# Definition:Gamma Function

## Definition

### Integral Form

The gamma function $\Gamma: \C \setminus \Z_{\le 0} \to \C$ is defined, for the open right half-plane, as:

$\ds \map \Gamma z = \map {\MM \set {e^{-t} } } z = \int_0^{\to \infty} t^{z - 1} e^{-t} \rd t$

where $\MM$ is the Mellin transform.

For all other values of $z$ except the non-positive integers, $\map \Gamma z$ is defined as:

$\map \Gamma {z + 1} = z \map \Gamma z$

### Weierstrass Form

The Weierstrass form of the gamma function is:

$\ds \frac 1 {\map \Gamma z} = z e^{\gamma z} \prod_{n \mathop = 1}^\infty \paren {\paren {1 + \frac z n} e^{-z / n} }$

where $\gamma$ is the Euler-Mascheroni constant.

The Weierstrass form is valid for all $\C$.

### Hankel Form

The Hankel form of the gamma function is:

$\ds \frac 1 {\map \Gamma z} = \dfrac 1 {2 \pi i} \oint_\HH \frac {e^t \rd t} {t^z}$

where $\HH$ is the contour starting at $-\infty$, circling the origin in an anticlockwise direction, and returning to $-\infty$.

The Hankel form is valid for all $\C$.

### Euler Form

The Euler form of the gamma function is:

$\ds \map \Gamma z = \frac 1 z \prod_{n \mathop = 1}^\infty \paren {\paren {1 + \frac 1 n}^z \paren {1 + \frac z n}^{-1} } = \lim_{m \mathop \to \infty} \frac {m^z m!} {z \paren {z + 1} \paren {z + 2} \cdots \paren {z + m} }$

which is valid except for $z \in \set {0, -1, -2, \ldots}$.

## Partial Gamma Function

Let $m \in \Z_{\ge 0}$.

The partial gamma function at $m$ is defined as:

$\ds \map {\Gamma_m} z := \frac {m^z m!} {z \paren {z + 1} \paren {z + 2} \cdots \paren {z + m} }$

which is valid except for $z \in \set {0, -1, -2, \ldots, -m}$.

## Graph of Gamma Function

The graph of the gamma function is illustrated here for real arguments.

The gamma function:

$\map \Gamma z$ (red solid line)
$\dfrac 1 {\map \Gamma z}$ (blue broken line)

## Also known as

Some authors call this function Euler's gamma function, after Leonhard Paul Euler.

## Examples

### Gamma Function of $4$

$\map \Gamma 4 = 6$

### Gamma Function of $\dfrac 1 2$

$\map \Gamma {\dfrac 1 2} = \sqrt \pi$

Its decimal expansion starts:

$\map \Gamma {\dfrac 1 2} = 1 \cdotp 77245 \, 38509 \, 05516 \, 02729 \, 81674 \, 83341 \, 14518 \, 27975 \ldots$

### Gamma Function of $\dfrac 1 3$

$\map \Gamma {\dfrac 1 3} = 2 \cdotp 67893 \, 85347 \, 07747 \, 63 \ldots$

### Gamma Function of $\dfrac 1 4$

$\map \Gamma {\dfrac 1 4} = 3 \cdotp 62560 \, 99082 \, 21908 \ldots$

## Also see

• Results about the gamma function can be found here.

## Historical Note

The symbol $\map \Gamma z$ for the gamma function was introduced by Adrien-Marie Legendre.