function pdf( x ) {
		var lnl;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return 0.0;
		}
		lnl = firstTerm - (( alpha + 1.0 ) * ln( x )) - (beta / x);
		return exp( lnl );
	}
Evaluates the probability density function (PDF) for an inverse gamma distribution.

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

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