Note: logs below are base 2. (Not sure how to do subscripts here)

Wondering if the below equation is true when thinking asymptotically (Computer Science)

$log_2((n!)^n) = O(n \sin(n \frac{\pi}{2}) + \log_2{n})$

But I'm not sure how to compute this.

I'm guessing we need to take the log of both sides of the following equation:

$log_2((n!)^n) < n sin(n (pi/2)) + log_2(n)$

getting us:

$log_2(log_2((n!)^n)) < log_2 (n sin(n (pi/2)) + log_2(n) )$

Not sure where to go from there.

  • 1
    $\begingroup$ You should know that all log have the same big O limit, as they differ by a constant. Secondly, an underscore (within math mode, that is $ signs) will get you to subscript. $\endgroup$
    – Asaf Karagila
    Sep 8 '10 at 0:57
  • 2
    $\begingroup$ Also, I'd try and use Stirling's approximation for $n!$ and the fact that $log(n!^n) = nlog(n!)$, and try to work it from there. Or something like that. $\endgroup$
    – Asaf Karagila
    Sep 8 '10 at 1:00
  • $\begingroup$ Thanks, I tried to make it look nicer with your suggestions...hope it helps the readability. $\endgroup$
    – Sev
    Sep 8 '10 at 1:04

If you are interested in computer science, a very useful thing to remember is that

$$ \log n! = \theta(n \log n) $$

Now given this, can you tell what $f(n)$ is, if $ \log (n!^{n}) = \theta(f(n))$?

How would that compare to the right hand side?

  • $\begingroup$ Theta(n log(n!)) -- but in my equation we need to take the log of that again, which gives us log(n log(n!))...any tips on where to go from there? $\endgroup$
    – Sev
    Sep 8 '10 at 2:46
  • $\begingroup$ @Sev: Why do you need to take log again? what is $\log (a^n)$? $\endgroup$
    – Aryabhata
    Sep 8 '10 at 2:50
  • $\begingroup$ because originally it was log (log((n!)^n)) -- now i see that log(n!^n) is n log n! but what about the outer log? $\endgroup$
    – Sev
    Sep 8 '10 at 2:53
  • $\begingroup$ @Sev: If log (n!) ~ nlogn, what is nlog(n!)? What is the log of that? $\endgroup$
    – Aryabhata
    Sep 8 '10 at 2:55
  • $\begingroup$ @Moron: nlog(n!) must be n* nlogn which is n^2 log n? $\endgroup$
    – Sev
    Sep 8 '10 at 2:58

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