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# Gamma Function

> [Gamma][gamma-function] function.

<section class="intro">

The [gamma function][gamma-function] extends the [factorial function][@stdlib/math/base/special/factorial] to [real][real] and [complex][complex] numbers. If `n` is a positive `integer`,

<!-- <equation class="equation" label="eq:gamma_function_positive_integers" align="center" raw="\Gamma ( n ) = (n-1)!" alt="Gamma function for positive integers."> -->

<div class="equation" align="center" data-raw-text="\Gamma ( n ) = (n-1)!" data-equation="eq:gamma_function_positive_integers">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/gamma/docs/img/equation_gamma_function_positive_integers.svg" alt="Gamma function for positive integers.">
    <br>
</div>

<!-- </equation> -->

Generalized to all complex numbers `z`, except for nonpositive integers, the [gamma function][gamma-function] can be expressed as an infinite product

<!-- <equation class="equation" label="eq:gamma_function_infinite_product" align="center" raw="\Gamma ( z ) = \frac{e^{-\gamma z}}{z} \prod^{\infty}_{n=1} \left ( 1+\frac{z}{n}\right )^{-1} e^{z/n}" alt="Gamma function for all complex numbers."> -->

<div class="equation" align="center" data-raw-text="\Gamma ( z ) = \frac{e^{-\gamma z}}{z} \prod^{\infty}_{n=1} \left ( 1+\frac{z}{n}\right )^{-1} e^{z/n}" data-equation="eq:gamma_function_infinite_product">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/gamma/docs/img/equation_gamma_function_infinite_product.svg" alt="Gamma function for all complex numbers.">
    <br>
</div>

<!-- </equation> -->

where `γ ≈ 0.577216` is the  [Euler–Mascheroni constant][@stdlib/constants/float64/eulergamma].

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var gamma = require( '@stdlib/math/base/special/gamma' );
```

#### gamma( x )

Evaluates the [gamma function][gamma-function].

```javascript
var v = gamma( 4.0 );
// returns 6.0

v = gamma( -1.5 );
// returns ~2.363

v = gamma( -0.5 );
// returns ~-3.545

v = gamma( 0.5 );
// returns ~1.772

v = gamma( 0.0 );
// returns Infinity

v = gamma( -0.0 );
// returns -Infinity

v = gamma( NaN );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var linspace = require( '@stdlib/array/linspace' );
var gamma = require( '@stdlib/math/base/special/gamma' );

var x = linspace( -10.0, 10.0, 100 );
var v;
var i;

for ( i = 0; i < x.length; i++ ) {
    v = gamma( x[ i ] );
    console.log( 'x: %d, f(x): %d', x[ i ], v );
}
```

</section>

<!-- /.examples -->

<section class="links">

[gamma-function]: https://en.wikipedia.org/wiki/Gamma_function

[@stdlib/math/base/special/factorial]: https://github.com/stdlib-js/math/tree/main/base/special/factorial

[real]: https://en.wikipedia.org/wiki/Real_number

[complex]: https://en.wikipedia.org/wiki/Complex_number

[@stdlib/constants/float64/eulergamma]: https://github.com/stdlib-js/constants-float64-eulergamma

</section>

<!-- /.links -->
