factorial2

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.

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 ] ) );
}