function kurtosis( k ) {
	if ( isnan( k ) || k <= 0.0 ) {
		return NaN;
	}
	return 12.0 / k;
}
Returns the excess kurtosis of a chi-squared distribution.

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

@example
var v = kurtosis( 9.0 );
// returns ~1.333

@example
var v = kurtosis( 1.0 );
// returns 12.0

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

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