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# factorialln

> Natural logarithm of the [factorial function][factorial-function].

<section class="intro">

The natural logarithm of the factorial function may be expressed

<!-- <equation class="equation" label="eq:factorialln_function" align="center" raw="f(n)=\ln (n!)" alt="Equation of the natural logarithm of the factorial."> -->

<div class="equation" align="center" data-raw-text="f(n)=\ln (n!)" data-equation="eq:factorialln_function">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/factorialln/docs/img/equation_factorialln_function.svg" alt="Equation of the natural logarithm of the factorial.">
    <br>
</div>

<!-- </equation> -->

The [factorial function][factorial-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/factorialln/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/factorialln/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/factorialln/docs/img/equation_zero_factorial.svg" alt="Zero factorial">
    <br>
</div>

<!-- </equation> -->

The [Gamma][gamma-function] function extends the [factorial function][factorial-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/factorialln/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.

Evaluating the natural logarithm of [factorial function][factorial-function] is useful as the [factorial function][factorial-function] can overflow for large `n`. Thus, `factorialln( n )` is generally preferred to `ln( n! )`.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### factorialln( x )

Evaluates the natural logarithm of the [factorial function][factorial-function]. For input values other than negative integers, the function returns `ln( x! ) = ln( Γ(x+1) )`, where `Γ` is the [Gamma][gamma-function] function. For negative integers, the function returns `NaN`.

```javascript
var v = factorialln( 3.0 );
// returns ~1.792

v = factorialln( 2.4 );
// returns ~1.092

v = factorialln( -1.0 );
// returns NaN

v = factorialln( -1.5 );
// returns ~1.266
```

If provided `NaN`, the function returns `NaN`.

```javascript
var v = factorialln( NaN );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

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

var x;
var v;
var i;

x = incrspace( -10.0, 50.0, 0.5 );
for ( i = 0; i < x.length; i++ ) {
    v = factorialln( 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

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

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

</section>

<!-- /.links -->
