function logpdf( x, alpha, beta ) {
	var denom;
	var num;
	if (
		isnan( x ) ||
		isnan( alpha ) ||
		isnan( beta ) ||
		alpha <= 0.0 ||
		beta <= 0.0
	) {
		return NaN;
	}
	if ( x >= beta ) {
		num = ln( alpha ) + ( alpha * ln( beta ) );
		denom = ( alpha + 1.0 ) * ln( x );
		return num - denom;
	}
	return NINF;
}
Evaluates the natural logarithm of the probability density function (PDF) for a Pareto (Type I) distribution with shape parameter `alpha` and scale parameter `beta` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - scale parameter
@returns {number} evaluated logPDF

@example
var y = logpdf( 4.0, 1.0, 1.0 );
// returns ~-2.773

@example
var y = logpdf( 20.0, 1.0, 10.0 );
// returns ~-3.689

@example
var y = logpdf( 7.0, 2.0, 6.0 );
// returns ~-1.561

@example
var y = logpdf( 7.0, 6.0, 3.0 );
// returns ~-5.238

@example
var y = logpdf( 1.0, 4.0, 2.0 );
// returns -Infinity

@example
var y = logpdf( 1.5, 4.0, 2.0 );
// returns -Infinity

@example
var y = logpdf( 0.5, -1.0, 0.5 );
// returns NaN

@example
var y = logpdf( 0.5, 0.5, -1.0 );
// returns NaN

@example
var y = logpdf( NaN, 1.0, 1.0 );
// returns NaN

@example
var y = logpdf( 0.5, NaN, 1.0 );
// returns NaN

@example
var y = logpdf( 0.5, 1.0, NaN );
// returns NaN