npm package: @stdlib/math-base-special-factorial2
Description: Evaluate the double factorial.
Author: The Stdlib Authors
Latest version: 0.3.1

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# factorial2

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> [Double factorial][double-factorial] function.

<section class="intro">

The [double factorial][double-factorial] of a number `n`, denoted `n!!`, is defined as the product of all the positive integers up to `n` that have the same parity (odd or even) as `n`.

Thus, for example, `5!!` is `5 * 3 * 1 = 15` and `8!!` is `8 * 6 * 4 * 2 = 384`.

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/math-base-special-factorial2
```

</section>

<section class="usage">

## Usage

```javascript
var factorial2 = require( '@stdlib/math-base-special-factorial2' );
```

#### factorial2( n )

Evaluates the [double factorial][double-factorial] of `n`.

```javascript
var v = factorial2( 2 );
// returns 2

v = factorial2( 3 );
// returns 3

v = factorial2( 0 );
// returns 1

v = factorial2( 4 );
// returns 8

v = factorial2( 5 );
// returns 15

v = factorial2( NaN );
// returns NaN

v = factorial2( 301 );
// returns Infinity
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var oneTo = require( '@stdlib/array-base-one-to' );
var factorial2 = require( '@stdlib/math-base-special-factorial2' );

var values = oneTo( 300 );

var i;
for ( i = 0; i < values.length; i++ ) {
    console.log( 'f(%d): %d', values[ i ], factorial2( values[ i ] ) );
}
```

</section>

<!-- /.examples -->

<!-- C interface documentation. -->

* * *

<section class="c">

## C APIs

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- C usage documentation. -->

<section class="usage">

### Usage

```c
#include "stdlib/math/base/special/factorial2.h"
```

#### stdlib_base_factorial2( n )

Evaluates the [double factorial][double-factorial] of `n`.

```c
double out = stdlib_base_factorial2( 3.0 );
// returns 3.0
```

The function accepts the following arguments:

-   **n**: `[in] double` input value.

```c
double stdlib_base_factorial2( const double n );
```

</section>

<!-- /.usage -->

<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

</section>

<!-- /.notes -->

<!-- C API usage examples. -->

<section class="examples">

### Examples

```c
#include "stdlib/math/base/special/factorial2.h"
#include <stdio.h>

int main( void ) {
    const double x[] = { 1.0, 10.0, 100.0, 301.0, 302.0 };

    double b;
    int i;
    for ( i = 0; i < 5; i++ ){
        b = stdlib_base_factorial2( x[ i ] );
        printf ( "factorial2(%lf) = %lf\n", x[ i ], b );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/math-base/special/factorial`][@stdlib/math/base/special/factorial]</span><span class="delimiter">: </span><span class="description">evaluate a factorial.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->


<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

<!-- /.stdlib -->

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/math-base-special-factorial2/main/LICENSE

[double-factorial]: https://en.wikipedia.org/wiki/Double_factorial

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[@stdlib/math/base/special/factorial]: https://www.npmjs.com/package/@stdlib/math-base-special-factorial

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</section>

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