find f that depends on g and c, and (ii) c must be greater than 1. $\endgroup$ Sep 3, 2010 at 16:53
  • $\begingroup$ I interpreted the question as: for every g and c s.t. ... there is an f s.t. ... $\endgroup$
    – Kaveh
    Sep 3, 2010 at 16:56
  • $\begingroup$ But Noam's interpretation also makes sense: is there g, c, and f s.t. ... . and is more reasonable considering the title. $\endgroup$
    – Kaveh
    Sep 3, 2010 at 17:04
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    $\begingroup$ This can be a nice exercise in a "computability course": show that for some g there exist such f and for others not... $\endgroup$
    – Noam
    Sep 3, 2010 at 18:01
  • $\begingroup$ @Kaveh: I don't see it. At least I see the closeness to the title, but "Let g:N→N be an uncomputable function" is well-established terminology unambiguously indicating that g is an unknown. $\endgroup$ Sep 3, 2010 at 19:26

2 Answers 2

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Sure: just take g(n) = n + halt(n) (where halt(n)=1 if TM number n halts, and 0 ow).

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  • $\begingroup$ I think you don't get to choose g. $\endgroup$ Sep 3, 2010 at 16:50
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    $\begingroup$ I take "can there be" to ask whether for some g there exists an f. For the interpretation of whether for every g there exists an f, you get a negative answer by taking g=n^{halt(n)}. $\endgroup$
    – Noam
    Sep 3, 2010 at 17:52
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It’s not clear to me whether you want a particular pair of functions f and g with the desired property, or if you’re asking if, for every uncomputable g, there exists a computable f ∈ Θ(g).

In the first case, Noam’s answer applies. Otherwise, the answer is no: just let g be growing faster than every computable function (standard example: the busy beaver function).

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  • $\begingroup$ +1 Thanks, for the answer. The interpretation I had intended was the former, whether such a $f$ and $g$ exist $\endgroup$ Sep 3, 2010 at 18:57

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