function skewness( k ) {
	var sigma3;
	var sigma2;
	var sigma;
	var mu;
	if ( isnan( k ) || k <= 0.0 ) {
		return NaN;
	}
	mu = mean( k );
	sigma = sqrt( variance( k ) );
	sigma2 = sigma * sigma;
	sigma3 = sigma2 * sigma;
	return ( mu / sigma3 ) * ( 1.0 - ( 2.0*sigma2 ) );
}
Returns the skewness of a chi distribution.

@param {PositiveNumber} k - degrees of freedom
@returns {PositiveNumber} skewness

@example
var v = skewness( 9.0 );
// returns ~0.252

@example
var v = skewness( 1.0 );
// returns ~0.995

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

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