Foxmaths! 2.0

January 9, 2008

Approximating Factorials: A Neat Limit

Filed under: Maths — Tags: , , , , — Fox @ 7:52 am

It becomes of interest, in a number of areas, to be able to approximate, for a given value of n, the value of

\frac{(2n)!}{ {n!}^2 }

This is a difficult expression, as are most all expressions involving factorial, so it is convenient to have a good approximation.

Playing around with logs and integrals, I was able to derive the following approximation, that

\frac{(2n)!}{ {n!}^2 } \approx 2^n (1+n)^{-1-n} (1+2 n)^{\frac{1}{2}+n}

Indeed, for large n, the two functions are practically proportional to each other. And, most interestingly of all, according to Mathematica, calculating out that limit,

lim_{n \rightarrow \infty} \frac{ 2^n (1+n)^{-1-n} (1+2 n)^{\frac{1}{2}+n} }{ (2n)!/{n!}^2 } = \sqrt{ \frac{2 \pi}{e} }

Neat, no?

3 Comments »

  1. Hi, just wanted to comment on the new look. your white-on-black math expressions look kind of funny in e.g. google reader when shown against a white background.

    any chance the white-on-black appearance of latex formulas could be put in the css stylesheet of your blog so that readers of the RSS feed would get the ’standard’ black-on-white appearance?

    Comment by anders — January 9, 2008 @ 2:01 pm

  2. I’m not exactly sure how I would go about that. CSS isn’t really my thing, and I’m not exactly sure how LaTeX is implemented on wordpress, so … the spirit is willing, but I sort of fail.

    Comment by Fox — January 10, 2008 @ 7:19 am

  3. You can derive that ratio limit pretty easily using Stirling’s approximation.

    Comment by twistedone151 — May 11, 2008 @ 9:40 am


RSS feed for comments on this post. TrackBack URI

Leave a comment

Blog at WordPress.com.