function factory( sigma ) {
	var s2;
	if ( isnan( sigma ) || sigma < 0.0 ) {
		return constantFunction( NaN );
	}
	if ( sigma === 0.0 ) {
		return degenerate( 0.0 );
	}
	s2 = pow( sigma, 2.0 );
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) for a Rayleigh distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 2 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		return 1.0 - exp( -pow( x, 2.0 ) / ( 2.0 * s2 ) );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a Rayleigh distribution with scale parameter `sigma`.

@param {NonNegativeNumber} sigma - scale parameter
@returns {Function} CDF

@example
var cdf = factory( 2.0 );
var y = cdf( 3.0 );
// returns ~0.675

y = cdf( 1.0 );
// returns ~0.118