function pdf( 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 0.0;
	}
	s2 = pow( sigma, 2.0 );
	A = 1.0 / ( sqrt( 2.0 * s2 * PI ) );
	B = -1.0 / ( 2.0 * s2 );
	return (1.0/x) * A * exp( B * pow( ln(x) - mu, 2.0 ) );
}
Evaluates 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 PDF

@example
var y = pdf( 2.0, 0.0, 1.0 );
// returns ~0.157

@example
var y = pdf( 1.0, 0.0, 1.0 );
// returns ~0.399

@example
var y = pdf( 1.0, 3.0, 1.0 );
// returns ~0.004

@example
var y = pdf( -1.0, 4.0, 2.0 );
// returns 0.0

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

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

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

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