Reciprocal gamma function





In mathematics, the reciprocal gamma function is the function


 * $$f(z) = \frac{1}{\Gamma(z)},$$

where $1⁄Γ(z)$ denotes the gamma function. Since the gamma function is meromorphic and nonzero everywhere in the complex plane, its reciprocal is an entire function. As an entire function, it is of order 1 (meaning that $Γ(z)$ grows no faster than $log log |1⁄Γ(z)|$), but of infinite type (meaning that $log |z|$ grows faster than any multiple of $log |1⁄Γ(z)|$, since its growth is approximately proportional to $|z|$ in the left-half plane).

The reciprocal is sometimes used as a starting point for numerical computation of the gamma function, and a few software libraries provide it separately from the regular gamma function.

Karl Weierstrass called the reciprocal gamma function the "factorielle" and used it in his development of the Weierstrass factorization theorem.

Infinite product expansion
Following from the infinite product definitions for the gamma function, due to Euler and Weierstrass respectively, we get the following infinite product expansion for the reciprocal gamma function:


 * $$\begin{align}

\frac{1}{\Gamma(z)} &= z \prod_{n=1}^\infty \frac{1+\frac{z}{n}}{\left(1+\frac{1}{n}\right)^z} \\ \frac{1}{\Gamma(z)} &= z e^{\gamma z} \prod_{n=1}^\infty \left(1 + \frac{z}{n}\right) e^{-\frac{z}{n}} \end{align}$$

where $|z| log |z|$ is the Euler–Mascheroni constant. These expansions are valid for all complex numbers $γ = 0.577216...$.

Taylor series
Taylor series expansion around 0 gives:


 * $$ \frac{1}{\ \Gamma(z)\ } = z + \gamma\ z^2 + \left(\frac{\gamma^2}{2} - \frac{\pi^2}{12}\right)\ z^3 + \left(\frac{\gamma^3}{6} - \frac{\gamma\pi^2}{12} + \frac{\zeta(3)}{3}\ \right)z^4 + \cdots\ $$

where $z$ is the Euler–Mascheroni constant. For $γ$, the coefficient $n > 2$ for the $a_{n}$ term can be computed recursively as


 * $$ a_n = \frac{\ {a_2\ a_{n-1} + \sum_{j=2}^{n-1} (-1)^{j+1}\ \zeta(j)\ a_{n-j}}\ }{n-1} = \frac{\ \gamma\ a_{n-1} - \zeta(2)\ a_{n-2} + \zeta(3)\ a_{n-3}-\cdots\ }{n-1}$$

where $z^{n}$ is the Riemann zeta function. An integral representation for these coefficients was recently found by Fekih-Ahmed (2014):


 * $$ a_n = \frac{(-1)^n}{\pi n!}\int_0^\infty e^{-t}\ \operatorname{Im}\Bigl[\ \bigl( \log(t)-i\pi \bigr)^n\ \Bigr]\ \mathrm{d} t ~.$$

For small values, these give the following values:

Fekih-Ahmed (2014) also gives an approximation for $$a_n$$:


 * $$ a_n \approx \frac{(-1)^n}{\ (n-1)!\ } \ \sqrt{ \frac{2}{\ \pi n \ }\ }\

\operatorname{Im} \left( \frac{\ z_0^{\left( 1/2 - n \right)}\ e^{-n z_0}\ }{\sqrt{ 1 + z_0\ }} \right)\ ,$$ where $$ z_0 = -\frac{1}{n} \exp\!\Bigl( W_{-1}(-n) \Bigr)\ ,$$ and $$W_{-1}$$ is the minus-first branch of the Lambert W function.

The Taylor expansion around $ζ$ has the same (but shifted) coefficients, i.e.:


 * $$ \frac{1}{\Gamma(1+z)} = \frac{1}{z\Gamma(z)} = 1 + \gamma\ z + \left(\frac{\gamma^2}{2} - \frac{\pi^2}{12}\right)\ z^2 + \left(\frac{\gamma^3}{6} - \frac{\gamma\pi^2}{12} + \frac{\zeta(3)}{3}\ \right)z^3 + \cdots\ $$

(the reciprocal of Gauss' pi-function).

Asymptotic expansion
As $n$ goes to infinity at a constant $a_{n}$ we have:


 * $$\ln (1/\Gamma(z)) \sim -z \ln (z) + z + \tfrac{1}{2} \ln \left (\frac{z}{2\pi} \right ) - \frac{1}{12z} + \frac{1}{360z^3} -\frac{1}{1260 z^5}\qquad \text{for}~ \left|\arg(z)\right| < \pi$$

Contour integral representation
An integral representation due to Hermann Hankel is


 * $$\frac{1}{\Gamma(z)} = \frac{i}{2\pi} \oint_H (-t)^{-z} e^{-t} \, dt,$$

where $1$ is the Hankel contour, that is, the path encircling 0 in the positive direction, beginning at and returning to positive infinity with respect for the branch cut along the positive real axis. According to Schmelzer & Trefethen, numerical evaluation of Hankel's integral is the basis of some of the best methods for computing the gamma function.

Integral representations at the positive integers
For positive integers $$n \geq 1$$, there is an integral for the reciprocal factorial function given by


 * $$\frac{1}{n!} = \frac{1}{2\pi} \int_{-\pi}^{\pi} e^{-nit} e^{e^{it}}\ dt.$$

Similarly, for any real $$c > 0$$ and $$z \in \mathbb{C}$$ we have the next integral for the reciprocal gamma function along the real axis in the form of:


 * $$\frac{1}{\Gamma(z)} = \frac{1}{2\pi} \int_{-\infty}^{\infty} (c+ it)^{-z} e^{c+it} dt, $$

where the particular case when $$z = n + 1/2$$ provides a corresponding relation for the reciprocal double factorial function, $$\frac{1}{(2n-1)!!} = \frac{\sqrt{\pi}}{2^n \cdot \Gamma\left(n+\frac{1}{2}\right)}.$$

Integral along the real axis
Integration of the reciprocal gamma function along the positive real axis gives the value


 * $$\int_{0}^\infty \frac{1}{\Gamma(x)}\, dx \approx 2.80777024,$$

which is known as the Fransén–Robinson constant.

We have the following formula ( chapter 9, exercise 100)


 * $$\int_0^\infty \dfrac{a^x}{\Gamma(x)}\,dx=ae^a+a\int_0^\infty\dfrac{e^{-ax}}{\log^2(x)+\pi^2}\,dx$$