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@license Apache-2.0
Copyright (c) 2018 The Stdlib Authors.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
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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.jsdelivr.net/gh/stdlib-js/stdlib@bb29798906e119fcb2af99e94b60407a270c9b32/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.jsdelivr.net/gh/stdlib-js/stdlib@591cf9d5c3a0cd3c1ceec961e5c49d73a68374cb/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 -->
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