function factory( sigma ) {
	var s2i;
	var s2;
	if ( isnan( sigma ) || sigma < 0.0 ) {
		return constantFunction( NaN );
	}
	if ( sigma === 0.0 ) {
		return degenerate( 0.0 );
	}
	s2 = pow( sigma, 2.0 );
	s2i = 1.0 / s2;
	return pdf;

	/**
	* Evaluates the probability density function (PDF) for a Rayleigh distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated PDF
	*
	* @example
	* var y = pdf( 2.3 );
	* // returns <number>
	*/
	function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 || x === PINF ) {
			return 0.0;
		}
		return s2i * x * exp( -pow( x, 2.0 ) / ( 2.0 * s2 ) );
	}
}
Returns a function for evaluating the probability density function (PDF) for a Rayleigh distribution with scale parameter `sigma`.

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

@example
var pdf = factory( 0.5 );
var y = pdf( 1.0 );
// returns ~0.541

y = pdf( 0.1 );
// returns ~0.392