In mathematics, the gamma function (Γ(z)) is a key topic in the field of special functions. Γ(z) is an extension of the factorial function to all complex numbers except negative integers. For positive integers, it is defined as ${\displaystyle \Gamma (n)=(n-1)!}$^[1][2]

The gamma function is defined for all complex numbers, but it is not defined for negative integers and zero. For a complex number whose real part is a positive integer, the function is defined by:^[2][3]

${\displaystyle \Gamma (z)=\int _{0}^{\infty }t^{z-1}e^{-t}\,{\rm {d))t.}$

Properties

Particular values

Some particular values of the gamma function are:

${\displaystyle {\begin{array}{lll}\Gamma (-3/2)&={\tfrac {4}{3)){\sqrt {\pi ))&\approx 2.363271801207\\\Gamma (-1/2)&=-2{\sqrt {\pi ))&\approx -3.544907701811\\\Gamma (1/2)&={\sqrt {\pi ))&\approx 1.772453850905\\\Gamma (1)&=0!&=1\\\Gamma (3/2)&={\tfrac {1}{2)){\sqrt {\pi ))&\approx 0.88622692545\\\Gamma (2)&=1!&=1\\\Gamma (5/2)&={\tfrac {3}{4)){\sqrt {\pi ))&\approx 1.32934038818\\\Gamma (3)&=2!&=2\\\Gamma (7/2)&={\tfrac {15}{8)){\sqrt {\pi ))&\approx 3.32335097045\\\Gamma (4)&=3!&=6\\\end{array))}$

Pi function

Gauss introduced the Pi function. This is another way of denoting the gamma function. In terms of the gamma function, the Pi function is

${\displaystyle \Pi (z)=\Gamma (z+1)=z\;\Gamma (z)=\int _{0}^{\infty }e^{-t}t^{z+1}\,{\frac ((\rm {d))t}{t)),}$

so that

${\displaystyle \Pi (n)=n!}$

for every non-negative integer n.

Applications

Analytic number theory

The gamma function is used to study the Riemann zeta function. A property of the Riemann zeta function is its functional equation:

${\displaystyle \Gamma \left({\frac {s}{2))\right)\zeta (s)\pi ^{-s/2}=\Gamma \left({\frac {1-s}{2))\right)\zeta (1-s)\pi ^{-(1-s)/2}.}$

Bernhard Riemann found a relation between these two functions. This was published in his 1859 paper "Über die Anzahl der Primzahlen unter einer gegebenen Grösse" ("On the Number of Prime Numbers less than a Given Quantity")

${\displaystyle \zeta (z)\;\Gamma (z)=\int _{0}^{\infty }{\frac {t^{z)){e^{t}-1))\;{\frac {dt}{t)).}$

Related pages

• Gamma distribution