function skewness( alpha, beta ) {
	if (
		isnan( alpha ) ||
		isnan( beta ) ||
		alpha <= 3.0 ||
		beta <= 0.0
	) {
		return NaN;
	}
	return 4.0 * sqrt( alpha - 2.0 ) / ( alpha - 3.0 );
}
Returns the skewness of an inverse gamma distribution.

@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {PositiveNumber} skewness

@example
var v = skewness( 4.0, 12.0 );
// returns ~5.657

@example
var v = skewness( 8.0, 2.0 );
// returns ~1.96

@example
var v = skewness( 2.0, 1.0 );
// returns NaN

@example
var v = skewness( 4.0, -0.1 );
// returns NaN

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

@example
var v = skewness( 4.0, NaN );
// returns NaN

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