function logpdf( x, mu, sigma ) {
	var s2;
	var A;
	var B;
	if (
		isnan( x ) ||
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	if ( x <= 0.0 ) {
		return NINF;
	}
	s2 = pow( sigma, 2.0 );
	A = -0.5 * ln( 2.0 * s2 * PI );
	B = -1.0 / ( 2.0 * s2 );
	return A - ln( x ) + ( B * pow( ln(x) - mu, 2.0 ) );
}
Evaluates the natural logarithm of the probability density function (PDF) for a lognormal distribution with location parameter `mu` and scale parameter `sigma` at a value `x`.

@param {number} x - input value
@param {number} mu - location parameter
@param {PositiveNumber} sigma - scale parameter
@returns {number} evaluated logPDF

@example
var y = logpdf( 2.0, 0.0, 1.0 );
// returns ~-1.852

@example
var y = logpdf( 1.0, 0.0, 1.0 );
// returns ~-0.919

@example
var y = logpdf( 1.0, 3.0, 1.0 );
// returns ~-5.419

@example
var y = logpdf( -1.0, 4.0, 2.0 );
// returns -Infinity

@example
var y = logpdf( NaN, 0.0, 1.0 );
// returns NaN

@example
var y = logpdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = logpdf( 0.0, 0.0, NaN );
// returns NaN

@example
// Negative scale parameter:
var y = logpdf( 2.0, 0.0, -1.0 );
// returns NaN