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

	/**
	* Evaluates the cumulative distribution function (CDF) for a Student's t distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 2.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		var x2;
		var p;
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === 0.0 ) {
			return 0.5;
		}
		x2 = pow( x, 2.0 );
		if ( v > 2.0*x2 ) {
			z = x2 / ( v + x2 );
			p = betainc( z, 0.5, v/2.0, true, true ) / 2.0;
		} else {
			z = v / ( v + x2 );
			p = betainc( z, v/2.0, 0.5, true, false ) / 2.0;
		}
		return ( x > 0.0 ) ? 1.0 - p : p;
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a Student's t distribution with degrees of freedom `v`.

@param {PositiveNumber} v - degrees of freedom
@returns {Function} CDF

@example
var cdf = factory( 0.5 );
var y = cdf( 3.0 );
// returns ~0.816

y = cdf( 1.0 );
// returns ~0.699