function variance( alpha, beta ) {
	if ( alpha <= 2.0 || beta <= 0.0 ) {
		return NaN;
	}
	return ( beta*beta ) / ( pow( alpha-1.0, 2.0 ) * ( alpha-2.0 ) );
}
Returns the variance of an inverse gamma distribution.

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

@example
var v = variance( 3.0, 5.0 );
// returns ~6.25

@example
var v = variance( 4.0, 12.0 );
// returns 8.0

@example
var v = variance( 8.0, 2.0 );
// returns ~0.014

@example
var v = variance( 3.0, -0.1 );
// returns NaN

@example
var v = variance( 1.5, 1.0 );
// returns NaN

@example
var v = variance( 3.0, NaN );
// returns NaN

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