function kurtosis( alpha, beta ) {
	var out;
	if (
		isnan( alpha ) ||
		alpha <= 4.0 ||
		isnan( beta ) ||
		beta <= 0.0
	) {
		return NaN;
	}
	out = 6.0 * ( pow( alpha, 3.0 ) + pow( alpha, 2.0 ) - ( 6.0*alpha ) - 2.0 );
	out /= alpha * ( alpha-3.0 ) * ( alpha-4.0 );
	return out;
}
Returns the excess kurtosis of a Pareto (Type I) distribution.

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

@example
var v = kurtosis( 5.0, 1.0 );
// returns ~70.8

@example
var v = kurtosis( 7.0, 12.0 );
// returns ~24.857

@example
var v = kurtosis( 8.0, 2.0 );
// returns ~19.725

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

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

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

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