Stirling's Formula for the Gamma Function

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Theorem

Let

$D_\epsilon = \{z \in \C : |\arg(z)| < \pi - \epsilon,\ |z| > 1\}$

Then for any $\epsilon > 0$ the gamma function satisfies:

$\displaystyle \Gamma(z) = \sqrt{\frac{2\pi}z}\left( \frac ze\right)^z\left(1 + \mathcal O\left( z^{-1} \right) \right)$

for all $z \in D_\epsilon$.


In logarithmic form the error term is given in the following:

$\displaystyle \log \Gamma(z) = \left(z - \frac12\right)\log z - z + \frac{\log 2\pi}2 + \sum_{n = 1}^{d-1} \frac{B_{2n}}{2n(2n-1)z^{2n-1}} + \mathcal O\left( z^{1-2d} \right)$

Where $B_{2k}$ are the Bernoulli numbers.

Proof



Source of Name

This entry was named for James Stirling.

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