function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 || x === PINF ) {
			return 0.0;
		}
		return gammaDeriv( alpha, x * beta ) * beta;
	}
Evaluates the probability density function (PDF) for a gamma distribution.

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

@example
var y = pdf( -1.2 );
// returns <number>