function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < a || x > b ) {
			return 0.0;
		}
		return A * exp( B * pow( x - mu, 2.0 ) ) / C;
	}
Evaluates the probability density function (PDF) for a truncated normal distribution.

@private
@param {number} x - input value
@returns {number} evaluated PDF