# Falling Factorial

Compute the falling factorial.

## Usage

``````var fallingFactorial = require( '@stdlib/math/base/special/falling-factorial' );
``````

#### fallingFactorial( x, n )

Evaluates the falling factorial of `x` and `n`.

``````var v = fallingFactorial( 0.9, 5 );
// returns ~0.644

v = fallingFactorial( -9.0, 3 );
// returns -990.0

v = fallingFactorial( 0.0, 2 );
// returns 0.0

v = fallingFactorial( NaN, 3 );
// returns NaN

v = fallingFactorial( 5.0, NaN );
// returns NaN

v = fallingFactorial( NaN, NaN );
// returns NaN
``````

The function returns `NaN` if not provided a non-negative integer for `n`.

``````var v = fallingFactorial( 2.0, 1.5 );
// returns NaN

v = fallingFactorial( 3.0, -2 );
// returns NaN
``````

## Examples

``````var randu = require( '@stdlib/random/base/randu' );
var ceil = require( '@stdlib/math/base/special/ceil' );
var fallingFactorial = require( '@stdlib/math/base/special/falling-factorial' );

var n;
var x;
var i;

for ( i = 0; i < 100; i++ ) {
x = ( randu()*40.0 ) - 20.0;
n = ceil( randu()*20.0 );
console.log( 'fallingFactorial(%d,%d) = %d', x, n, fallingFactorial( x, n ) );
}
``````