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 = -0.5 * ln( 2.0 * s2 * PI );
	B = -1.0 / ( 2.0 * s2 );
	return logpdf;

	/**
	* Evaluates the natural logarithm of the probability density function (PDF) for a lognormal distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logPDF
	*
	* @example
	* var y = logpdf( 2.5 );
	* // returns <number>
	*/
	function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return NINF;
		}
		return A - ln( x ) + ( B * pow( ln(x) - mu, 2.0 ) );
	}
}
Returns a function for evaluating the natural logarithm of 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} logPDF

@example
var logpdf = factory( 4.0, 2.0 );
var y = logpdf( 10.0 );
// returns ~-4.275

y = logpdf( 2.0 );
// returns ~-3.672