function factory( mu, sigma ) {
	var s2;
	var A;
	var B;
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return constantFunction( NaN );
	}
	s2 = pow( sigma, 2.0 );
	A = 1.0 / ( sqrt( 2.0 * s2 * PI ) );
	B = -1.0 / ( 2.0 * s2 );
	return pdf;

	/**
	* Evaluates the probability density function (PDF) for a lognormal distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated PDF
	*
	* @example
	* var y = pdf( 2.5 );
	* // returns <number>
	*/
	function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return 0.0;
		}
		return (1.0/x) * A * exp( B * pow( ln(x) - mu, 2.0 ) );
	}
}
Returns a function for evaluating the probability density function (PDF) for a lognormal distribution with location parameter `mu` and scale parameter `sigma`.

@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {Function} PDF

@example
var pdf = factory( 4.0, 2.0 );
var y = pdf( 10.0 );
// returns ~0.014

y = pdf( 2.0 );
// returns ~0.025