function kurtosis( mu, sigma ) {
	var out;
	var s2;
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	s2 = sigma * sigma;
	out = exp( 4.0*s2 );
	out += 2.0 * exp( 3.0*s2 );
	out += 3.0 * exp( 2.0*s2 );
	out -= 6.0;
	return out;
}
Returns the excess kurtosis for a lognormal distribution with location `mu` and scale `sigma`.

@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {number} excess kurtosis

@example
var y = kurtosis( 0.0, 1.0 );
// returns ~110.936

@example
var y = kurtosis( 5.0, 2.0 );
// returns ~9220556.977

@example
var y = kurtosis( NaN, 1.0 );
// returns NaN

@example
var y = kurtosis( 0.0, NaN );
// returns NaN

@example
var y = kurtosis( 0.0, 0.0 );
// returns NaN