function chi2( k ) {
		if (
			isnan( k ) ||
			k <= 0.0
		) {
			return NaN;
		}
		return sqrt( rchisq( k ) );
	}
Returns a random number drawn from a chi distribution with degrees of freedom `k`.

@private
@param {PositiveNumber} k - degrees of freedom
@returns {number} pseudorandom number

@example
var v = chi2( 2 );
// returns <number>