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

	/**
	* Evaluates the probability density function (PDF) for a Pareto (Type I) distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated PDF
	*
	* @example
	* var y = pdf( 4.0 );
	* // returns <number>
	*/
	function pdf( x ) {
		var denom;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x >= beta ) {
			denom = pow( x, alpha + 1.0 );
			return num / denom;
		}
		return 0.0;
	}
}
Returns a function for evaluating 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} PDF

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

var y = mypdf( 0.8 );
// returns ~0.494

y = mypdf( 2.0 );
// returns ~0.125