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# Factorial

> [Factorial][factorial-function] function.

<section class="intro">

The [factorial][factorial-function] function may be defined as the product

<!-- <equation class="equation" label="eq:factorial_function" align="center" raw="n! = \prod_{k=1}^n k" alt="Factorial function definition"> -->

<div class="equation" align="center" data-raw-text="n! = \prod_{k=1}^n k" data-equation="eq:factorial_function">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/factorial/docs/img/equation_factorial_function.svg" alt="Factorial function definition">
    <br>
</div>

<!-- </equation> -->

or according to the recurrence relation

<!-- <equation class="equation" label="eq:factorial_recurrence_relation" align="center" raw="n! = \begin{cases}1 & \textrm{if } n = 0,\\(n-1)! \times n & \textrm{if } n > 1\end{cases}" alt="Factorial function recurrence relation"> -->

<div class="equation" align="center" data-raw-text="n! = \begin{cases}1 &amp; \textrm{if } n = 0,\\(n-1)! \times n &amp; \textrm{if } n &gt; 1\end{cases}" data-equation="eq:factorial_recurrence_relation">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/factorial/docs/img/equation_factorial_recurrence_relation.svg" alt="Factorial function recurrence relation">
    <br>
</div>

<!-- </equation> -->

Following the convention for an [empty product][empty-product], in all definitions,

<!-- <equation class="equation" label="eq:zero_factorial" align="center" raw="0! = 1" alt="Zero factorial"> -->

<div class="equation" align="center" data-raw-text="0! = 1" data-equation="eq:zero_factorial">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/factorial/docs/img/equation_zero_factorial.svg" alt="Zero factorial">
    <br>
</div>

<!-- </equation> -->

The [Gamma][@stdlib/math/base/special/gamma] function extends the [factorial][factorial-function] function for non-integer values.

<!-- <equation class="equation" label="eq:factorial_function_and_gamma" align="center" raw="n! = \Gamma(n+1)" alt="Factorial function extension via the Gamma function"> -->

<div class="equation" align="center" data-raw-text="n! = \Gamma(n+1)" data-equation="eq:factorial_function_and_gamma">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/factorial/docs/img/equation_factorial_function_and_gamma.svg" alt="Factorial function extension via the Gamma function">
    <br>
</div>

<!-- </equation> -->

The [factorial][factorial-function] of a **negative** integer is not defined.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### factorial( x )

Evaluates the [factorial][factorial-function] function.

```javascript
var v = factorial( 3.0 );
// returns 6.0

v = factorial( -1.5 );
// returns ~-3.545

v = factorial( -0.5 );
// returns ~1.772

v = factorial( 0.5 );
// returns ~0.886

v = factorial( -10.0 );
// returns NaN

v = factorial( 171.0 );
// returns Infinity

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

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

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

var x = incrspace( -10.0, 100.0, 1.0 );
var v;
var i;

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

</section>

<!-- /.examples -->

<section class="links">

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

[factorial-function]: https://en.wikipedia.org/wiki/Factorial

[empty-product]: https://en.wikipedia.org/wiki/Empty_product

</section>

<!-- /.links -->
