function factory( d1, d2 ) {
	if (
		isnan( d1 ) ||
		isnan( d2 ) ||
		d1 <= 0.0 ||
		d2 <= 0.0
	) {
		return constantFunction( NaN );
	}
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) for an F distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 10.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return 0.0;
		}
		if ( x === PINF ) {
			return 1.0;
		}
		if ( d1 * x > d2 ) {
			return betainc( (d1*x)/(d2+(d1*x)), d1/2.0, d2/2.0, true, false );
		}
		return betainc( d2/(d2+(d1*x)), d2/2.0, d1/2.0, true, true );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for an F distribution with numerator degrees of freedom `d1` and denominator degrees of freedom `d2`.

@param {PositiveNumber} d1 - numerator degrees of freedom
@param {PositiveNumber} d2 - denominator degrees of freedom
@returns {Function} CDF

@example
var cdf = factory( 10.0, 2.0 );

var y = cdf( 10.0 );
// returns ~0.906

y = cdf( 8.0 );
// returns ~0.884