@stdlib/math-base-special-factorial2 NPM

factorial2

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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 ] ) );
}

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.