function factory( k ) {
	return gammaFactory( k/2.0, 0.5 );
}
Returns a function for evaluating the cumulative distribution function (CDF) for a chi-squared distribution with degrees of freedom `k`.

@param {NonNegativeNumber} k - degrees of freedom
@returns {Function} CDF

@example
var cdf = factory( 3.0 );

var y = cdf( 6.0 );
// returns ~0.888

y = cdf( 1.5 );
// returns ~0.318