function kurtosis( k ) {
	var sigma2;
	var sigma;
	var g1;
	var mu;
	if ( isnan( k ) || k <= 0.0 ) {
		return NaN;
	}
	sigma2 = variance( k );
	sigma = sqrt( sigma2 );
	mu = mean( k );
	g1 = skewness( k );
	return ( 2.0/sigma2 ) * ( 1.0 - ( mu*sigma*g1 ) - sigma2 );
}
Returns the excess kurtosis of a chi distribution.

@param {PositiveNumber} k - degrees of freedom
@returns {PositiveNumber} excess kurtosis

@example
var v = kurtosis( 9.0 );
// returns ~0.011

@example
var v = kurtosis( 1.0 );
// returns ~0.869

@example
var v = kurtosis( -0.2 );
// returns NaN

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