Prove that $\log(n!)\leq(\log(n))!$

My attempt:

I read somewhere that $n\leq\log(n!)\leq(\log(n))!$. But when I used calculator $\log(n!)$ can not be less than or equal to $(\log(n))!$.

Can you explain it in a formal way, please?

  • 2
    $\begingroup$ If you meant $\;\left(\log n\right)!\;$ then this number can only be taken via the Gamma Function, as it usually isn't a natural number. Did you mean this? Or perhaps there's a floor function there? $\endgroup$ – DonAntonio Jul 1 '16 at 9:22
  • $\begingroup$ Yes, $(\log(n))!$ $\endgroup$ – 1 0 Jul 1 '16 at 9:24
  • $\begingroup$ @Joanpemo I think it does not make great difference when $n$ is large. The bound is quite loose. $\endgroup$ – Brian Cheung Jul 1 '16 at 9:25
  • 1
    $\begingroup$ To be precise we should either write $\lfloor log(n) \rfloor !$ or $\Gamma(log(n) + 1)$ but I don't think it'll really matter. $\endgroup$ – Zubzub Jul 1 '16 at 9:28
  • 1
    $\begingroup$ I do think that the way to go here is en.wikipedia.org/wiki/Stirling%27s_approximation . Note that the first inequality is an immediate consequence $\endgroup$ – b00n heT Jul 1 '16 at 9:30

Let $x=\log(n)$


and the results follow by Factorial grows faster than Exponential.

  • 1
    $\begingroup$ Thanks for nice explanation. $\endgroup$ – 1 0 Jul 1 '16 at 9:41

As I suggested, using Strirling approximation is a very loose way, one immediately gets $$\log(n!)\approx n\log n$$ and $$\log(n)! \approx (\log(n))^{\log(n)}.$$ Cleraly the second dominates the first term, giving the claim.

  • $\begingroup$ Please, clear little bit this $$\log(n)! \sim (\log(n))^{\log(n)}.$$ $\endgroup$ – 1 0 Jul 1 '16 at 9:45
  • $\begingroup$ Just set $n=\log n$ in Stirling's formula $n! \sim \sqrt{2 \pi n}\left(\frac{n}{e}\right)^n$ and forget about the other terms $\endgroup$ – b00n heT Jul 1 '16 at 9:47
  • $\begingroup$ What is interesting (at least to me) is that $\log(x!) \gt \log(x)!$ holds if $x \gt 2.24836$. $\endgroup$ – Claude Leibovici Jul 1 '16 at 9:49
  • $\begingroup$ Wait... If what you claim is true, then this should contradict what I am saying... or would it? $\endgroup$ – b00n heT Jul 1 '16 at 9:50
  • $\begingroup$ @b00nheT I think you are correct. Put some numbers much larger to check it. This inequality describes asymptotic behavior so some small numbers may contradict the result. $\endgroup$ – Brian Cheung Jul 1 '16 at 9:56

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