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 logpdf;

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

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

@example
var logpdf = factory( 0.5 );
var y = logpdf( 1.0 );
// returns ~-0.614

y = logpdf( 0.1 );
// returns ~-0.936