function kurtosis( alpha, beta ) {
	var abm1;
	var bm1;
	var out;
	if (
		isnan( alpha ) ||
		alpha <= 0.0 ||
		isnan( beta ) ||
		beta <= 4.0
	) {
		return NaN;
	}
	abm1 = alpha + beta - 1.0;
	bm1 = beta - 1.0;
	out = ( alpha * abm1 * ( (5.0*beta )-11.0 ) ) + ( ( bm1*bm1 )*( bm1-1.0 ) );
	out *= 6.0;
	out /= alpha * abm1 * ( beta-3.0 ) * ( beta-4.0 );
	return out;
}
Returns the excess kurtosis of a beta prime distribution.

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

@example
var v = kurtosis( 2.0, 6.0 );
// returns ~26.143

@example
var v = kurtosis( 4.0, 12.0 );
// returns ~5.764

@example
var v = kurtosis( 12.0, 6.0 );
// returns ~19.49

@example
var v = kurtosis( 2.0, 4.0 );
// returns NaN

@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