function variance( k ) {
	if ( isnan( k ) || k < 0.0 ) {
		return NaN;
	}
	return 2.0 * k;
}
Returns the variance of a chi-squared distribution.

@param {NonNegativeNumber} k - degrees of freedom
@returns {NonNegativeNumber} variance

@example
var v = variance( 9.0 );
// returns 18.0

@example
var v = variance( 1.0 );
// returns 2.0

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

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