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;
	}
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>