function factory( alpha, beta ) {
	var num;
	if (
		isnan( alpha ) ||
		isnan( beta ) ||
		alpha <= 0.0 ||
		beta <= 0.0
	) {
		return constantFunction( NaN );
	}
	num = ln( alpha ) + ( alpha * ln( beta ) );
	return logpdf;

	/**
	* 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>
	*/
	function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x >= beta ) {
			return num - ( ( alpha + 1.0 ) * ln( x ) );
		}
		return NINF;
	}
}
Returns a function for evaluating the natural logarithm of the probability density function (PDF) for a Pareto (Type I) distribution with shape parameter `alpha` and scale parameter `beta`.

@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - scale parameter
@returns {Function} logPDF

@example
var mylogpdf = factory( 0.5, 0.5 );

var y = mylogpdf( 0.8 );
// returns ~-0.705

y = mylogpdf( 2.0 );
// returns ~-2.079