Inverse gamma function

Not to be confused with Inverse-gamma distribution or Reciprocal gamma function.

Graph of an inverse gamma function

Plot of inverse gamma function in the complex plane

In mathematics, the inverse gamma function $\Gamma ^{-1}(x)$ is the inverse function of the gamma function. In other words, $y=\Gamma ^{-1}(x)$ whenever ${\textstyle \Gamma (y)=x}$. For example, $\Gamma ^{-1}(24)=5$. Usually, the inverse gamma function refers to the principal branch with domain on the real interval $\left[\beta ,+\infty \right)$ and image on the real interval $\left[\alpha ,+\infty \right)$, where $\beta =0.8856031\ldots$ is the minimum value of the gamma function on the positive real axis and $\alpha =\Gamma ^{-1}(\beta )=1.4616321\ldots$ is the location of that minimum.

Definition

The inverse gamma function may be defined by the following integral representation

$$\Gamma ^{-1}(x)=a+bx+\int _{-\infty }^{\Gamma (\alpha )}\left({\frac {1}{x-t}}-{\frac {t}{t^{2}-1}}\right)d\mu (t)\,,$$

where $\mu (t)$ is a Borel measure such that

$$\int _{-\infty }^{\Gamma \left(\alpha \right)}\left({\frac {1}{t^{2}+1}}\right)d\mu (t)<\infty \,,$$

and $a$ and $b$ are real numbers with $b\geqq 0$.

Approximation

To compute the branches of the inverse gamma function one can first compute the Taylor series of $\Gamma (x)$ near $\alpha$. The series can then be truncated and inverted, which yields successively better approximations to $\Gamma ^{-1}(x)$. For instance, we have the quadratic approximation:

$$\Gamma ^{-1}\left(x\right)\approx \alpha +{\sqrt {\frac {2\left(x-\Gamma \left(\alpha \right)\right)}{\Psi \left(1,\ \alpha \right)\Gamma \left(\alpha \right)}}}.$$

The inverse gamma function also has the following asymptotic formula

$$\Gamma ^{-1}(x)\sim {\frac {1}{2}}+{\frac {\ln \left({\frac {x}{\sqrt {2\pi }}}\right)}{W_{0}\left(e^{-1}\ln \left({\frac {x}{\sqrt {2\pi }}}\right)\right)}}\,,$$

where $W_{0}(x)$ is the Lambert W function. The formula is found by inverting the Stirling approximation, and so can also be expanded into an asymptotic series.

Series expansion

To obtain a series expansion of the inverse gamma function one can first compute the series expansion of the reciprocal gamma function ${\frac {1}{\Gamma (x)}}$ near the poles at the negative integers, and then invert the series.

Setting $z={\frac {1}{x}}$ then yields, for the n th branch $\Gamma _{n}^{-1}(z)$ of the inverse gamma function ($n\geq 0$)

$$\Gamma _{n}^{-1}(z)=-n+{\frac {\left(-1\right)^{n}}{n!z}}+{\frac {\psi ^{(0)}\left(n+1\right)}{\left(n!z\right)^{2}}}+{\frac {\left(-1\right)^{n}\left(\pi ^{2}+9\psi ^{(0)}\left(n+1\right)^{2}-3\psi ^{(1)}\left(n+1\right)\right)}{6\left(n!z\right)^{3}}}+O\left({\frac {1}{z^{4}}}\right)\,,$$

where $\psi ^{(n)}(x)$ is the polygamma function.