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?

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