function logpdf( x ) {
		var out;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return NINF;
		}
		out = firstTerm - ( ( alpha + 1.0 ) * ln( x ) ) - ( beta / x );
		return out;
	}
Evaluates the natural logarithm of the probability density function (PDF) for an inverse gamma distribution.

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

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