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If $x! = N^{\log N}\;,$ How can I estimate $x$ in terms of $N$?

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    You can try [Stirling's approximation formula](http://en.wikipedia.org/wiki/Stirling%27s_approximation) for $x!$ and take logarithms (which will reduce the right hand side to $(\log N)^2$).2012-02-20

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If we had an approximation for the inverse gamma function $\Gamma^{-1}$, we could apply it to both sides to get $x \approx \Gamma^{-1} \left( N^{\log{N}} \right)$

An approximation of $\Gamma^{-1}$ can be found here.