factorial2

Double factorial function.

The 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.

Installation

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

Usage

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

factorial2( n )

Evaluates the double factorial of n.

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

Examples

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 ] ) );
}

C APIs

Usage

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

stdlib_base_factorial2( n )

Evaluates the double factorial of n.

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

The function accepts the following arguments:

• n: [in] double input value.

double stdlib_base_factorial2( const double n );

Examples

#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 );
    }
}

See Also

• @stdlib/math-base/special/factorial: evaluate a factorial.

Notice

This package is part of 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, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2026. The Stdlib Authors.