function variance( alpha, beta ) {
	if (
		isnan( alpha ) ||
		alpha <= 0.0 ||
		isnan( beta ) ||
		beta <= 0.0
	) {
		return NaN;
	}
	if ( alpha < 2.0 ) {
		return PINF;
	}
	return ( beta*beta * alpha ) / ( pow( alpha-1.0, 2.0 ) * ( alpha-2.0 ) );
}
Returns the variance of a Pareto (Type I) distribution.

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

@example
var v = variance( 4.0, 12.0 );
// returns 32.0

@example
var v = variance( 8.0, 2.0 );
// returns ~0.109

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

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

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

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

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