function skewness( alpha, beta ) {
	var out;
	if (
		isnan( alpha ) ||
		alpha <= 0.0 ||
		isnan( beta ) ||
		beta <= 3.0
	) {
		return NaN;
	}
	out = 2.0 * ( ( 2.0*alpha ) + beta - 1.0 ) / ( beta-3.0 );
	out *= sqrt( ( beta-2.0 ) / ( alpha * ( alpha+beta-1.0 ) ) );
	return out;
}
Returns the skewness of a beta prime distribution.

@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {number} skewness

@example
var v = skewness( 2.0, 4.0 );
// returns ~6.261

@example
var v = skewness( 4.0, 12.0 );
// returns ~1.724

@example
var v = skewness( 8.0, 4.0 );
// returns ~5.729

@example
var v = skewness( 2.0, 2.0 );
// returns NaN

@example
var v = skewness( 1.0, -0.1 );
// returns NaN

@example
var v = skewness( -0.1, 1.0 );
// returns NaN

@example
var v = skewness( 2.0, NaN );
// returns NaN

@example
var v = skewness( NaN, 2.0 );
// returns NaN