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 ) );
	}
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>