function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x >= beta ) {
			return num - ( ( alpha + 1.0 ) * ln( x ) );
		}
		return NINF;
	}
Evaluates the natural logarithm of the probability density function (PDF) for a Pareto (Type I) distribution.

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

@example
var y = logpdf( 4.0 );
// returns <number>