function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return A * exp( B * pow(x-mu, 2.0) );
	}
Evaluates the probability density function (PDF) for a normal distribution.

@private
@param {number} x - input value
@returns {number} evaluated probability density function

@example
var y = pdf( -3.14 );
// returns <number>